Nanometer-range communication of stereochemical information by reversible switching of molecular helicity

Article abstract of DOI:10.1002/anie.201001130

(Figure Presented) A long-distance call: The inversion of configuration at a stereogenic center led to a detectable switch in the position of a ¹³C stereochemical probe 40 bonds (2.5 nm) away. Information was relayed between the sites by an inversion of screw sense in the intervening helix as illustrated (¹³C NMR signals were read as output).

Full text of DOI:10.1002/anie.201001130

Communications
DOI: 10.1002/anie.201001130
Conformational Communication
Nanometer-Range Communication of Stereochemical Information by
Reversible Switching of Molecular Helicity**
Jordi Solꢀ, Stephen P. Fletcher, Alejandro Castellanos, and Jonathan Clayden*
Stable helical molecular structures are ubiquitous in both
biological and synthetic polymers,[1] and in general the
handedness (screw-sense) of a helical polymer is dictated by
the chirality of the monomers from which it is built (for
example, DNA built from d-deoxyribose is typically right-
handed, as are a helices built from l-amino acids).[2] It
occurred to us that the switching of a molecular helix between
a left- and a right-handed form could constitute a very simple
means of communicating information along the axis of the
helix over distances commensurate with the helical cooper-
ativity length.[3] Changes in conformation are commonly used
by biological systems to communicate information, for
example, in receptors or in allosteric proteins.[4] However,
simple switching between two helical forms of a typical
peptide is not suitable for such a purpose because of the
substantial difference in stability between diastereoisomeric
helices built from chiral monomers.[2] There are, however,
many synthetic oligomers which favor helical structures even
though they are constructed from nonchiral subunits.[1,5] We
exploited one of these classes of oligomers in this study.
Pfeiffer and Quehl showed in 1931 that a configurational
preference could be induced in a stereolabile cation by a
chiral anion.[6] A screw-sense preference in a stereolabile
helical oligomer or polymer[7] can likewise be induced by
coordination to a chiral counterion,[8] or by the incorporation
of (even a very weak) chiral influence into the oligomer
itself.[3,9] At the limit, the screw sense of a helical oligomer
may be induced solely by thermodynamic control from a
chiral influence located at the terminus of the helix.[8c,10,11]
With a switchable chiral influence, helix inversion has been
achieved under the influence of factors such as temper-
ature,[12a] solvent polarity,[12b] irradiation,[12c] and electroche-
mistry.[12d]
information over multi-nanometer distances. We envisaged a
molecule (Figure 1) in which such a helix was attached at one
end to a switchable controller—a chiral “input” which would
induce a local left- or right-handed preference. The propen-
sity of the molecule to adopt a consistent helical structure
Figure 1. A mechanism for communication on the basis of molecular
helicity. L and R correspond to left- and right-handed helicity, respec-
tively.
would cause this local influence to propagate through the
helix (provided the rate of inversion of helicity were
sufficiently fast); signal transduction in this way would
enable the information encoded in the handedness of the
controller to be read remotely by a detector of helix chirality
located at the helix terminus. Interactions within the helicity
detector would result in measurably different outputs from
left- and right-handed helices, so that information could be
communicated over the distance d (Figure 1).
Peptide-like oligomers of the achiral amino acid amino-
isobutyric acid (Aib) adopt stable hydrogen-bonded helical
structures (usually 310 helices)[14] with a low barrier to
inversion between enantiomeric left- and right-handed
forms.[15] Since we also knew that Aib mediates the prop-
agation of a given helical preference with a high degree of
fidelity,[11] we chose to use Aib oligomers of the general
structure shown in Scheme 1 to mediate the information
Signal transduction can result from conformational
changes in artificial molecular structures as a result of a
chemical stimulus;[13] however, the switching of a helix has not
previously been coupled with the local detection of helical
screw sense at a remote site for the communication of
[*] Dr. J. Solꢀ, Dr. S. P. Fletcher, Dr. A. Castellanos, Prof. J. Clayden
School of Chemistry, University of Manchester
Oxford Road, Manchester M13 9PL (UK)
Fax: (+44)161-275-4939
E-mail: clayden@man.ac.uk
[**] We are grateful to the EPSRC and the Leverhulme Trust for financial
support of this research, and to the Departament d’Innovaciꢁ
Universitats i Empresa de la Generalitat de Catalunya for a
postdoctoral fellowship to J.S.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001130.
Scheme 1. A switchable helix of Aib monomers, capped by a 3-phenyllactate
controller and an isotopically labeled Aib detector.
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transfer. For the stereo-
chemical input, we selected
the benzoate ester of 3-phe-
nyllactic acid (2-hydroxy-3-
phenylpropionic acid), first
because of its structural sim-
ilarity to protected phenyl-
alanine (which is known to
be able to control helicity in
oligo-Aib systems[11]), and
second because the ben-
zoate esters can be conven-
iently switched between
their two mirror-image
forms by means of the Mit-
sunobu reaction.[16]
Scheme 2. Synthesis of the capped helices. Bz=benzoyl, Cbz=carbobenzyloxy, py=pyridine, PyBOP=benzo-
triazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate.
Information about the
stereochemical input would
be encoded in the screw
sense of the helix; a remote
asymmetric probe was required to decode this information.
For this purpose, we chose to incorporate an isotopic label
into the terminal Aib monomer: the stereochemical environ-
ment of the label could then be conveniently read by using the
noninvasive technique of NMR spectroscopy. The use of a
chemically silent isotopic label minimizes the influence of the
probe on the configuration of the helix.[17]
To identify the location of the two diastereotopic
13C NMR signals for the C-terminal Aib residue of the
helix, and to establish that the two signals exhibited a
detectable chemical shift difference, we used doubly 13C
labeled Aib, 1a,[18] to open the azlactone[19] 3 derived from the
Aib tetramer 2[11] with its N terminus protected as an azide
(Scheme 2). The doubly 13C labeled helically racemic pen-
tamer 4a displayed a single strong peak at d = 25.37 ppm in its
13C NMR spectrum in CD3OD. This result indicated that the
two methyl groups of the C-terminal Aib
fication of the method of Kouklovsky and co-workers.[21]
Ligation of the tetra-Aib azlactone 3 with 1b (to give 4b)
and then with l-3-phenyllactic acid (5b) gave the helical
hexapeptide 7a with a switchable stereocenter at the N ter-
minus and a spectroscopic probe at the C terminus.
Sequential Mitsunobu esterification and hydrolysis ena-
bled stereochemical switching, as shown structurally and
schematically in Scheme 3. Thus, invertive esterification with
benzoic acid in the presence of triphenylphosphane and
diisopropylazodicarboxylate (DIAD) gave the ester (R)-13a,
which displayed in its 13C NMR spectrum two strong diaste-
reotopic peaks at d = 26.01 and 24.69 ppm, precisely coinci-
dent with those of 6c, but in a 78:22 ratio (Figure 2b). The
ester (R)-13a was transformed into the alcohol (R)-7a by
methanolysis, and the N-terminal controller was converted
back into the ester (S)-13a by a second inversion under
residue are in enantiomeric environments
(i.e. they are not diastereotopic). Oligomer
4a must therefore, as expected,[15] undergo
rapid helix inversion at ambient temper-
ature on the NMR timescale.
Ligation with either Cbz-l-Phe (5a) or
l-3-phenyllactate (5b) gave peptides 6a,b;
peptide 6b was benzoylated to yield 6c.
Compounds 6a and 6c showed two strong
signals centered at 25.35 ppm owing to the
now diastereoisomeric environments of the
methyl groups of the C-terminal Aib residue
(Figure 2a). Fourfold serial dilution of 6a
did not change the peak separation. We
could therefore conclude that the anisochro-
nicity of the two 13C NMR signals was due to
the intramolecular influence of the N-ter-
minal controller rather than any supra-
molecular association.[20]
A singly 13C labeled Aib probe 1b was
synthesized as an unequal mixture of enan-
tiomeric isotopomers (R/S 77:23) by a modi-
Scheme 3. Helix switching. DMAP=4-dimethylaminopyridine.
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Communications
the benzoate ester (R)-13b by invertive Mitsunobu esterifi-
cation as before. Use of an unequal mixture of enantiomers
enabled us not only to decode the sense of helicity of the
oligomer, but also to quantify the extent to which helicity is
controlled as the chain is lengthened.[23] Peak separation in
the 13C NMR spectrum (Figure 2d) was less than for 13a as a
result of erosion of the signal with the lengthening chain,[11]
but induction of helicity through the 12 Aib residues between
the N-terminal controller and C-terminal reporter resulted in
a clear 75:25 ratio of 13C NMR signals at d = 25.76 and
25.02 ppm (Figure 2d). Upon the inversion of (R)-13b to (S)-
13b by methanolysis and Mitsunobu esterification, the
information about the switch was transmitted through all 40
bonds lying between the N- and C-terminal residues of the
peptides, and the population of the 13C reporter was inverted
(Figure 2e). The identity of (S)-13b formed in this way, and
hence the completion of the switching cycle, was confirmed by
comparing its spectrum with that of (S)-13b formed by simple
acylation of (S)-7b with benzoyl chloride (Figure 2 f). The
essentially identical spectra furthermore confirm the fully
invertive nature of the Mitsunobu steps (> 97% inversion for
each of the two steps).
The transmission of stereochemical information from the
N-terminal residue of 14-mer 13b to the C-terminal probe of
helicity is, as far as we are aware, the first example of the use
of the handedness of a helical structure to convey quantified
information between spatially separated points, whether on a
molecular scale or otherwise. Typically, 310 helices have a
1.94 ꢀ rise per residue,[24] and the 2.5 nm length of 13b is
commensurate with the thickness of a typical phospholipid
bilayer.[25] Aib oligomers 13 have considerable homology with
a family of membrane-spanning oligomers known as the
peptaibols.[26] We expect future modification and general-
ization of the encoding and decoding steps, with control of
helicity retained as the means of propagating the information,
to advance the development of artificial mimics of biological
receptors capable of transmitting information across chemi-
cally impermeable membrane barriers.
Figure 2. Portion of the 13C NMR spectrum of a) 6c (50:50 ratio);
b) (R)-13a (after one inversion of (S)-7a; 78:22 ratio); c) (S)-13a (after
two inversions of (S)-7a; 28:72 ratio implying coincidence of the
minor peak and some “unlabeled” Me signals); d) (R)-13b (after one
inversion of (S)-7b; 75:25 ratio); e) (S)-13b (after two inversions of
(S)-7b; 20:80 ratio); f) (S)-13b (after benzoylation of (S)-7b; 23:77
ratio).
Received: February 24, 2010
Revised: June 18, 2010
Published online: August 4, 2010
Keywords: conformation analysis · helicity · molecular switches ·
.
nanotechnology · signal transduction
Mitsunobu conditions. Figure 2c shows the response of the
remote reporter: a switch in the location of the major signal
from 26.05 to 24.65 ppm. Although we were unable to deduce
the magnitude of the helical excess observed, N-terminal l-
amino acids typically induce P helicity.[22]
Given the low incidence of helix reversal in Aib oligo-
mers,[11] extension of the Aib-oligomer segment of the
peptides should enable communication over longer distances.
To prove this principle, we synthesized longer peptides by
dimerizing 2 via 3 and 8 to yield 9, which was converted into
its azlactone 10 and then coupled with 11, the reduction
product of 4b. Reduction of the azide and ligation with 5b
gave the tetradecapeptide (S)-7b, which was converted into
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