A switching cascade of hydrazone-based rotary switches through coordination-coupled proton relays

Article abstract of DOI:10.1038/nchem.1408

Imidazole, a subunit of histidine, plays a crucial role in proton-relay processes that are important for various biological activities, such as metal efflux, viral replication and photosynthesis. We show here how an imidazolyl ring incorporated into a rotary switch based on a hydrazone enables a switching cascade that involves proton relay between two different switches. The switching process starts with a single input, zinc(II), that initiates an E/Z isomerization in the hydrazone system through a coordination-coupled proton transfer. The resulting imidazolium ring is unusually acidic and, through proton relay, activates the E/Z isomerization of a non-coordinating pyridine-containing hydrazone switch. We hypothesize that the reduction in the acid dissociation constant of the imidazolium ring results from a combination of electrostatic and conformational effects, the study of which might help elucidate the proton-coupled electron-transfer mechanism in photosynthetic bacteria.

Full text of DOI:10.1038/nchem.1408

ARTICLES
PUBLISHED ONLINE: 29 JULY 2012 | DOI: 10.1038/NCHEM.1408
A switching cascade of hydrazone-based rotary
switches through coordination-coupled
proton relays
Debdas Ray, Justin T. Foy, Russell P. Hughes and Ivan Aprahamian
*
Imidazole, a subunit of histidine, plays a crucial role in proton-relay processes that are important for various biological
activities, such as metal efflux, viral replication and photosynthesis. We show here how an imidazolyl ring incorporated
into a rotary switch based on a hydrazone enables a switching cascade that involves proton relay between two different
switches. The switching process starts with a single input, zinc(II), that initiates an E/Z isomerization in the hydrazone
system through a coordination-coupled proton transfer. The resulting imidazolium ring is unusually acidic and, through
proton relay, activates the E/Z isomerization of a non-coordinating pyridine-containing hydrazone switch. We hypothesize
that the reduction in the acid dissociation constant of the imidazolium ring results from a combination of electrostatic and
conformational effects, the study of which might help elucidate the proton-coupled electron-transfer mechanism in
photosynthetic bacteria.
roton relay is ubiquitous in biological systems1 because it (QIH-Me). More importantly, we show that once the imidazolium
plays a crucial part in many processes, including viral replica- ring is formed by the zinc(II)-initiated CCD process, it can itself act
P
tion2–4, photosynthesis5–8, metal efflux9–11 and enzymatic cata- as a proton source that can activate a different non-coordinating
lysis12,13. The proton translocation in these systems is directed pyridine-containing hydrazone switch (PPH)24 through a dynami-
mainly by an intricate network of water channels and/or flexible cally assisted proton relay (Fig. 1). The key factor behind this switch-
protonatable protein side chains14. Although it is very challenging ing cascade is a synergistic effect that involves the imidazolyl ring
to mimic such complex proton-conduction pathways15, one can and zinc(II) and leads to a substantial reduction in the pKa of the
still be inspired by these systems and build prototypes that can imidazolium ring through allosteric and electrostatic regulation.
attempt to replicate their function16. A common motif in most of To the best of our knowledge this is the first example of such a
the biological proton-relay processes is the imidazolyl ring2–16
,
multistep switching cascade, and a first instance of a dynamically
which is a subunit of the amino acid histidine. For example, switched compound that acts as the input to another. This is some-
proton conduction in influenza M2 proton channels is regulated what reminiscent of the biological molecular motor adenosine tri-
by a dynamically assisted proton-shuttling process8 (that is, depro- phosphate synthase26, which through its mechanical operation
tonation of imidazolium units assisted by ring flipping)4, which is a produces the fuel to power other biological machines.
crucial step in the virus’s replication cycle. Alternatively, a coordi-
nation-coupled deprotonation (CCD) process that involves histidine Results
residues provides the energetic basis for zinc(II) efflux in a cation- The synthesis of QIH-Me is achieved in four steps (Fig. 2). First,
diffusion facilitator4–9. However, the coordination of zinc(II) 1-benzyl-2-methyl imidazole is treated with ethyl chloroformate to
within bacterial photosynthetic reaction centres (RCs) lowers the obtain 2 (ref. 27). Debenzylation of 2 (10% Pd/C) in ethanol
acid dissociation constant (pKa) of the surrounding histidine resi- followed by coupling with the quinoline-8-diazonium tetrafluoro-
dues, which leads to the disruption of proton-transfer pathways borate salt and purification by column chromatography (hexanes:
and subsequently diminishes electron-transfer rates in the RCs6–8
.
ethyl acetate, 9:1) gives QIH in 25% isolated yield. Treatment of
Biological processes play an integral role in the field of molecular QIH with methyl iodide in the presence of K2CO3 gives QIH-Me
1
switches and machines17–20 because they are a constant source of as a yellow solid in 48% yield. QIH-Me was characterized by H
inspiration. For some time now we have looked actively into and 13C NMR spectroscopies, high-resolution mass spectrometry
mimicking biological processes22 in the activation of hydrazone- and X-ray crystallography (see Supplementary Information).
based22–25 molecular switches. Our efforts in this direction led to
The fully characterized 1H NMR spectrum of QIH-Me in
the development of a molecular switch that undergoes configura- CD3CN (see Supplementary Fig. S4a) shows the presence of a
tional switching (that is, E/Z isomerization), which is activated by distinct intramolecular hydrogen bond between the hydrazone
zinc(II)-initiated CCD25. In the first generation of these switches, a proton and the imidazolyl and quinolinyl nitrogens. This hydrogen
pyridyl ring was used as the proton acceptor. We reasoned that bond manifests itself by a deshielded NH proton signal at d ¼
replacing the pyridyl ring with an imidazolyl ring will enable us 13.4 ppm. Nuclear Overhauser enhancement spectroscopy (NOESY)
to investigate the diverse mechanisms and pathways by which this experiments clearly show an interaction between proton H8 from
group is involved in biological proton relay (for example, dynamic the imidazole ring and the hydrazone NH proton, and thus establish
proton shuttling, CCD, pKa modulations).
that the E isomer is the major configuration (.99%) in solution.
Crystals of QIH-Me suitable for X-ray analysis were obtained
Here we report the synthesis and acid- and zinc(II)-initiated
E/Z isomerization of an imidazole-containing hydrazone switch from a 1:1 mixture of CH2Cl2 and hexanes. The crystal structure
Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, Hanover, New Hampshire 03755, USA. e-mail: ivan.aprahamian@dartmouth.edu
*
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1

NATURE CHEMISTRY DOI: 10.1038/NCHEM.1408
ARTICLES
N
O
N
H
O
N
N
N
H
H
N
+
+
O
O
N
N
O
N
N
O
N
N
N
H
O
N
N
O
H
O
Zn2+
O
Zn2+
Zn2+
QIH-Me
+
+
+
N
N
N
N
O
N
N
N
O
N
H
N
Zn(QIH-Me(Z-H+))
Zn(QIH-Me(Z))
PPH(Z-H+)
PPH
PPH
nBu4NCN
Figure 1 | Modulating two switches using a single input. The addition of zinc(II) to a solution of QIH-Me and PPH initiates a CCD process that switches
QIH-Me into a system with an unusually acidic imidazolium ring (Zn(QIH-Me(Z-H1))). The latter, in turn, acts as a trigger that activates PPH through proton
relay. This whole process is reversible, as the systems revert to their original state on the addition of nBu4NCN to the reaction mixture.
...
(Fig. 3a) shows an intramolecular hydrogen bond (N–H N, us to conclude that the quinolinyl ring is in conjugation with the
2.731(1) Å, 129.7(0)8) between the imidazolyl nitrogen (N5) and imidazolyl and/or ester moieties.
the hydrazone NH proton. The quinolinyl nitrogen atom N3 also
The ultraviolet/visible spectrum of QIH-Me (1025 M, CH3CN)
...
forms an intramolecular hydrogen bond (N–H N, 2.670(1) Å, shows an absorption maximum (lmax) at 385 nm (Fig. 4). On titra-
103.7(4)8) with the hydrazone proton. The measured dihedral tion with zinc(II) perchlorate (Zn(ClO4)2), the lmax shifts batho-
angles indicate that the naphthyl and imidazolyl rings deviate chromically with an accompanying hypsochromic change. No
from planarity (–178.11(1)8 and 31.47(18)8 when viewed along difference in absorption was observed on the addition of more
the C(1)–N(1)–N(2)–C(9) atoms and the N(5)–C(5)–C(1)¼N(1) than 1.0 equiv. of the metal ions. An isosbestic point is observed
atoms, respectively). Comparison of the bond lengths with those at l ¼ 405 nm, which indicates the interconversion of only two
of similar intramolecularly hydrogen-bonded hydrazones28 leads species in solution. A Job’s plot analysis (see Supplementary
Fig. S32) shows a vertex around 0.55, which suggests a 1:1
O
Ph
Ph
binding stoichiometry between QIH-Me and Zn2þ. The binding
constant between QIH-Me and Zn2þ was determined to be K1 ¼
2.7 × 105 M21. At a higher concentration of QIH-Me (1024 M,
CH3CN) clear isosbestic points were not observed and the Job’s
plot analysis shows a vertex at 0.70. This indicates that other
species are present in the solution at higher concentrations (see
Supplementary Figs S33 and S34).
O
Cl
O
O
N
N
Et3N, MeCN, 0 o
C
N
N
1
2
N2+ BF4
1
The binding of Zn2þ with QIH-Me was also probed using H
N
NMR spectroscopy. On the addition of Zn(ClO4)2 to QIH-Me the
1H NMR spectrum changed drastically: initially, a complicated spec-
trum was obtained (see Supplementary Fig. S9), which simplified
when 8.0 equiv. of Zn2þ were added (Fig. 5e). Based on our analysis
of the data (1D and 2D NMR spectroscopy) the coordination of Zn2þ
with QIH-Me leads to a coordination-coupled proton transfer that
initiates an E/Z isomerization to give the zinc-bound and protonated
complex Zn(QIH-Me(Z-H1)) (ref. 22). This conclusion is based on
(i) a NOESY correlation between H2 and the NMe hydrogen
atoms, which indicates that an E to Z isomerization occurred on
binding with Zn2þ (see Supplementary Fig. S14), and (ii) a correlation
spectroscopy (COSY) interaction between the proton at d ¼
11.6 ppm (Im-NHþ) and proton H8 on the imidazolyl ring, which
indicates that the ring was protonated. This assignment is confirmed
by the dissolution of the crystals (see Supplementary Fig. S11) that
were used in the analysis of the solid-state structure of the 1:1
HCO2NH4,
Pd-C (10%)
O
H
O
N
Ethanol, 80 o
C
NaOAc, H2O,
N
Ethanol, 0 o
C
3
9
H
N
N
N
O
O
8
N
H
N
H
O
O
N
N
MeI, K2CO3
N
N
N
DMF, 60 oC, 3 h
7
6
2
3
5
4
QIH
QIH-Me
1
complex between Zn2þ and QIH-Me (Fig. 3b). The H NMR spec-
Figure 2 | Synthesis of QIH-Me. The alkylation of 1-benzyl-2-
trum obtained from these crystals contains the same signals as
those when excess Zn2þ was added to QIH-Me (Fig. 5e). The spec-
trum of the crystals also contains signals that arise from a small
amount of protonated QIH-Me (QIH-Me(Z-H1) (Fig. 5f)), which
indicates that the two species are in equilibrium. The coordination-
methylimidazole (1) with ethyl chloroformate to afford 2 (29%) was
followed by debenzylation (10% Pd/C) in ethanol to give 3 in 95% yield.
Compound 3 was then deprotonated with sodium acetate and coupled with
the appropriate diazonium salt to give QIH-Me in 25% yield. The latter was
treated with methyl iodide in the presence of K2CO3 to give QIH-Me in 48% coupled isomerization process is fully reversible, as illustrated by the
n
yield. The methylated derivative used in the studies to simplify the NMR
1H NMR spectrum obtained after the addition of excess Bu4NCN
spectra. DMF ¼ dimethyl formamide.
to the 1:1 complex (see Supplementary Fig. S23).
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1408
ARTICLES
a
c
b
d
N5
C5
C2
C5
N5
N4
C11
N5
C2
C14
C1
N1
C12
C10
C1
N1
C1
N2
Zn1
Zn1
N2
C9
N2
N2
C9
N3
N3
N1
C1
N1
C5
N3
N3
Figure 3 | Oak Ridge thermal ellipsoid plots (50% probability ellipsoids) of the X-ray crystal structures of the hydrazone-based switches. a–d, QIH-Me
(a), zinc(II)-bound and protonated QIH-Me (b), protonated QIH-Me (c) and zinc(II)-bound and protonated QPH-Me (d). The protons are placed in calculated
positions, except for those on the hydrazone N(2), imidazolyl N(5) and pyridyl N(4) atoms, in addition to the water molecule in (b), which were refined. The
zinc atom in (b) is also coordinated with water and acetonitrile molecules, whereas in (d) it is coordinated with two acetonitriles. The counterions are
removed for the sake of clarity.
At lower concentrations of Zn2þ (for example, 0.5 equiv.) three H7. The broadness of the signal at d ¼ 11.9 ppm precluded the
different species were present in solution: QIH-Me(Z-H1), observation of any COSY or NOESY correlations. We assume that
Zn(QIH-Me(Z-H1)) and Zn(QIH-Me(Z)) (see Supplementary this signal originates from the imidazolium ring (Im-NHþ),
Figs S9 and S10). This assignment is based on comparison with which would explain the downfield shift of the imidazolyl hydrogen
the spectra of the protonated QIH-Me system (Fig. 5f) and the atoms H8 and H9. Moreover, a NOESY interaction between the H2
zinc(II)-bound QIH-Me complex (Fig. 5g and Supplementary and Im-Me protons indicates that an E/Z isomerization has taken
Fig. S18). It seems that at lower concentrations of Zn2þ a fraction of place at this stage, as shown by the X-ray analysis of QIH-Me(Z-H1)
the latter acts as an acid to protonate the unbound QIH-Me in sol- (Fig. 3c). The absence of the low-field signals at d ¼ 14.1 and
ution. This process is an indication that the imidazolium nitrogen in 11.9 ppm in Fig. 5c is an indication that this species is not present
Zn(QIH-Me(Z-H1)) is less basic than that in QIH-Me (and in pyr- in the two-switch solution mixture. We come to the same
idine, vide infra). The three species are all in equilibrium as the conclusion when we compare the mixture with the spectrum of
addition of excess Zn2þ pushes the equilibrium towards the Zn(QIH-Me(Z-H1)) (Fig. 5e). However, the imidazole signals at
zinc(II)-bound and protonated QIH-Me complex (Fig. 5e).
d ¼ 7.4 (H8) and 7.2 (H9) ppm, and the aromatic proton signals
The self-protonation of QIH-Me in the presence of Zn2þ is very at d ¼ 8.9 (H7), 8.5 (H5), 7.7–7.8 (H2/6), 7.6 (H4) and 7.5 (H3)
promising as far as proton relay is concerned. To take advantage of ppm in Fig. 5b, closely match those of Zn(QIH-Me(Z)) (Fig. 5g),
this process, a second non-coordinating rotary switch, PPH which indicates that this species is the second compound
(ref. 24), was used to probe the possibility of proton relay from in solution.
Zn(QIH-Me(Z-H1)). Based on our experimental results we con-
The most crucial part of the relay process is the lowering of the
clude that a coordination-coupled proton transfer initially switches pKa of the imidazolium ring by ꢀ3 units8 in Zn(QIH-Me(Z-H1)),
QIH-Me into Zn(QIH-Me(Z-H1)) (Fig. 1), which in turn relays because without it the multistep switching cascade could not
its acidic imidazolium proton to PPH and transforms it into occur. In biological systems, such pKa modulations are prevalent1
PPH(Z-H1). This whole process is reversible; the addition of and usually they are attributed to conformational changes as a
nBu4NCN to the mixture reverts the two switches back to their result of metal binding. Another relevant mechanism8,29,30, albeit
original state (see Supplementary Fig. S24). Our conclusion with
regard to the proton-relay process is based on the following NMR
spectroscopy experiments. The 1H NMR spectrum of the equimolar
mixture of QIH-Me and PPH (Fig. 5b) shows sharp and non-
overlapping signals for the hydrazone and aromatic protons,
except for protons H10, H11 and H14, which are merged together
H
+
O
N
N
O
0.24
0.20
0.16
0.12
0.08
0.04
0.00
N
N
H
O
O
2+
N
N
Zn
N
N
N
N
1
at d ¼ 7.38 ppm. The H NMR spectrum of the mixture changes
+
QIH-Me
Zn(QIH-Me(Z-H ))
drastically, especially for the hydrazone and aromatic hydrogen
atoms, when 1.0 equiv. of Zn2þ is added to the mixture (Fig. 5c);
the aromatic signals of the two switches are well separated, except
for protons H5, H16, H2, H3 and H6, and a new signal appears
at d ¼ 12.9 ppm. The general trend is of a downfield shift of the
aromatic signals, except for the hydrogen atom H13, which is
shifted to a higher field. Based on our earlier report24 the signal at
d ¼ 12.9 ppm is an indication that the pyridyl ring in PPH is
protonated at this stage. A comparison with the separately
protonated species (Fig. 5d) shows that this is, indeed, the case.
To identify the other species in the solution mixture we
conducted a number of control experiments. The addition of 1.0
Zn2+ (equiv.)
0.0
0.2
0.4
0.6
0.8
1.0
250
300
350
400
450
500
550
600
λ (nm)
equivalents of trifluoroacetic acid to QIH-Me (Fig. 5f and Figure 4 | Spectroscopic studies of the binding between zinc(II) and
Supplementary Fig. S22) resulted in a downfield shift of the aro- QIH-Me (1.0 3 1025 M, CH3CN, 294 K). The ultraviolet/visible spectra
matic signals, and the imidazolyl hydrogen atoms H8 and H9 show a gradual bathochromic shift on titration of QIH-Me with Zn(ClO4)2:
appear together at d ¼ 7.45 ppm. In addition, two new signals are lmax shifts from 385 nm (black trace) to 438 nm (red trace). An isosbestic
observed at d ¼ 14.1 and 11.9 ppm. The hydrazone NH signal point is visible at l ¼ 405 nm. The process reaches saturation after the
(d ¼ 14.1 ppm) shows the typical NOESY correlation with proton addition of 1.0 equiv. of zinc(II) to QIH-Me.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1408
ARTICLES
a
13
15
8
8
8
H
H
N
H
9
14
15
14
13
16
O
N
O
N
O
N
N
N
O
N
4
O
N
4
O
N
4
9
9
9
8
O
H
N
N
N
N
H
N
H
O
O
Zn2+
O
Zn2+
O
H
O
16
N
N
N
N
N
N
N
N
N
N
N
N
5
10
11
2
3
7
6
2
3
7
6
10
11
2
3
7
6
7
6
2
3
12
5
4
5
5
12
QIH-Me(E)
PPH(E)
Zn(QIH-Me(Z-H+))
Zn(QIH-Me(Z))
QIH-Me(Z-H+)
PPH(Z-H+)
H10/11/14
b
QIH-Me + PPH
QIH-Me(NH)
H8
H9
PPH-NH
H16
H4/6
H3
H7 H13
H12
H5
H15
H2
PPH-NH(Z)
PPH(Z)
PPH(Z)
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
Zn(QIH-Me(Z)) + PPH(Z-H+)
c
H11
H5/16
H13
H10
H4
PPH-NH
H12
H8
H9
H14
H2/6
H15
H7
H3
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
PPH(Z-H+)
PPH-NH
d
H10
H11
H12
H13/16/15
H14
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
Zn(QIH-Me(Z-H+))
e
Im-NH+
H8/9
H6/4
H3
H7
H5
H2
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
QIH-Me(Z-H+)
QIH-Me(NH)(Z)
Im-NH+
f
H8 H9
H3/6
H4
H2
H7
H5
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
+
g
Zn(QIH-Me(Z)) + DBP-H
H11(DBP)
H10(DBP)
H4
H9
H8
(DBP)Py-NH+
(DBP)H11'
H3
H7
H2/6
H5 H10' (DBP)
15 14 13 12 11
9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0
ppm
Figure 5 | The 1H NMR spectra (500 MHz, CD3CN, 294 K) used in confirming the CCD-initiated proton transfer that leads to the successive activation of
two switches. a, The structures of the different hydrazone systems identified using NMR spectroscopy. b, QIH-Me and PPH (1:1) (the signals of the minor Z
isomer of PPH are also labelled). c, The mixture of zinc(II)-bound QIH-Me and protonated PPH obtained after the addition of 1.0 equiv. of Zn(ClO4)2.
d–f, Protonated PPH (d), zinc(II)-bound and protonated QIH-Me (e) and protonated QIH-Me (f). g, Zinc(II)-bound QIH-Me and protonated 2,6-di-tert-
butylpyridine (DBP), DBP-H1 (the spectrum was recorded after the addition of 1.0 equiv. of Zn(ClO4)2 to an equimolar mixture of QIH-Me and DBP).
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1408
ARTICLES
combination of allosteric and electrostatic effects brought about
by the coordination of the switch with zinc(II). The binding event
causes the imidazolium ring to tilt out of planarity by 25.58
(Fig. 6), such that the intramolecular hydrogen bond between the
imidazolium ring and ester oxygen is weakened relative to that of
the other two systems. The lack of a NOESY interaction between
the NMe and H2 protons in Zn(QIH-Me(Z-H1)) is an indication
that the imidazolium ring also tilts away from planarity when in sol-
ution. However, this by itself cannot explain the change in pKa values
and so we hypothesized that the combination of the weakened hydro-
gen bond with the electrostatic repulsion between the imidazolium
proton and zinc(II) results in the observed decrease in the pKa of
the imidazolium ring8.
*
To test this hypothesis we synthesized QPH-Me (see
Supplementary Fig. S6), in which the pyridyl ring is substituted by
a methyl group at the 3-position inducing the pyridyl ring to go out
of the plane of the molecule (see Supplementary Fig. S39). This sub-
stitution drastically changes the isomer ratio in solution (E:Z ¼ 1:2),
which complicates the proton-transfer experiment; the coordination
of zinc(II) with the major Z isomer will deprotonate the hydrazone
NH, which can be picked up directly by PPH without switching
QPH-Me. Nevertheless, Zn(QPH-Me(Z-H1)) can be obtained by
adding excess zinc(II) to the latter (see Supplementary Fig. S28). The
addition of PPH to this solution results in its complete protonation,
contrary to no protonation when it is added to the non-methylated
version. This experiment shows that the non-planarity (dihedral
angle of 132.6(2)8 when viewed along the N(4)–C(12)–C(10)¼N(2)
Figure 6 | Superimposition of the optimized density functional theory
(B3LYP/LACV3P**11) structures of QIH-Me(Z-H1) (blue) and Zn(QIH-
Me(Z-H1)) (bronze). The figure shows the distortion of the molecular
framework on coordination to zinc(II), and the tipping of the imidazolium ring
out of the plane by 25.58. The principal site of angular distortion is marked
with an asterisk. For simplicity, the calculations were carried out with two
acetonitriles coordinated to the metal.
controversial31,32, that was put forward to explain the lowering of the atoms) of the pyridyl group in Zn(QPH-Me(Z-H1)) (Fig. 3d) is a
pKa of histidine residues in RCs on metal binding (by ꢀ2 units) prerequisite for proton relay to occur. These results indicate that a
involves electrostatic repulsion of protons by the bound metal ion. combination of electrostatic and allosteric effects can be responsible
As mentioned earlier, this change in pKa leads to the slowing for the change in pKa observed in RCs on coordination with zinc(II).
down of the photosynthetic cycle in RCs6–8. To assess whether con- Moreover, they show that the protons have to be weakly hydrogen
formational changes, electrostatic interactions or other effects are bonded for them to be deprotonated on binding of the metal with
responsible for the behaviour in Zn(QIH-Me(Z-H1)), we compared the RCs, that is, not all titratable protein residues are affected
closely its X-ray structure with that of QIH-Me(Z-H1) and that of equally by the metal coordination.
the previously reported pyridinium system25, Zn(QPH(Z-H1)),
neither of which are capable of subsequent proton relay (see
Supplementary Figs S26 and S27). There seems to be a synergistic
O
effect at play, as both zinc(II) and the imidazolyl ring are required
for the reduction in pKa and switching cascade to take effect.
Moreover, we calculated (B3LYP/LACV3P**þþ) (refs 33–36) the
charge distribution in these systems using the natural bond orbital
(NBO) method (see Supplementary Information for full details),
and used the calculated electron densities at the bond critical
points of the intramolecular hydrogen bonds37 to assess their
strengths (see Supplementary Figs S35–S38).
O
H
N
N
N
N
QPH-Me
Conclusions
The field of molecular switches and machines has come a long way in
the past two decades38, but nevertheless quite a few challenges still
need to be addressed in the coming years. Of these challenges,
using switches to drive systems out of equilibrium39–41 and integrat-
ing them into periodic and robust architectures42 seem to be the pre-
dominant ones. We postulate that the step-wise and congruent
activation of different molecular switches is a challenge of equal
importance, as this is how complexity43 is brought about in biological
systems. In this article we show that through mimicking biologi-
cal processes it is possible to effect, using a single input, a multistep
H
+
O
N
O
Zn2+
N
N
N
Zn(QPH(Z-H+))
Comparison of the X-ray and NBO data of the three compounds switching process (that is, E/Z isomerizations) that involve two
shows no substantial differences in bond distances or partial different molecular switches. We show that the switching process
charges between them (see Supplementary Table S2), which is fully reversible, as the addition of cyanide to the hydrazone
suggests that inductive and/or resonance effects can be excluded switch mixture induced a reversal back to its original state.
as reasons for the change in pKa. However, the bond critical point
This reversible multistep switching process opens the way to effi-
calculations show that the hydrogen bond in the non-planar cient switching cascades that have minimal side reactions/products
Zn(QIH-Me(Z-H1)) is weaker than that in the zinc(II)-bound and as one switch acts as the input to another. The crucial step in the
protonated pyridinium system and that in protonated QIH-Me. switching process is the lowering of the pKa of the imidazolyl ring
All these data suggest that the change in acidity results from a in QIH-Me on binding with zinc(II), which results from the
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1408
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Acknowledgements
The authors dedicate this manuscript to Sir Fraser Stoddart on the occasion of his 70th
birthday. This work was supported by Dartmouth College, the Burke Research Initiation
Award and the American Chemical Society Petroleum Research Fund. The authors thank
R. Staples for the X-ray analysis, S. Voskian for his help with the graphic for the table of
contents and D.S. Glueck for his input.
Author contributions
All experiments were conducted by D.R. and J.T.F. with input from I.A. The calculations
were carried out and interpreted by R.P.H. The manuscript was co-written by I.A. and D.R.
with input from R.P.H.
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Additional information
The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://www.
nature.com/reprints. Correspondence and requests for materials should be addressed to I.A.
6
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