Hydrogen-bonded Wire Research Articles

Proton Release Reactions in the Inward H+ Pump NsXeR.

Directional ion transport across biological membranes plays a central role in many cellular processes. Elucidating the molecular determinants for vectorial ion transport is key to understanding the functional mechanism of membrane-bound ion pumps. The extensive investigation of the light-driven proton pump bacteriorhodopsin from Halobacterium salinarum(HsBR) enabled a detailed description of outward proton transport. Although the structure of inward-directed proton pumping rhodopsins is very similar to HsBR, little is known about their protonation pathway, and hence, the molecular reasons for the vectoriality of proton translocation remain unclear. Here, we employ a combined experimental and theoretical approach to tracking protonation steps in the light-driven inward proton pump xenorhodopsin from Nanosalina sp. (NsXeR). Time-resolved infrared spectroscopy reveals the transient deprotonation of D220 concomitantly with deprotonation of the retinal Schiff base. Our molecular dynamics simulations support a proton release pathway from the retinal Schiff base via a hydrogen-bonded water wire leading to D220 that could provide a putative gating point for the proton release and with allosteric interactions to the retinal Schiff base. Our findings support the key role of D220 in mediating proton release to the cytoplasmic side and provide evidence that this residue is not the primary proton acceptor of the proton transiently released by the retinal Schiff base.

Water Networks in Photosystem II Using Crystalline Molecular Dynamics Simulations and Room-Temperature XFEL Serial Crystallography.

Structural dynamics of water and its hydrogen-bonding networks play an important role in enzyme function via the transport of protons, ions, and substrates. To gain insights into these mechanisms in the water oxidation reaction in Photosystem II (PS II), we have performed crystalline molecular dynamics (MD) simulations of the dark-stable S1 state. Our MD model consists of a full unit cell with 8 PS II monomers in explicit solvent (861 894 atoms), enabling us to compute the simulated crystalline electron density and to compare it directly with the experimental density from serial femtosecond X-ray crystallography under physiological temperature collected at X-ray free electron lasers (XFELs). The MD density reproduced the experimental density and water positions with high fidelity. The detailed dynamics in the simulations provided insights into the mobility of water molecules in the channels beyond what can be interpreted from experimental B-factors and electron densities alone. In particular, the simulations revealed fast, coordinated exchange of waters at sites where the density is strong, and water transport across the bottleneck region of the channels where the density is weak. By computing MD hydrogen and oxygen maps separately, we developed a novel Map-based Acceptor-Donor Identification (MADI) technique that yields information which helps to infer hydrogen-bond directionality and strength. The MADI analysis revealed a series of hydrogen-bond wires emanating from the Mn cluster through the Cl1 and O4 channels; such wires might provide pathways for proton transfer during the reaction cycle of PS II. Our simulations provide an atomistic picture of the dynamics of water and hydrogen-bonding networks in PS II, with implications for the specific role of each channel in the water oxidation reaction.

Evolutionary diversity of proton and water channels on the oxidizing side of photosystem II and their relevance to function.

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the 'water wheel' to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421-425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831-15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).

Insights into atomic‐electronegativity‐controlled excited state intramolecular proton transfer behavior for the novel oxazolonapthoimidazo[1,2‐a]‐pyridine (ONIP) compound: A time‐dependent density functional theory study

AbstractIn this work, we mainly focus on probing into the effects of atomic electronegativity on photo‐induced hydrogen bond (i.e., O–H···N) effects and excited state intramolecular proton transfer (ESIPT) processes for a novel fluorescent chemical probe ONIP derivatives (i.e., ONIP‐O, ONIP‐S, and ONIP‐Se). First, insights into optimized structural geometries and related infrared vibrational spectra, we clarify that the hydrogen bond O–H···N interaction could be strengthened via photoexcitation. Further, via predicting hydrogen bonding energy EHB, we quantificationally present that the S1‐state hydrogen bonding strengthening behavior should be more distinct with lower atomic electronegativity. When it comes to vertical photo‐induced excitation, the intramolecular charge transfer process happens and the charge redistribution plays roles in promoting ESIPT behavior. It is worth mentioning the energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital show that low atomic electronegativity should be more favorable to ESIPT process. Finally, based on the restricted optimization method, we construct the potential energy curves along with hydrogen bond wire for ONIP‐O, ONIP‐S, and ONIP‐Se. By exploring the conformation of potential energy curves and potential energy barriers, we disclose the detailed atomic‐electronegativity‐controlled ESIPT dynamical behaviors for ONIP system.

Structural Basis for the Function of the C-Terminal Proton Release Pathway in the Calcium Pump.

The calcium pump (sarco/endoplasmic reticulum Ca2+-ATPase, SERCA) plays a major role in calcium homeostasis in muscle cells by clearing cytosolic Ca2+ during muscle relaxation. Active Ca2+ transport by SERCA involves the structural transition from a low-Ca2+ affinity E2 state toward a high-Ca2+ affinity E1 state of the pump. This structural transition is accompanied by the countertransport of protons to stabilize the negative charge and maintain the structural integrity of the transport sites and partially compensate for the positive charges of the two Ca2+ ions passing through the membrane. X-ray crystallography studies have suggested that a hydrated pore located at the C-terminal domain of SERCA serves as a conduit for proton countertransport, but the existence and function of this pathway have not yet been fully characterized. We used atomistic simulations to demonstrate that in the protonated E2 state and the absence of initially bound water molecules, the C-terminal pore becomes hydrated in the nanosecond timescale. Hydration of the C-terminal pore is accompanied by the formation of water wires that connect the transport sites with the cytosol. Water wires are known as ubiquitous proton-transport devices in biological systems, thus supporting the notion that the C-terminal domain serves as a conduit for proton release. Additional simulations showed that the release of a single proton from the transport sites induces bending of transmembrane helix M5 and the interaction between residues Arg762 and Ser915. These structural changes create a physical barrier against full hydration of the pore and prevent the formation of hydrogen-bonded water wires once proton transport has occurred through this pore. Together, these findings support the notion that the C-terminal proton release pathway is a functional element of SERCA and also provide a mechanistic model for its operation in the catalytic cycle of the pump.

Proton-Coupled Electron Transfer in the Aqueous Nanochannels of Lyotropic Liquid Crystals: Interplay of H-Bonding and Polarity Effects.

A molecular-level description of the aqueous nanochannels in lyotropic liquid crystals (LLCs) is crucial for their widespread utilization in diverse fields. Herein, the polarity and hydrogen bonding effects of LLC water molecules have been simultaneously explored using a single probe, 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF), by the unique multiparametric sensitivity of the excited state proton-coupled electron transfer (PCET) phenomenon. The decreased ESIPT efficiency and the significantly retarded ESIPT dynamics (>20 times) of DMA3HF in the LLC phases suggests the dominant influence of strong hydrogen-bonded solute-solvent complexes that leads to a high activation barrier for ESIPT in the mesophases. The effects of hydrogen bonding on ESIPT have been elucidated by enhanced sampling techniques based on classical MD simulations of DMA3HF in explicit water. ESIPT via an extended hydrogen-bonded water wire is associated with a significantly high ESIPT activation barrier, substantiating the experimentally observed slow ESIPT dynamics inside the LLCs.

Proton strings and rings in atypical nucleation of ferroelectricity in ice

Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.

Transesterification activity by a zinc(II)-Schiff base complex with theoretical interpretation

The article demonstrates the synthesis, structural characterization and transesterification activity of a mononuclear zinc(II) complex, [Zn(HL)(H2O)]·(H2O) (1) containing a previously reported Schiff base ligand, H3L = N,N′-bis(salicylidene)-1,3-diamino-2-propanol]. X-ray structural analysis of 1 reveals that Zn(II)-Schiff base complex crystallizes in hexagonal crystal system with P61 space group and adopts a distorted tetrapyramidal geometry. Self-assembled molecular units of 1 exhibit a beautiful construction of 3D crystalline architecture through intermolecular hydrogen bond wire. Zinc(II) complex displays an enhancement of fluorescence intensity in comparison to HL in methanol medium. Catalytic behaviour of 1 towards disodium salt of 2,4-dinitrophenylphosphate (PNPP) in aqueous-methanol medium exhibits good transesterification activity with initial rate constant value of 1.73 × 10−4 min−1. Detailed DFT calculations are also employed to cope with the geometrical parameters of 1 as well as to explore the proposed catalytic mechanism of transesterification activity.

A Proton Wire Mediates Proton Coupled Electron Transfer from Hydroxyurea and Other Hydroxamic Acids to Tyrosyl Radical in Class Ia Ribonucleotide Reductase.

Proton-coupled electron transfer (PCET) is fundamental to many important biological reactions, including solar energy conversion and DNA synthesis. For example, class Ia ribonucleotide reductases (RNRs) contain a tyrosyl radical-diiron cofactor with one aspartate ligand, D84. The tyrosyl radical, Y122•, in the β2 subunit acts as a radical initiator and oxidizes an active site cysteine in the α2 subunit. A transient quaternary α2/β2 complex is induced by substrate and effector binding. The hydroxamic acid, hydroxyurea (HU), reduces Y122• in a PCET reaction involving an electron and proton. This reaction is associated with the loss of activity, a conformational change at Y122, and a change in hydrogen bonding to the Fe1 ligand, D84. Here, we use isotopic labeling, solvent isotope exchange, proton inventories, and reaction-induced Fourier transform infrared (RIFT-IR) spectroscopy to show that the PCET reactions of hydroxamic acids are associated with a characteristic spectrum, which is assignable to electrostatic changes at nonligating aspartate residues. Notably, RIFT-IR spectroscopy reveals this characteristic spectrum when the effects of HU, hydroxylamine, and N-methylhydroxylamine are compared. A large solvent isotope effect is observed for each of the hydroxamic acid reactions, and proton inventories predict that the reactions are associated with the transfer of multiple protons in the transition state. The reduction of Y122• with 4-methoxyphenol does not lead to these characteristic carboxylate shifts and is associated with only a small solvent isotope effect. In addition to studies of the effects of hydroxamic acids on β2 alone, the reactions involving the quaternary α2β2 complex were also investigated. HU treatment of the quaternary complex, α2/β2/ATP/CDP, leads to a similar carboxylate shift spectrum, as observed with β2 alone. The use of globally labeled 13C chimeras (13C α2, 13C β2) confirms the assignment. Because the spectrum is sensitive to 13C β2 labeling, but not 13C α2 labeling, the quaternary complex spectrum is assigned to electrostatic changes in β2 carboxylate groups. Examination of the β2 X-ray structure reveals a hydrogen-bonded network leading from the protein surface to Y122. This predicted network includes nonligating aspartates, glutamate ligands to the iron cluster, and predicted crystallographically resolved water molecules. The network is similar when class Ia RNR structures from Escherichia coli, human, and mouse are compared. We propose that the PCET reactions of hydroxamic acids are mediated by a hydrogen-bonded proton wire in the β2 subunit.

A theoretical study on excited state proton transfer in 2‐(2′‐dihydroxyphenyl) benzoxazole

AbstractThis research investigates the dynamic excited state process for a novel system 2‐(2′‐dihydroxyphenyl) benzoxazole (DHBO) for excited state proton transfer (ESPT) process based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. Because 2 intramolecular hydrogen bonds (O1―H2···N3 and O4―H5···O6) in DHBO molecules may trigger proton transfer process in the S1 state, we focus on these 2 hydrogen bonds. Our results show that only the O1―H2···N3 bond has obvious changes in both bond length and bond angle upon photoexcitation. Charge redistribution also confirms that hydrogen bond wire (O1―H2···N3) is the best way to achieve the ESPT process in the S1 state. Considering the ESPT mechanism, our theoretical potential energy curves of DHBO indicate that only the excited state single‐proton transfer process occurs via O1―H2···N3 rather than O4―H5···O6. We believe that our work not only clarifies the excited state dynamical behavior of DHBO but also promotes the investigations about ESPT reactions in intramolecular or intermolecular hydrogen bonded chemical systems.

Excited-State Proton Transfer Mechanism of 2,6-Diazaindoles·(H2O) n ( n = 2-4) Clusters.

This paper identified a new excited-state proton transfer (ESPT) mechanism for 2,6-diazaindoles (2,6-DAI) in aqueous (H2O) solution based on time-dependent density functional theory. The calculated results show that the excited-state three proton transfer reaction cannot occur because the 2,6-DAI with two water molecules do not form hydrogen bond wires; this finding was different from those reported in previous experiments (Chung et al. J. Am. Chem. Soc. 2017, 139, 6396-6402). 2,6-DAI with three water molecules form 2,6-DAI·(H2O)3 clusters, whereas 2,6-DAI with four water molecules form 2,6-DAI·(H2O)4 cluster. These clusters participate in the ESPT reaction. To determine the ESPT mechanism of 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters, we constructed the potential energy curves of S1 and S0 states. The results confirmed the simultaneous presence of both 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters and only one proton transfer pathway. By calculating the transition states of 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters, we found that the ESPT reaction is a consistent mechanism. Our work investigated the number of water molecules involved in the ESPT and paved the way to further study the intermolecular hydrogen bonding interactions in the biological field.

Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin.

As the most important component of deep red pigments, alkannin is investigated theoretically in detail based on time-dependent density functional theory (TDDFT) method. Exploring the dual intramolecular hydrogen bonds (O1-H2···O3 and O4-H5···O6) of alkannin, we confirm the O1-H2···O3 may play a more important role in the first excited state than the O4-H5···O6 one. Infrared (IR) vibrational analyses and subsequent charge redistribution also support this viewpoint. Via constructing the S1-state potential energy surface (PES) and searching transition state (TS) structures, we illuminate the excited state double proton transfer (ESDPT) mechanism of alkannin is the stepwise process that can be first launched by the O1-H2···O3 hydrogen bond wire in gas state, acetonitrile (CH3CN) and cyclohexane (CYH) solvents. We present a novel mechanism that polar aprotic solvents can contribute to the first-step proton transfer (PT) process in the S1 state, and nonpolar solvents play important roles in lowering the potential energy barrier of the second-step PT reaction.

Direct dynamics calculations of multiple proton transfer through hydrogen-bonded wire and the role of micro-solvation in ClONO2 + H2O → HNO3 + HOCl reactions

The hydrolysis of ClONO2 on polar stratospheric ice has been considered as a major factor causing stratospheric ozone depletion. We have theoretically investigated the reaction dynamics of hydrolysis on ice surface and the role of micro-solvation. No theoretical studies have been performed for the micro-solvent effect of multiple proton transfer in the hydrolysis of ClONO2. Rate constants and tunneling coefficients were calculated using variational transition state theory including multidimensional tunneling. The dispersion corrected, spin-component scaled, double hybrid PBE functional with the P86 correlation that can reproduce the MP2/CBS + ∆CCSD(Q) result was used to generate potential energy surfaces. No more than three water molecules could form a cyclic hydrogen (H)-bonded chain to catalyze the reaction and the other is bound to the chain to act as a micro-solvent. The catalytic water reduced not only the barrier but also the tunneling effect significantly. The micro-solvent effect of lowering the barrier is smaller and depends on the position. The multiple proton transfer path through H-bonded chain, in some cases, varied with the position of micro-solvent, and consequently the H-bonded structure of HOCl–HNO3 cluster became completely different from the reactant and TS structures. The predicted rate constant was 0.671 at 193 K, and the Arrhenius activation energy was 8 kcal/mol. This rate constant was smaller by three orders of magnitude than that of ClONO2 + HCl on ice, which is consistent with the experimental observations that at low HCl concentration conditions ClONO2 hydrolysis competes with ClONO2 + HCl reaction.

Elaborating the excited state behavior of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine coupling with methanol solvent

AbstractWe present a theoretical investigation about the excited state dynamical mechanism of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine (DMAPIP‐c). Within the framework of density functional theory and time‐dependent density functional theory methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, 3 methanol (MeOH) solvent molecules should be connected with DMAPIP‐c forming DMAPIP‐c‐MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O7‐H8···N9 one should be largely strengthened in the S1 state, which plays an important role in facilitating the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals based on the photoexcitation, we do find tendency of ESIPT reaction due to the most charges located around N9 atom in the lowest unoccupied molecular orbital. Based on constructing the potential energy curves of both S0 and S1 states, we not only confirm that the ESIPT process should firstly occur along with hydrogen bond wire O7‐H8···N9, but also find a low potential energy barrier 8.898 kcal/mol supports the ESIPT reaction in the S1 state forming DMAPIP‐c‐MeOH‐PT configuration. Subsequently, DMAPIP‐c‐MeOH‐PT could twist its dimethylamino moiety with a lower barrier 3.475 kcal/mol forming DMAPIP‐c‐MeOH‐PT‐TICT structure. Our work not only successfully explains previous experimental work but also paves the way for the further applications about DMAPIP‐c sensor in future.

Explaining the excited state behavior of t-DMASIP-b sensor: A theoretical study

In this present work, we theoretically investigate the excited state dynamical mechanism of a novel sensor trans-2-[4′-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine (t-DMASIP-b). Within the framework of density functional theory (DFT) and time dependent DFT (TDDFT) methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, two methanol (MeOH) solvent molecules should be connected with t-DMASIP-b forming t-DMASIP-b-MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O5-H6⋯N7 one should be largely strengthened in the S1 state, which might facilitate the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared (IR) vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals (MOs) based on the photo-excitation, we do find tendency of ESIPT reaction due to the most charges located around N7 atom in the LUMO orbital. Even though some indications reveal the ESIPT, we find the moderate potential energy barriers in the S1-state potential energy curves. The moderate potential energy barrier 12.89kcal/mol along with O5-H6⋯N7 hydrogen bonding wire indeed hinders the proceeding of ESIPT, so only one fluorescence peak was reported in previous experiment, which has been reasonably explained in our work. As a whole, we deem our work not only successfully explains previous experimental work, but also paves the way for the further applications about t-DMASIP-b sensor in future.

A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

AbstractIn the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O1‐H2⋯O5 and O3‐H4⋯O5) have been supported to be formed in the S0 state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S1 state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S1 state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O1‐H2 and O3‐H4 coordinates demonstrate that the proton transfer process should be more likely to occur in the S1 state via hydrogen bond wire O1‐H2⋯O5 rather than O3‐H4⋯O5 because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.

Theoretical insights into photoinduced proton transfer of 7-hydroxyquinoline via intermolecular hydrogen-bonded wire of mixed methanol and water

Proton transfer (PT) reactions of 7-hydroxyquinoline (7HQ) via intermolecular hydrogen-bonded wire of methanol, water and mixed methanol–water in ground (S0) and first excited (S1) states, [7HQ(X)3 when X = M-methanol and W-water], have been studied by using density functional theory (DFT) at B3LYP and its time-dependent DFT with 6-31+G(d,p) basis set. For all complexes, the intermolecular hydrogen bonds become shorter and the O–H stretching vibrational frequencies shift to lower frequencies in the S1 state, which confirm that the hydrogen bonding interaction is stronger in the S1 state. Moreover, the frontier molecular orbitals of all complexes were analyzed to confirm the PT reactions (ππ*). The simulated absorption and emission spectra of 7HQ(MMM) are in good agreement with the experimental data. In addition, the potential energy curves along the PT reaction coordinates of all complexes both in S0 and S1 states were scanned by constrained optimizations fixing the O–H bond distance (a proton donor site of 7HQ) to investigate the effect of different intermolecular hydrogen-bonded solvent wires surrounding 7HQ. PT reactions are found to be favorable in S1 state due to the low PT energy barrier. For pure solvent, the excited-stated proton transfer (ESPT) occurs faster via methanol than water. For mixed solvent, when replacing methanol with one up to three water molecules, PT energy barrier is found to be higher than that of 7HQ(MMM); therefore, water may block the ESPT reaction.

Evidence for hydrogen-bonded ammonia wire influencing the ESMPT process of the 7-hydroxy-4-methylcoumarin·(NH3)3 cluster

The different types of excited-state hydrogen-bonded dynamics mechanisms orderly provide two driving forces for the excited-state multiple proton transfer (ESMPT) reaction of the 7H4MC·(NH3)3 cluster.

Long-range proton relay shows an inverse linear free energy relationship depending on the pKa of the hydrogen-bonded wire

The long-range proton transfer dependence on the pKa of hydroxyl molecules in hydrogen (H)-bonded wires was investigated using quantum mechanical calculations.

Ground-state and excited-state multiple proton transfer via a hydrogen-bonded water wire for 3-hydroxypyridine

The multiple proton transfer reactions of 3-hydroxypyridine-(H2O)3have been demonstrated, and a perfect proton transfer cycle has been revealed in the ground and excited states.