Modeling, preparation, and characterization of a dipole moment switch driven by Z/E photoisomerization

Article abstract of DOI:10.1021/ja906733q

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene- pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a Reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Full text of DOI:10.1021/ja906733q

Published on Web 06/22/2010
Modeling, Preparation, and Characterization of a Dipole
Moment Switch Driven by Z/E Photoisomerization
†
,9
†
†
,‡
Alfonso Melloni, Riccardo Rossi Paccani, Donato Donati, Vinicio Zanirato,*
†
†
|
§
Adalgisa Sinicropi, Maria Laura Parisi, Elena Martin, Mikhail Ryazantsev,
Wan Jian Ding, Luis Manuel Frutos, Riccardo Basosi, Stefania Fusi,
§,#
O
†
†
⊥
∇
,†,§
Loredana Latterini, Nicolas Ferr e´ , and Massimo Olivucci*
Dipartimento di Chimica, UniVersit a` degli Studi di Siena, Via Aldo Moro 2, I-53100 Siena, Italy,
Dipartimento di Scienze Farmaceutiche, UniVersit a` di Ferrara, Via Fossato di Mortara 17-19,
I-44100 Ferrara, Italy, Chemistry Department, Bowling Green State UniVersity, Bowling Green
Ohio 43403, Departamento de Ingenier ´ı a Qu ´ı mica y Qu ´ı mica F ´ı sica, UniVersidad de
Extremadura AVenida de ElVas, E-06071 Badajoz, Spain, Centro di Eccellenza Materiali
InnoVatiVi Nanostrutturati and Dipartimento di Chimica, UniVersit a` degli Studi di Perugia, Via
Elce di Sotto 8, I-06123 Perugia, Italy, College of Chemistry, Beijing Normal UniVersity, Beijing
1
00875, People’s Republic of China, Laboratoire de Chimie Th e´ orique et de Mod e´ lisation
Mol e´ culaire, UMR 6517-CNRS UniVersit e´ de ProVence, Case 521-Facult e´ de Saint-J e´ r oˆ me, AV.
Esc. Normandie Niemen, 13397 Marseille Cedex 20, France, and Departamento de Qu ´ı mica
F ´ı sica, UniVersidad de Alcal a´ , 28871 Alcal a´ de Henares, Madrid, Spain
Received August 23, 2009; E-mail: znv@unife.it; molivuc@bgnet.bgsu.edu
Abstract: We report the results of a multidisciplinary research effort where the methods of computational
photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel
N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent
zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a
system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible
light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating
the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate
the fluorescence of a tryptophan probe.
Introduction
We have recently shown that N-alkylated indanylidene-
pyrroline (NAIP) Schiff bases provide a class of biomimetic
switches that replicate different aspects of the Z/E photoisomer-
ization of rhodopsin (i.e., the visual pigment of superior
1
,2
animals). Below we report the results of a multidisciplinary
effort where the methods of computational photochemistry and
retrosynthetic analysis/synthesis have contributed to the prepara-
tion of the NAIP switch 1, featuring a stable zwitterionic head.
We show that, due to its large dipole moment, 1 may constitute
the prototype of a generation of electrostatic switches achieving,
at the single-molecule level, a light-induced dipole moment
inversion on the order of 30 D.
Photochemical switches (from now on “photoswitches”) are
bistable compounds that can be interconverted between two
different isomers (states) via light irradiation. The most widely
used photoswitch is azobenzene, which has been employed to
control different biomolecular functions (see for instance refs
3-5). However, owing to the significant change in molecular
length (ca. 3.5 Å between the two para carbons) but limited
†
Universit a` degli Studi di Siena.
Universit a` di Ferrara.
Bowling Green State University.
Universidad de Extremadura Avenida de Elvas.
Beijing Normal University.
Universidad de Alcal a´ .
Universit a` degli Studi di Perugia.
Universit e´ de Provence.
‡
§
|
6
change in dipole moment (ca. 3 D) between its Z and E forms,
azobenzene is considered a mechanical rather than electrostatic
#
O
⊥
∇
(3) Harvey, J. H.; Trauner, D. Chembiochem 2008, 9, 191–193.
(4) Numano, R.; Szobota, S.; Lau, A. Y.; Gorostiza, P.; Volgraf, M.; Roux,
B.; Trauner, D.; Isacoff, E. Y. Proc. Natl. Acad. Sci. U.S.A. 2009,
106, 6814–6819.
9
Present address: Zambon Chemicals, via Davaro 2, I-36045 Almisano
di Lonigo (VI), Italy.
(
1) Sinicropi, A.; et al. Proc. Natl. Acad. Sci. U.S.A. 2008, 46, 17642–
1
7647.
(5) Schierling, B.; No e¨ l, A.-J.; Wende, W.; Hien, L. T.; Volkov, E.;
Kubareva, E.; Oretskaya, T.; Kokkinidis, M.; R o¨ mpp, A.; Spengler,
B.; Pingoud, A. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 1361–1366.
(6) Kumar, G. S.; Neckers, D. C. Chem. ReV. 1989, 89, 1915–1925.
(
2) Lumento, F.; Zanirato, V.; Fusi, S.; Busi, E.; Latterini, L.; Elisei, F.;
Sinicropi, A.; Andruni o` w, T.; Ferr e´ , N.; Basosi, R.; Olivucci, M.
Angew. Chem., Int. Ed. 2007, 119, 418–424.
9
310 9 J. AM. CHEM. SOC. 2010, 132, 9310–9319
10.1021/ja906733q  2010 American Chemical Society

Dipole Moment Switch Driven by Z/E Photoisomerization
A R T I C L E S
Scheme 1. Retrosynthetic Analysis for the NAIP Schiff Bases 1-P
and 1
Figure 1. (a) Schematic representation of an electrostatic switch based on
Z/E isomerization. The 180° rotation of unit A will invert the dipole moment
vector with respect to unit B. (b) Inversion (relative to the superimposed
indanylidene unit) of the dipole moment of S -Z-1 and S -E-1. The computed
0 0
dipole moment values are 15.7 and 14.8 D, respectively.
switch. Literature examples of electrostatic photoswitches are
compounds of the spiropyran type that can be changed between
a neutral state and a zwitterionic state via a photochemical ring-
opening reaction. This property has been employed in different
methods coupled with a molecular mechanics force field allow
a realistic modeling of the excited states of the system as well
as of other solution-phase NAIPs.
The zwitterion 1 is a derivative of 1-P in which a noncon-
jugating internal carboxylate (replacing the external chloride)
7
-9
experiments to reversibly modulate the activity of enzymes
10
11
and channel proteins and to achieve novel sensors. Further-
more, it has been shown that the strong permanent dipole
moment in the zwitterionic form of nitrospiropyran units
attached to specific peptide residues is responsible for light-
13
is substituted at C2. Nakanishi and co-workers have reported
the synthesis and characterization of different zwitterionic
alkylated Schiff bases. These were used to investigate the point
charge model for the control of the absorption wavelength in
visual receptors. However, to our knowledge, 1 represents the
first zwitterionic Schiff base where two relatively rigid frame-
works (i.e., five-membered rings) are connected by a single
exocyclic double bond, providing full conformational control.
In the following we report the result of a four-stage work
starting with the construction and analysis of a quantum-
mechanics/molecular-mechanics (QM/MM) model of 1. Since
the model indicates that 1 has the same favorable photochemical
and spectral features as 1-P, during the second stage of the work
we design and perform the synthesis of a zwitterionic derivative.
The characterization of the prepared compound shows that the
predicted features are supported by spectral and photochemical
data. Through the continuation of the synthesis work, we then
show that 1-P can be turned into an unnatural R-amino acid
featuring a quaternary R-carbon at C2′. Finally, since the results
above support the possibility of preparing semisynthetic proteins
incorporating 1 as a rigidly oriented side chain, we build a QM/
MM model of a tripeptide fragment incorporating a tryptophan
probe. The simulation of the tryptophan emission maximum for
the Z and E forms of the peptide is used to quantify the ability
of the switch to control the optical properties of the fragment.
1
2
induced R-helixfrandom coil conformational transitions.
In principle, the Z/E photoisomerization of olefins can provide
the basis for the development of electrostatic photoswitches
where a large dipole moment change is achieved via a ca. 180°
rotation of a functionalized alkylidene unit. As shown in Figure
1
a, the placement of opposite charges on unit A along an axis
orthogonal to the central double bond provides a permanent
dipole moment that is exactly inverted (with respect to unit B)
upon double-bond isomerization. Note that, provided that the
unit B is spatially constrained, the molecular dipole moment
inverts sign rather than changes intensity (as in spiropyrans,
where it goes from a small to a large value without direction
control), leading to a maximum electrostatic potential change
on the surrounding environment.
As a working hypothesis, we propose that a system of the
type described above can be achieved via functionalization of
2
the known NAIP photoswitch 1-P. The photochemical and
spectral characterization of 1-P (a chloride-alkylated Schiff base
ion pair) in methanol reveals a prompt wavelength control of
the Z/E photostationary composition as well as a ZfEfZ
1
photocycle that is completed in ca. 20 ps. The same studies
also reveal that ab initio multiconfigurational quantum-chemical
Methods
(
(
(
7) Hug, D. H.; O’Donnel, P. S.; Hunter, J. K. Photochem. Photobiol.
980, 32, 841–848.
8) Aizawa, M.; Namba, K.; Suzuki, S. Arch. Biochem. Biophys. 1977,
80, 41–48.
9) Mayer, G.; Heckel, A. Angew. Chem., Int. Ed. 2006, 45, 4900–4921.
Synthesis. The synthesis of the zwitterionic switch 1 was
1
accomplished using a procedure developed in our laboratories for
2
,14
the preparation of nonfunctionalized switches.
A cationic
1
cyclization reaction of a transient nitrilium ion is the featured step
leading to the indanylidene-pyrroline framework of 1-P. As
retrosynthetically depicted in Scheme 1, the homoallylic transposi-
(
(
10) Kocer, A.; Walko, M.; Meijberg, W.; Feringa, B. L. Science 2005,
3
09, 755–758.
11) F o¨ lling, J.; Belov, V.; Kunetsky, R.; Medda, R.; Sch o¨ nle, A.; Egner,
A.; Eggeling, C.; Bossi, M.; Hell, S. W. Angew. Chem., Int. Ed. 2007,
4
6, 6266–6270.
(13) Sheves, M.; Nakanishi, K. J. Am. Chem. Soc. 1983, 105, 4033–4039.
(14) Zanirato, V.; Pollini, G. P.; De Risi, C.; Valente, F.; Melloni, A.; Fusi,
S.; Barbetti, J.; Olivucci, M. Tetrahedron 2007, 63, 4975–4982.
(
12) Angelini, N.; Corrias, B.; Fissi, A.; Pieroni, O.; Lenci, F. Biophys. J.
998, 74, 2601–2610.
1
J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010 9311

A R T I C L E S
Melloni et al.
a
Scheme 2. Synthetic Strategy Used To Obtain the Molecular
Scheme 3
a
Zwitterionic Z-1
a
Reagents and conditions: (i) ethyl 2-iodocyclopropanecarboxylate,
iPrMgCl (54%); (ii) HBr/AcOH (95%); (iii) Me
NaN -DMF, (b) Ph P-H O, (c) AcCl Et N (65%, three steps); (v) PPSE
46%); (vi) MeOTf (98%); (vii) LiOH (53%).
3 2
SiCHN (91%); (iv) (a)
3
3
2
3
(
tion of cyclopropylindenylcarbenium triggered by the bromide or,
alternatively, by CH CN was the key step on the route to the pivotal
3
a
Reagents and conditions: (i) (a) NaH, (EtO)
, (b) Ph P, (c) (CF CO)
CN (81%); (v) PPSE (63%); (vi) MeOTf (95%).
2
CO, (b) NBS (90%); (ii)
O (90%); (iii) cPrMgBr (78%); (iv) Tf O,
2 2
electrophilic nitrilium ion. Thus, it was conjectured that 1 could
be obtained by triggering the rearrangement of the proper carboxyl-
functionalized cyclopropylindenylcarbenium intermediate.
(
a) NaN
3
3
3
CH
3
1
5
The homoallylic rearrangement has been widely exploited to
install a 3-bromopropenyl side chain onto different substrates by
reacting unsubstituted as well as aryl- or alkyl-substituted cyclo-
propylcarbinols with HBr. Instead, to our knowledge, only in a few
could precipitate the lithium triflate; after that the soluble fraction
containing the zwitterion was purified by silica gel column
chromatography. The targeted zwitterionic switch 1 was then
isolated.
16-18
cases
has the reaction been extended to carboxyl-functionalized
In order to demonstrate that the parent switch 1 can be
incorporated in an unnatural R-amino acid featuring a conforma-
tionally locked side chain, we have also carried out the synthesis
of compound 15 (Scheme 3). This is a derivative of 1-P where
protected amino and carboxylic functions have been inserted in
position C2′. The preparation of 15 required the indanone derivative
cyclopropylcarbinols. Moreover, it is known that a lack of regi-
oselectivity in the bromide attack at the R-carboxy cyclopropyl
carbon atom would lead unavoidably to a mixture of isomeric
compounds.
As shown in Scheme 2, to begin the synthesis, we needed to
add the functionalized cyclopropylmagnesium reagent to the already
reported 5-methoxy-2,2-dimethylindan-1-one (2). The goal was
1
1 as the key starting material, which was easily obtained from
the commercially available 5-methoxy-1-indanone (9) through
functionalization of the C2 carbon atom. In detail, the synthesis
commenced with the ethoxycarbonylation and R-bromination
reactions to give compound 10. The subsequent nitrogen introduc-
tion was achieved via bromide displacement with sodium azide,
Staudinger reduction, and trifluoroacetylation of the resulting
primary amine. We could ascertain the structure of the suitably
protected R-amino acid 11 by crystallographic data (CCDC 752436).
Having in hand the required indanone moiety, the stage was set
to accede one pot to the NAIP precursor 14 by submitting the
Grignard adduct 12 to the cyclopropyl ring-opening/nitrilium ion
1
9
achieved following the protocol recently introduced by Knochel,
entailing the metal-halogen exchange between i-PrMgCl and cis-
-iodocyclopropanecarboxylate followed by the in situ trapping of
2
the organometallic reagent with electrophiles. Indeed, employing
compound 2 as the electrophilic counterpart, the reaction went to
completion with a spontaneous lactonization, giving the spiro
compound 3. We were delighted to find out that the latter, when
treated with HBr/AcOH, underwent the crucial homoallylic rear-
rangement in a completely regioselective manner, the R-bromoin-
danylidene carboxylic acid 4 being the only isolable compound.
Restoration of the ester group by treatment of the carboxyl
derivative 4 (as a 3:1 mixture of Z/E isomers) with (trimethylsi-
lyl)diazomethane allowed the facile installation of the tethered
acetamido group. Thus, from the methyl ester 5, following our
reported synthetic sequence entailing bromide displacement with
sodium azide, Staudinger reduction, and acetylation, we acceded
the pivotal secondary amide for nitrilium ion generation. As
14
ring-closing tandem reaction. Unexpectedly, treatment of cyclo-
propyl indanol 12 with triflic anhydride in the presence of
acetonitrile led to compound 13, compelling us to put into practice
the alternative way of producing electrophilic nitrilium ion. Thus,
PPSE-promoted dehydration of the secondary amide 13 followed
by in situ cationic cyclization yielded the expected free base 14
that, in turn, provided the photoswitchable amino acid 15 after
quaternization with methyl triflate. Further details of the synthesis
of Scheme 3 will be published in a separate report.
Spectroscopy and Photochemistry. Photoisomerization was
carried out with a 900 W irradiator and an f/3.4 monochromator
Applied Photophysics) apparatus and was followed by H NMR
spectroscopy (Bruker AC 200 and 400 spectrometers at 200.13 and
00.13 MHz, respectively). The composition of the photostationary
20
expected, trimethylsilyl polyphosphate (PPSE)-promoted dehydra-
tion of compound 6 led directly to the free imine 7, which
subsequently was transformed to the corresponding iminium triflate
8
by treatment with methyl triflate. At this stage, in order to get
the zwitterionic compound, the methyl ester group was saponified
with LiOH. From the crude product taken up in acetonitrile we
1
(
4
state at different irradiation wavelengths was evaluated from the
area ratio of the signal of the aromatic proton in the ortho position
with respect to the exocyclic double bond (see above and the
Supporting Information). UV/vis measurements were performed by
using a Hewlett-Packard 8423 spectrophotometer. See the Sup-
porting Information for further details.
(
(
15) Julia, M.; Julia, S.; Gu e´ gan, R. Bull. Soc. Chim. Fr. 1960, 1072–
079.
16) Stoemer, F.; Schenk, F.; Buschmann, H. Chem. Ber. 1928, 61, 2312–
323.
17) Stoermer, R.; Keller, W. Chem. Ber. 1931, 64, 2783–2792.
18) Julia, M.; Mouzin, G.; Descoins, C. C. R. Chim. 1967, 264, 330–332.
19) Vu, V. A.; Marek, I.; Polborn, K.; Knochel, P. Angew. Chem., Int.
Ed. 2002, 41, 2535–2538.
1
2
(
(
(
Computations. The model of the Z and E switches in solution
was constructed by placing the chromophore in a rectangular box
(20) Gawley, R. E.; Chemburkar, S. R. Heterocycles 1989, 29, 1283–1292.
9
312 J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010

Dipole Moment Switch Driven by Z/E Photoisomerization
A R T I C L E S
Scheme 4. QM/MM Model of Ala-Zw-Trp
Table 1. Modified MM Point Charges for the Part of the Trp
Residue (-NH-C H-CO-) That We Treated at the MM Level
R
atom
N
C
R
C
carbonyl
H
N
O
carbonyl
H
R
charge -0.4157 0.0000 0.5973 0.24335 -0.5679 0.08375
2
9
the Trp residue and the generalized amber force field (GAFF)
point charges to describe the rest of the peptide at the MM level
including the unnatural Zw residue (see Results and Discussion).
Accordingly, the QM/MM frontier was set at the weakly polarized
C
R
-C
used to saturate the QM C
from C and along the C
overpolarization of the QM wave function by the close point
charges, the charge of the frontier C carbon atom was set to zero,
ꢀ
bond of Trp. The hydrogen link atom (HLA) scheme was
atom. The HLA was fixed at 1.0 Å
-C bond axis. In order to avoid
ꢀ
ꢀ
R
ꢀ
of methanol molecules positioned within 10 Å from any given atom
of the chromophore by using the xleap module of the Amber
R
and the charges of the other MM atoms of Trp were modified, since
Trp is a natural residue (Table 1). This procedure is allowed by
the small values of the original AMBER99 point charges, which
also make it possible to use the standard MM van der Waals and
bonded potentials parameters. The point charges of all other MM
atoms (except those of the standard Ala residue) were determined
via HF/6-31G* RESP calculations.
21
package. The average ground-state configuration of the methanol
molecules (that is, the solvent) was determined according to the
following procedure. The solvent was relaxed by 1000 conjugate-
gradient minimization steps using periodic boundary conditions
while keeping the chromophore (i.e., the solute) fixed in its gas-
phase configuration. In this step the partial charges of the chro-
2
2
mophore atoms were determined with GAUSSIAN03, using a
As for the solution system described above, the QM/MM
calculations were carried out with a modified version of GAUSS-
2
3
restrained electrostatic potential (RESP) procedure at the HF/6-
1G* level of theory. The minimized system was further relaxed
keeping the solute molecule fixed) using molecular dynamics
simulation within an isothermal-isobaric NPT ensemble (1 atm,
2
2
26
3
(
IAN03, linked with a modified version of TINKER4.2. The
putative structures were drawn in two conformations (Conf1 and
Conf2) that allowed a stabilizing stacking interaction between the
24
-
2
98 K) using the program NAMD. In the next step we performed
Zw and Trp side chains and hydrogen bonding between the -COO
CASSCF/6-31G*/AMBER geometry optimization to relax the
coordinates of the QM chromophore and of all solvent molecules
featuring at least one atom less than 4.5 Å away from any solute
atom. The positions of the remaining solvent molecules, more
distant from the chromophore, were kept frozen. The QM calcula-
tions were based on a CASSCF/6-31G* level, including an active
space of 12 electrons in 11 π orbitals (that is, the full π system of
the solute).
function of the Z form of Zw and the N-H group of the 3-MI
moiety. The corresponding equilibrium structures were computed
via QM/MM geometry optimization. On the basis of the QM/MM
optimized ground-state (GS) structure, a CASPT2 single-point
2
7
calculation was conducted with MOLCAS 6.7 to evaluate the
excitation energies as well the associated oscillator strength f. The
emission energies from the second excited state (that corresponds
1
1
to the emitting La state) were estimated by computing the GS- La
The chosen 6-31G* basis set represented a cost/accuracy
compromise, yielding an excitation energy error of less than 3 kcal
vertical energy gaps via CASPT2 computations using a zeroth-
order three-root state-average CASSCF wave function at the La
1
-
1
25
mol for rhod and solution-phase PSB11. CASSCF/6-31G*/
AMBER geometry optimization was carried out with GAUSSI-
equilibrium geometry.
2
2
26
Results and Discussion
25,30
AN03 and TINKER 4.2. To account for dynamic correlation
energy, four root-state average CASPT2 calculations were carried
out by using the MOLCAS 6.7 software. The quality of the
achieved CASSCF/6-31G*/AMBER equilibrium structures was
assessed by comparison with the structures computed using
averaged solvent electrostatic potential/molecular dynamics (ASEP/
Recently
we implemented the ab initio CASPT2//
2
7
CASSCF protocol (where equilibrium geometries and electronic
31
32
energies are determined at the CASSCF and CASPT2 levels,
respectively) in a QM/MM scheme allowing for the evaluation
of the excitation and emission energies of neutral or charged
chromophores (treated quantum mechanically) embedded in
protein or solution environments (described by the AMBER
2
8
MD) computations. As we explain below and in the Supporting
Information, this method, although computationally expensive,
represents the best QM/MM-based method available for producing
equilibrium structures by minimizing the Gibbs free energy.
The QM/MM model of the gas-phase tripeptide Ala-Zw-Trp
-
1
force field) with a few kcal mol errors. Using this ab initio
1,2
CASPT2//CASSCF/AMBER protocol, we were able to show
(
Scheme 4) was constructed using the CASSCF(10,9)/6-31G* QM
that the observed absorption and fluorescence maxima of 1-P
-
1
level to describe the 3-methylindole (3-MI) fluorescent moiety of
can be reproduced within a few kcal mol . In the following
the same methodology is used to achieve a description of the
excited state (e.g., vertical excitation energy, nature of the
spectroscopic state, geometric relaxation) of 1 in methanol
solution.
(
(
(
(
21) Case, D. A.; et al. Amber 7; University of California: San Francisco,
2
002.
22) Frisch, M. J.; et al. GAUSSIAN03, Revision B.04; Gaussian, Inc.:
Pittsburgh, PA, 2003.
23) Bayly, C. I.; Cieplak, P.; Cornell, W. D. J. Phys. Chem. 1993, 97,
In Table 2 we report the computed absorption maxima for
1
0269–10280.
the equilibrium S
0
structure of both the Z and E isomers of 1
24) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.;
Villa, E.; Chipot, C.; Skeel, R. D.; Kale, L.; Schulten, K. J. Comput.
Chem. 2005, 26, 1781–1802.
(
S
0
-Z-1 and S -E-1 in Figure 2a). Consistent with the oscillator
0
(
(
(
(
25) Andruniow, T.; Ferr e´ , N.; Olivucci, M. Proc. Natl. Acad. Sci. U.S.A.
(29) Wang, J.; Wolf, J. W.; Caldwell, R. M.; Kollman, P. A.; Case, D. A.
J. Comput. Chem. 2004, 25, 1157–1174.
2
004, 101, 17908–17913.
26) Ponder, J. W.; Richards, F. M. J. Comput. Chem. 1987, 8, 1016–
024.
27) Andersson, K.; et al. MOLCAS 6.7; Lund University: Lund, Sweden,
002.
(30) Ferr e´ , N.; Olivucci, M. J. Am. Chem. Soc. 2003, 125, 6868–6869.
(31) Roos, B. O. In Ab Initio Methods in Quantum Chemistry-II; Lawley,
K. P., Ed.; Advances in Chemical Physics 69; Wiley: New York, 1987;
pp 399-446.
1
2
28) S a´ nchez, M. L.; Mart ´ı n, M. E.; Galv a´ n, I. F.; Olivares del Valle, F. J.;
(32) Andersson, K.; Malmqvist, P.-A.; Roos, B. O.; Sadlej, A. J.; Wolinski,
K. J. Phys. Chem. 1990, 94, 5483–5488.
Aguilar, M. A. J. Phys. Chem. B 2002, 106, 4813–4817.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010 9313

A R T I C L E S
Melloni et al.
Table 2. CASPT2//CASSCF/AMBER Absorption (λmax), Change in
Dipole Moment (∆µ), Oscillator Strength (f), and Charge
Translocation (∆q) Values
(IRC) calculations starting at S
-Z-1 and reaching CI-Z-1 after
0
a barrierless path. Similarly, the second conical intersection is
located at the bottom of the S valley starting at S -E-1. As for
-P, these structures are analogues of the excited-state mini-
mum and conical intersection reported for the visual pigment
1
1
a
f
b
2
structure
exc
λmax (nm)
∆µ (D)
∆q (au)
1
S
0
-Z-1
S
S
S
S
S
S
0
fS
fS
fS
fS
fS
fS
1
2
3
1
2
3
390
304
250
371
301
248
9.4
0.8
1.1
10.0
0.8
1.5
0.646
0.026
0.210
0.667
0.033
0.255
-0.29
-0.02
-0.03
-0.27
-0.02
-0.04
2
5
0
0
0
0
0
rhodopsin (Rh). While the S
energy surface always has a
-Z-1 the S -S energy
gap is g10 kcal mol . Therefore, it is consistent with a weak
coupling between S and the diradical (dark) state S
1
dominating charge-transfer character, at S
1
1
2
S
0
-E-1
-
1
1
2
.
The data in Figure 3a suggest, for both the ZfE and EfZ
a
Oscillator strengths are calculated using the CASSCF zeroth-order
reactions, an isomerization mechanism (see full arrows for the
b
wave function and CASSCF energies. Charge transfer from the
pyrroline to the indanylidene ring for 1 in MeOH.
1
ZfE process) similar to that fully documented for 1-P and
2
5
Rh. After S
0 1
fS photoexcitation, 1 initially relaxes along an
S
1
path dominated by double-bond/single-bond inversion and
only a limited torsional deformation. This initial event is
followed by a torsional relaxation about the C1′dC4 bond.
0
Indeed, upon excitation of the S -Z-1 structure, which has a
-
13° pretwisted exocyclic C1′dC4 bond, it relaxes to an S
Z-1 structure featuring an inverted π-bond order and a -20°
twisted C1′dC4 bond. From the S -Z-1 structure, the system
1
-
1
evolves toward the conical intersection Z-1-CI, which is
approximately -80° twisted (in the modeled R enantiomer), and
it is accessed through a substantially barrierless path. After
1 0 0
S fS decay at the conical intersection, the S -E-1 photoproduct
(
featuring a +177° pretwisted C1′dC4 bond) can be reached
via further torsional relaxation in the same direction. At this
point a photocycle can be achieved via subsequent photoisomer-
ization of S
presented in Figure 3a support the existence of an S
excited-state energy surface similar to that of S -Z-1 and leading
to S evolution toward the E-1-CI intersection featuring an
approximately 90° twisted C1′dC4 bond.
Similar to what was done for the parent compound 1-P, the
relative stability of S -Z-1 and S -E-1 in solution has been
0
-E-1 (dashed arrows in Figure 3). Indeed, the data
Figure 2. (a) CASSCF/AMBER computed equilibrium structures (hydro-
gens not shown) of S -Z-1 and S -E-1 embedded in a box (not shown) of
0 0
-E-1
0
0
methanol molecules. The computations are carried out for the R enantiomer
-
1
and show that the -COO group is stable in an axial (Ax) rather than an
equatorial position in both forms (the equatorial positions appear to be
unstable in methanol). The values in degrees refer to the C9′-C1′-C4-C5
torsional angle. Gas-phase and preliminary solution computations indicate
that these values correspond to lowest energy conformations. (See also
below. Solution conformations may exist with a C9′-C1′-C4-C5 torsional
angle of opposite sign. These conformers will be investigated in a future
work.) Bond lengths are given in Å. (b) Change in the charge distribution
0
0
determined in terms of the difference in Gibbs free energy
between the solvated systems using averaged solvent electro-
28
static potential/molecular dynamics (ASEP/MD) computations
0 0 1
for the S -Z-1 structure upon S fS excitation.
(
2
see the Supporting Information). The results indicate that, at
98 K, S -E-1 is 4.1 kcal mol higher in free energy than S -
0 0
-
1
strength values, the spectroscopic state corresponds to S
b gives the S and S charge distribution computed for the
ground-state equilibrium structure S -Z-1, which points to the
charge-transfer nature of the spectroscopic state. Accordingly,
upon S fS vertical excitation, S -Z-1 undergoes a 29% charge
translocation through its reactive C1′dC4 bond from the
pyrroline to the indanylidene moiety. On S the positive charge
is stabilized in a region away from the Schiff base function
C5dN) by delocalization on the phenyl ring. As found for the
1
. Figure
Z-1. This difference can be ascribed to steric hindrance factors;
in fact, in the case of the E isomer the methyl substituent on
C5 of the pyrroline ring is closer to the methyl substituents on
C2′ of the indanylidene moiety. The larger difference in 1
relative to 1-P could be due to diaxial interaction of the
carboxylate and methyl groups.
2
0
1
0
0
1
0
1
As already mentioned above, 1 presents a permanent dipole
moment that, with respect to the indanylidene unit, can be
inverted via light-induced Z/E isomerization. The computed
(
2
parent photoswitch 1-P, the electron-releasing p-OMe group
further enhances the positive charge stabilization in the inda-
nylidene moiety.
solution S
0 0
dipole moments are 15.7 and 14.8 D for the S -Z-1
and S -E-1, respectively. As shown in Figure 1b, the superposi-
0
In Figure 3a, we report the relative CASPT2//CASSCF/
tion of the indanylidene units of the two isomers displays dipole
moment vectors nearly orthogonal to the C1′-C4 axis and
forming a 157° angle, thus yielding a difference module of 29.8
D. The dipole moment values, obtained within an explicit
solvent representation (i.e., with a suitably constructed box of
methanols), are confirmed by an implicit treatment of the
solvent. Indeed, when using an independent solution model
based on B3LYP/6-31+G* density functional theory with the
polarizable continuum model (PCM) implemented in GAUSS-
AMBER energies for S
0 0
-Z-1 and S -E-1 and for two associated
S
1
/S conical intersections (CI-Z-1 and CI-E-1) as a function
0
of the torsional deformation along the reactive C1′dC4 bond.
2
Similar to the case for the 1-P ion pair, we have located two
very shallow (actually unstable) S
1 1
energy minima (S -Z-1 and
S
1
-E-1) describing the region of the excited-state energy surface
reached immediately after relaxation from the corresponding
Franck-Condon points. In other words, CI-Z-1 is located at
2
2
the bottom of an initially flat S
reached via relaxation of the S
out of the S -Z-1 region. This is demonstrated in Figure 3b,
which incorporates the result of intrinsic reaction coordinate
1
potential energy surface valley
0 0
IAN03, the computed values of the S -Z-1 and S -E-1 dipole
0
-Z-1 structure and developing
moments are 15.8 and 13.5 D, respectively. Therefore, both
explicit and implicit solvent representations appear to yield large
and consistent dipole moment values.
1
9
314 J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010

Dipole Moment Switch Driven by Z/E Photoisomerization
A R T I C L E S
Figure 3. (a) S
1
(diamonds) and S
0
(squares) relative CASPT2//CASSCF/AMBER energies for the S
0
, S
1
, and CI points of 1 (solid symbols), characterizing
1
its ZfEfZ photocycle in methanol solution. The energies of the corresponding 1-P points (open symbols) are also given for comparison. The structures
(
hydrogens not shown) provide a perspective showing the helicity of the molecular framework. The values in degrees refer to the C9′-C1′-C4-C5 torsional
angle. Bond lengths are given in Å. Note that the difference between S -E-1 and S -Z-1 (and the corresponding 1-P isomers) is calculated using averaged
solvent electrostatic potential/molecular dynamics computations (see the Supporting Information). Note also that, due to the C2 stereogenic center, the ca.
0
0
9
0° twisted CI-Z-1 and the CI-E-1 structures are distinct distereoisomers that relax along stereochemically distinct S
0
paths connecting CI-Z-1 to S
-Z-1 equilibrium structures and the corresponding CI-Z-1 and the CI-E-1 is not related
energy profile
-Z-1, and CI-Z-1 are connected by a
energy profile (squares) along the computed S coordinate
-Z-1 conformer are also shown. In the formula at the top, R indicates the absolute configuration of the C2 center
0
-E-1 and
-
1
0 0 0
CI-E-1 to S -Z-1. The ca. 40 kcal mol gap between the S -E-1 and S
to the thermal isomerization barrier, which is smaller and requires the corresponding solvated transition structure to be evaluated. (b) S
diamonds) along the computed S IRC path for the ZfE photoisomerization of S -Z-1. It is shown that S -Z-1, S
barrierless reaction coordinate. The IRC value is given in atomic units (au, amu bohr). The S
and the M,M helicity of the computed (R)-S
1
(
1
0
0
1
1
/2
0
1
0
-
and Ax the axial position of the -COO substituent.
Since the dipole moments of 1 could only be predicted and
not measured, we benchmarked the employed computational
methods indirectly. Using the B3LYP/6-31+G* and PCM
methods, we have evaluated the dipole moments of 1 in dioxane,
finding slightly decreased values with respect to methanol (13.7
and 11.6 D for the Z and E form, respectively). With exactly
the same method, we also computed the dipole moments of an
experimentally investigated merocyanine-spirobenzopyran sys-
tem in dioxane and compared the results with the observed
33
values available in the literature. The observed dipole moments
17.7 and 4.3 D for the open merocyanine and closed spiropyran
(
forms, respectively) compare reasonably well with the computed
data (15.9 and 6.9 D, respectively), pointing to moderately
underestimated predictions.
The computational evidence for an efficient photoisomeriza-
tion, a ca. 30 D light-driven dipole moment change and,
ultimately, the possibility to achieve a novel class of electrostatic
switches, prompted the search for a viable preparation of 1. The
synthetic protocol adopted for the preparation of the parent
compound 1-P (see Scheme 1) involved establishing a poly-
conjugated iminium ion chromophore onto a rigid framework
consisting of an indanylidene nucleus connected to a pyrrolinium
Figure 4. Room-temperature experimental absorption spectra of the Z
isomer (blue line) and E isomer (red line) of 1 in MeOH.
observed λmax values and intensities were compared with the
predicted properties in Table 2 to validate our computational
models and, in turn, to assign the bands to given electronic
transitions. In fact, the computed S fS absorption maximum
0 1
for the Z isomer (λmax) falls within 10 nm of the intense band
2
,14
ring.
As detailed in the Methods section and shown in
Scheme 2, a cationic cyclization reaction of a transient nitrilium
ion with an internal olefin was successfully employed as the
key step in setting up the desired chromophore (for further
details and full spectroscopic characterizations of the new
compounds, see the Supporting Information).
at 397 nm, which yields a computational error of less than 2
-1
kcal mol in excitation energy.
Similarly, the observed weaker band at λmax ) 263 nm falls
close to the computed S
and S fS
respectively) are qualitatively consistent with the observed
absorbance pattern. The S fS transition is predicted to
0 3 0 1
fS transition. Furthermore, the S fS
0
3
values of the oscillator strength f (0.646 and 0.210,
As shown in Figure 4, the absorption spectra of Z-1 and E-1
in methanol display two bands absorbing above 250 nm. The
0
2
correspond to a weak band (f ) 0.026) most probably hidden
below the shoulder near 300 nm, as indicated in Figure 4.
(
33) Bletz, M.; Pfeifer-Fukumura, U.; Kolb, U.; Baumann, W. J. Phys.
Chem. A 2002, 106, 2232–2236.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010 9315

A R T I C L E S
Melloni et al.
The computational results displayed in Figure 3 indicate that
S
0
-Z-1 and S
barrierless, and therefore ultrafast (i.e., with a time scale below
ps), reactions. This conclusion is in agreement with the results
0
-E-1 can be photochemically interconverted via
1
34
of a recently reported combined femtosecond fluorescence up-
conversion, UV-vis, and IR transient absorption spectroscopic
study of 1. Indeed, the fluorescence lifetime for the ZfE process
is found to be 140 fs, with an excited-state absorption persisting
over 230 fs in the form of a vibrational wavepacket according
to twisting of the isomerizing double bond. It is also shown
that the hot photoproduct appears at the ground state on a 400
fs time scale and that the reaction is substantially completed
after 600 fs from photon absorption.
To assign the observed time scale, we have used a recently
proposed computational strategy to simulate the excited-state
motion of 1 in methanol. Accordingly, we computed a scaled
3
5
CASSCF/AMBER trajectory of our Z-1 solvated model (i.e.,
the same used to reproduce the reaction path of Figure 3b)
starting at a point close to the Franck-Condon structure and
describing the relaxation of the first solvent shell together with
the solute. In this calculation, the excited-state CASSCF gradient
is scaled in such a way as to simulate the more accurate (but
computationally impractical) CASPT2 gradient along the entire
reaction path of Figure 3b. In other words such a trajectory
simulates, at the cost of CASSCF gradients, the excited-state
3
5
evolution on a CASPT2-like potential energy surface. This
protocol has reproduced successfully the magnitude of the
observed excited-state lifetime of the retinal chromophore in
Rh and provided a mechanism in line with its ultrafast
3
5
isomerization. Notice that it has not been possible to start the
trajectory from the Franck-Condon structure due to the
incorrect order (with respect to the corresponding CASPT2
2 3
values) of the S and S states computed at the CASSCF level
of theory. The order becomes correct only at ca. 30° double
bond twisting deformation (i.e., ca. 20° more twisted than the
Franck-Condon structure). Thus, the computed trajectory
provides an approximated lower limit of the reaction time scale
that, according to Figure 5a and assuming a ca. 50 fs initial
relaxation, is predicted to be in the 300-400 fs range. In spite
of this uncertainty, the ultrafast nature of the isomerization and
Figure 5. (a) Scaled CASSCF/AMBER trajectory simulating the S
1
the magnitude of the predicted S
1
lifetime are consistent with
isomerization of 1 in methanol solution. The system is predicted to reach
the conical intersection region on a 300 fs time scale. The open triangles
indicate single-point CASPT2//CASSCF/AMBER energies, demonstrating
that the scaled CASSCF energy profile is only slightly off the CASPT2
energy. The CASSCF/AMBER value of the oscillator strength f along the
trajectory is also given (full circles). (b) Evolution of the two main
geometrical parameters (C1′-C4 length and C9′-C1′-C4-C5 dihedral)
34
the observed excited-state dynamics that points to an excited-
state lifetime between 250 and 400 fs. Notice that transient
fluorescence is predicted to occur, assuming a ca. 50 fs initial
relaxation time, during the first 200 fs as indicated by the values
of the S
0
1
-S oscillator strength (see Figure 5a).
1
characterizing the S structure of the switch.
The scaled CASSCF/AMBER trajectory also reveals that the
solvent cavity cannot stop the excited-state photoisomerization
motion that, according to our analysis, is facilitated by the ring
inversion of both the indanylidene and pyrrolinium envelope-
like five-membered rings. Indeed, we could observe (see movie
in the Supporting Information) an inversion of the axial/
equatorial position of the two methyl and hydrogen groups at
C2′ and C3, respectively. Because of the inversions, the double
bond can undergo a twisting deformation of 90° and decay at
the conical intersection without moving the bulkiest parts of
the molecular framework. Indeed, as shown in Scheme 5, both
Scheme 5. Conformational Changes Occurring during the
Excited-State Trajectory of Z-1 in Methanol and Allowing a
a
Space-Saving Isomerization Motion
-
+
the phenyl ring and the highly solvated -COO and N(Me)
a
See movie in the Supporting Information.
(
(
34) Briand, J.; Br a¨ m, O.; R e´ hault, J.; L e´ onard, J.; Cannizzo, A.; Chergui,
M.; Zanirato, V.; Olivucci, M.; Helbing, J.; Haacke, S. Phys. Chem.
Chem. Phys. 2010, 12, 3178–3187.
1
groups remain substantially fixed during the entire S motion.
The counterclockwise rotation about the reactive C1′-C4 double
bond is accompanied by clockwise twisting (associated with
35) Frutos, L. M.; Andruni o´ w, T.; Santoro, F.; Ferr e´ , N.; Olivucci, M.
Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 7764–7769.
9
316 J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010

Dipole Moment Switch Driven by Z/E Photoisomerization
A R T I C L E S
3
6
Table 3. Composition of the Photostationary State of 1 in
Methanol as a Function of the Irradiation Wavelength, Determined
by NMR Analysis
Landau-Zener picture. This could be connected with the
multimode nature of the excited-state reaction path, involving
both ring puckering and bond twisting, and/or with the restrains
imposed, after the excited-state decay, by the solvent shell on
the isomerization motion. Furthermore, the limited magnitude
of the quantum yields may be related to the fact that, im-
abs
λ
max (nm)
Z ((0.1)
E ((0.1)
3
3
4
40
90
40
1
1
1
0.5
0.6
1.2
1 0
mediately after the S decay and during the S relaxation, the
system needs to reshape the solvent shell to accomplish full
double bond isomerization.
the ring inversion) of the C2′-C3′ and C2-C3 bonds. These
directions are imposed by the pretwisting of the C1′-C4 double
bond and initial ring conformation (i.e., imposing a framework
The successful photoisomerization documented above for 1
and the successful synthesis of the unnatural amino acid 15 (in
a protected form), featuring a photoswitchable side chain,
strongly support the possibility to achieve semisynthtetic
peptides and proteins incorporating a dipole switch in a
conformationally locked orientation. While we expect that the
photoisomerization dynamics of these systems will differ from
the dynamics calculated/observed for Z-1 in methanol, the results
above point to a facile process. More specifically, these
achievements indicate that the amino acid Zw of Figure 6a is
a realistic synthesis target that, in principle, only requires
the tuning of the chemistry seen in Schemes 2 and 3. While
the preparation of such a system goes beyond the scope
of the present research, below we report on the computational
design of a tripeptide (Ala-Zw-Trp) that incorporates Zw as
the central residue and tryptophan as the N-terminal residue.
The tryptophan residue constitutes an internal fluorescence
probe sensing the changes in the electrostatic field produced
in its environment. By virtue of the fact that 15, and therefore
Zw, features a quaternary R-carbon, the side chain of this
residue is conformationally rigid and has a well-defined
orientation with respect to the peptide backbone. In particular,
as displayed in Figure 6b (right), this side chain would be
orthogonal to the local backbone axis and is positioned off
to the left or to the right of the same axis as a function of
the stereochemistry (R or S) of the R-carbon.
0
with M,M helicity) of S -Z-1 and, according to our computations,
are a consequence of the system R configuration. This is
reminiscent of the type of cooperative bond-twisting, space-
saving mechanism documented for the retinal chromophore
3
5
embedded in the chiral cavity of Rh.
Since, according to Figure 3 and the experimental data
presented in ref 34, both the ZfE and EfZ isomerizations are
found to be ultrafast and the absorption λmax values of Z-1 and
E-1 are close but not identical (see Table 2), it is predicted that,
upon continuous irradiation, a photostationary state will be
rapidly generated whose composition depends on the irradiation
wavelength, leading to the possibility to control the dipole
moment value. Comparison of the Z-1 and E-1 spectra (see the
Supporting Information) suggests, consistent with the computed
λ
max data, a decrease of the Z/E ratio upon an increase in the
wavelength (Table 3), since the S fS band of E-1 appears to
0
1
be slightly blue-shifted and more intense than the corresponding
Z band. In particular, at 440 nm, the E form becomes dominant.
On the other hand, due to a relatively low EfZ thermal
isomerization barrier, which has not yet been measured or
computed, the photogenerated ratio slowly returns to the original
Z/E mixture (a Z/E ratio larger than 99:1) when the irradiation
is interrupted at room temperature (see the Supporting Informa-
-
1
tion). This is consistent with the more than 4 kcal mol
computed energy difference between S -Z-1 and S -E-1. Notice
Here our target is to assess if the Z/E photoisomerization
reversing the dipole moment of the central residue would
significantly change the tryptophan emission wavelength. In
other words, we want to find out if the dipole moment inversion
would expose the 3-MI fluorophore of tryptophan to a different
0
0
that, while the photostationary state is rapidly reached (e.g., 12
min irradiation at the isosbestic point), this room-temperature
thermal relaxation/interconversion is not a fast process.
A relaxation time of about 8 h could be estimated, allowing
the isomers to be fully characterized (see the Supporting
Informationfordetails).Thesystemscouldundergoirradiation-dark
relaxation cycles several times without showing any degradation.
This indicates that 1 can indeed be employed in applications
where the photoswitch triggers molecular events that are
completed on a time scale shorter than a few hours or when
one wants to recover the original Z/E composition (and be ready
to repeat a photoswitching cycle) after several hours or more.
On the other hand, the 8 h relaxation time points to a thermal
isomerization barrier smaller than the one expected for a CdC
double bond. The origin of this low barrier is currently under
investigation in our laboratory. The similarity between the
reaction path in Figure 3 and that of the parent NAIP
photoswitch and rhodopsin suggests that Z-1 and E-1 could
potentially display good photoisomerization quantum yields.
These quantities were measured via HPLC and spectroscopic
analysis and found to be 0.20 and 0.23 for the Z-1 and E-1
isomerizations, respectively. The EfZ quantum yield was
determined by considering the effect of the thermal EfZ
isomerization on the isomer concentrations determined by
HPLC. The fact that an ultrafast reaction driven by a barrierless
path leads to a moderate quantum yield points to a decay
mechanism that deviates from the simple one-dimensional
1
electrostatic field, thus modulating its GS- La energy gap. In
fact, as shown in Figure 6b, the fluorescent 1La state of
tryptophan has a charge-transfer character (ca. 30% of the
pyrrole ring π-electron density is shifted toward the phenyl ring
3
7
upon photoexcitation). As a consequence, an electrostatic
potential that stabilizes a positive charge on the 3-MI pyrrole
ring or a negative charge on the phenyl ring will stabilize the
1
La state with respect to the GS, leading to a red shift of the
fluorescence λmax
.
In the following the tripeptide is assumed to represent a
fragment of the interior of a protein that is not in contact with
the solvent; therefore, the calculations are carried out using
isolated models (i.e., no solvent molecules are considered). As
displayed in Figure 6, we have investigated one stereoisomer
of the switch (the configuration of the C2′ quaternary R-carbon
of Zw is R and the configuration of the C2 carbon is S), allowing
for a shorter distance between the Zw and Trp residues. We
consider (see the Supporting Information for details) two
specifically designed conformers (Conf1-Ala-Zw-Trp and Conf2-
(
36) Desouter-Lecomte, M.; Lorquet, J. C. J. Chem. Phys. 1977, 66, 4006–
4
017.
(37) Pistolesi, S.; Sinicropi, A.; Pogni, R.; Basosi, R.; Ferre e´ , N.; Olivucci,
M. J. Phys. Chem. B 2009, 113, 16082–16090.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010 9317

A R T I C L E S
Melloni et al.
Figure 7. Structure of the designed Ala-Zw-Trp tripeptide in its fluorescent
1
state (i.e., at the La equilibrium geometry). The incorporated unnatural amino
-
acid Zw is in its R configuration (see straight arrow), while the -COO group
resides on an asymmetric carbon with S configuration. The curly arrows indicate
the position of the isomerizing double bond. The hydrogen bonds present in
the structures are indicated with dashed lines. The incipient or broken hydrogen
bonds are indicated with dotted lines. The distances are given in Å. The labels
A and B facilitate the reading of the hydrogen-bonding change in Conf2.
Table 4. Computed Fluorescence Maxima (λ mfl ax) for the Conf1 and
Conf2 Conformers of the Ala-Zw-Trp Tripeptide and for the
a
Gas-Phase Fluorophore 3-MI (from ref 37)
-1
b
)
fl
∆E (kcal mol
λ
max (nm)
f
∆µ (D)
3
-MI
97.1
295
318
290
311
286
0.11
0.10
0.11
0.11
0.12
5.9
5.2
4.9
5.2
4.8
Z-Conf1
E-Conf1
Z-Conf2
E-Conf2
90.0 (96.4)
98.5 (97.3)
91.9 (96.5)
100.0 (98.0)
a
The active space used in the calculation comprises the full π-system
of the 3-MI fluorophore. The oscillator strength (f) and the change in the
dipole moment (∆µ) upon the vertical electronic transition are also
b
given. Values in parentheses were calculated in the absence of MM
point charges of the models; see text.
Figure 6. (a) Structure of the designed Ala-Zw-Trp tripeptide. The
incorporated unnatural amino acid Zw is framed. The position of the 3-MI
fluorophore (ball-and-stick representation) treated at the QM level with
respect to the peptide backbone and to the rigidly oriented zwitterionic side
chain of the Zw residue (tube representation) of Conf1-Ala-Zw-Trp is
section for details). The central residue corresponding to the
unnatural amino acid Zw and the Ala residue are described at
the MM level with a GAFF potential and features parametrized
RESP charges (computed at the HF/6-31G* level) for both the
Z and E forms.
1
shown. (b) The ground-state (GS) and fluorescent-state ( La) charged
distribution of 3-MI from ref 37.
The energy gaps between the fluorescent state and the ground
state are computed via CASPT2//CASSCF/6-31G*/AMBER
single-point computations and correspond to the S fS transi-
Ala-Zw-Trp) of the stereoisomer, both featuring stacked and
hydrogen-bonded Trp and Zw side chains (the Z forms of Figure
0
2
1
7
). This situation is reminiscent of the documented Arg-Trp
tion (the La state features the largest oscillator strength with
respect to the ground state). In Table 4 we show that, according
to our QM/MM models, the tryptophan fluorescence can be
modulated via ZfE photoisomerization. In particular, the
Conf1-Ala-Zw-Trp and Conf2-Ala-Zw-Trp emissions are sig-
nificantly blue-shifted (28 and 25 mm, respectively) upon
photoisomerization. Notice that these effects correspond to an
3
8
stacking documented for cytokine-binding proteins where,
loosely, the positively charged arginine unit is replaced by the
pyrrolinium moiety.
Conf1 features an extended backbone, while Conf2 has a
ꢀ-turn-like backbone. As mentioned above, Conf1-Ala-Zw-Trp
and Conf2-Ala-Zw-Trp are assumed to represent different
realistic arrangements of the chosen tripeptide fragment in a
protein. The effect of the photoisomerization on the tryptophan
fluorescence is determined in the following way. The structure
1
-1
increase in the S - La energy gap of more than 8 kcal mol .
0
It is also remarkable to find out that the emission maxima of
the Z and E forms are predicted to be respectively strongly red-
shifted and blue-shifted with respect to that of the isolated 3-MI.
In order to demonstrate that the above changes are dominated
by the different electrostatic field created by the change in
orientation of the Zw-head moiety, we have repeated the energy
of the corresponding fluorescent conformers (i.e., featuring the
1
3
3
-MI moiety in its La state) is optimized at the CASSCF/6-
1G*/AMBER QM/MM level. Accordingly, the QM part
corresponds to the 3-MI side chain of tryptophan, while the
rest of the molecule is described at the MM level (see Methods
1
gap calculation using the same La equilibrium peptide structures
in the absence of the MM point charges of the models, which
are responsible for the electrostatic field acting on the fluoro-
phore. The results of these calculations (see values in parentheses
(
38) Bravo, J.; Staunton, D.; Heath, J. K.; Jones, E. Y. EMBO J. 1998, 17,
665–1674.
1
9
318 J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010

Dipole Moment Switch Driven by Z/E Photoisomerization
A R T I C L E S
in Table 4) fully confirm that the energy gap change is due to
a change in electrostatics. Indeed, the resulting energy gaps are
all within 1 kcal mol- of the corresponding isolated 3-MI value.
The results above can be readily explained considering the
typical GS and La charge distributions seen in Figure 6b as
well as the geometry of the peptide conformers seen in Figure
leads to easily detectable (>20 nm) changes in the fluorescence
maximum, thus providing an excellent basis for future experi-
mental work. Furthermore, given the presence of the charged
carboxylate and imminium centers in the rotating head (Zw-
head in Figure 6a), the same photoisomerization could be used
to break and reconstitute hydrogen bonds and salt bridges in
suitably designed peptides. However, as mentioned above, to
implement these concepts one needs further functionalization of 1
at the level of the indanylidene ring (introducing, for instance,
1
1
7
. In fact, both Z-isomers feature a hydrogen bond between the
-
ionized -COO group of the switch, placed directly above the
3
strongly stabilize the excited state with respect to the ground
state due to the short distance between the -COO negative
charge and the pyrrole moiety positive charge of La 3-MI. The
-MI unit and the indole N-H bond. This situation would
-
2
NH and -COOH groups) to graft the molecule to the macro-
-
molecule backbone. For instance, one could repeat the synthesis
of Scheme 2 using the intermediate 11 rather than the substrate 2
as the indanone precursor. Synthesis work in this direction is
currently being carried out in our laboratories.
1
removal of this interaction in both E-isomers must therefore
lead to a blue-shift of the absorption due to a less stabilized
1
La state.
A different perspective may be opened by the preparation of
enantiomerically pure 1 (e.g., via chiral resolution of the
available racemate). Similar to the well-known chiral dia-
rylidenes, the R and S enantiomers of 1 display molecular
helicity. According to our computation, the lowest energy
solution conformers of Z-1 and E-1 have axial carboxylates and
pretwisted double bonds in the same direction (clockwise). This
establishes that the R (S) configuration at C2 induces an M,M
The analysis of the E and Z conformers also provide
information on the possible structural effects induced upon
photoisomerization and can be due to both mechanical (i.e.,
bond-breaking) and electrostatic changes. For instance, in Conf1-
-
Ala-Zw-Trp the photoisomerization breaks the -COO · · ·H-N
hydrogen bond, which also results in the formation of an intra-
backbone hydrogen bond stabilizing the conformer. Interestingly
-
the breaking of the -COO · · ·H-N hydrogen bond in Conf2-
(
P,P) helicity (see Figure 3 for the case of the R enantiomer) in
Ala-Zw-Trp leads, in our model, to a backbone rearrangement
involving the breaking of the hydrogen bond A and the
formation of the new hydrogen bond B shown in Figure 7.
the lower energy diastereomeric conformer of both the Z and E
forms. This leads to the possibility of a preferential direction
of isomerization and, ultimately, to a two-photon-induced
unidirectional rotary motion of the Zw-head with respect to
the indanylidene unit. Indeed, the IRC of Figure 3b displays a
Conclusions
The photoinduced dipole moment change of spiropyran-
clockwise rotation of the S
continue upon relaxation to the S
0
-Z-1 pyrrolinium head that would
-Z-1 intermediate. Future
merocyanine electrostatic photoswitches has been measured for
3
3
0
the case of indolino-spirobenzopyrans. The results indicate
a maximum dipole moment change of ca. 17 D due to the low
dipole moment (ca. 3 D) of the spiropyran closed form with
respect to the merocyanine open form (ca. 15 D). In a situation
where the indanylidene ring is rigidly oriented, the presented
NAIP Schiff base 1 provides an electrostatic photoswitch
displaying a nearly 2-fold dipole change. This is due to the
spatial inversion (reorientation) of a large (ca. 15 D) permanent
dipole moment (see Figure 1).
computational work will explore this perspective.
Acknowledgment. We are grateful to Jan Helbing and Stefan
Haacke for valuable discussions. This work was supported by the
Universit a` di Siena (PAR02/04), FIRB (RBAU01EPMR), CO-
FIN2006 (Prot. 200431072_002), and Bowling Green State Uni-
versity. M.O. is grateful to the Center for Photochemical Sciences
and the School of Arts & Sciences of Bowling Green State
University for start-up funds. L.-M.F. acknowledges the financial
support from a “Ramon y Cajal” contract and project CTQ2009-
The ca. 30 D dipole moment change of 1 opens up a new
perspective for the light-driven conformational control of
macromolecular structures determined by polar interactions. In
a situation where the indanylidene ring is held in a fixed
orientation (e.g., when part of a protein backbone or supramo-
lecular scaffold), light can be used to invert the dipole, yielding
a dramatic change in the local electrostatic field and, ultimately,
a conformational change thus leading to a destabilization of the
original equilibrium conformation. The construction of QM/
MM models of the Z and E forms of the Ala-Zw-Trp tripeptide
strongly supports this hypothesis. In fact, we have provided
computational evidence that the Ala-Zw-Trp photoisomerization
0
7120 of the Spanish MICINN.
Supporting Information Available: Details of the synthesis,
QM/MM computations, ASEP/MD calculation, spectroscopy
and photochemistry, and complete refs 1, 21, 22, and 27; a
movie displaying the computed excited-state isomerization
motion of Z-1 in methanol solution (the solvation shell is
displayed). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA906733Q
J. AM. CHEM. SOC. 9 VOL. 132, NO. 27, 2010 9319