Excited-state proton transfer in 7-hydroxy-4-methylcoumarin along a hydrogen-bonded water wire

TDDFT, RI-CC2, and CIS calculations have been performed for the nondissociative excited-state proton transfer (ESPT) in the S1 state of 7-hydroxy-4-methylcoumarin (7H4MC) along a H-bonded water wire of three water molecules bridging the proton donor (OH) and the proton acceptor (C[double bond]O) groups (7H4MC.(H2O)3). The observed structural reorganization in the water-wire cluster is interpreted as a proton-transfer (PT) reaction along the H2O solvent wire. The shift of electron density within the organic chromophore 7H4MC due to the optical excitation appears to be the driving force for ESPT. All the methods used show that the reaction path occurs in the 1pipi* state, and no crossing with a Rydberg-type 1pisigma* state is found. TDDFT and RI-CC2 calculations predict an exoergic reaction of the excited-state enol-to-keto transformation. The S1 potential energy curve reveals well-defined Cs minima of enol- and keto-clusters, separated by a single barrier with a height of 17-20 kcal/mol. After surmounting this barrier, spontaneous PT along the water wire is observed, leading without any further barrier to the keto structure. The TDDFT and RI-CC2 methods appear to be reliable approaches to describe the energy surfaces of ESPT. The CIS method predicts an endoergic ESPT reaction and an energy barrier, which is too high.     

Energetics and dynamics of proton transfer reactions along short water wires

Proton transfer (pT) reactions in biochemical processes are often mediated by chains of hydrogen-bonded water molecules. We use hybrid density functional calculations to study pT along quasi one-dimensional water arrays that connect an imidazolium-imidazole proton donor-acceptor pair. We characterize the structures of intermediates and transition states, the energetics, and the dynamics of the pT reactions, including vibrational contributions to kinetic isotope effects. In molecular dynamics simulations of pT transition paths, we find that for short water chains with four water molecules, the pT reactions are semi-concerted. The formation of a high-energy hydronium intermediate next to the proton-donating group is avoided by a simultaneous transfer of a proton from the donor to the first water molecule, and from the first water molecule into the water chain. Lowering the dielectric constant of the environment and increasing the water chain length both reduce the barrier for pT. We study the effect of the driving force on the energetics of the pT reaction by changing the proton affinity of the donor and acceptor groups through halogen and methyl substitutions. We find that the barrier of the pT reaction depends linearly on the proton affinity of the donor but is nearly independent of the proton affinity of the acceptor, corresponding to Br?nsted slopes of one and zero, respectively.     

Exploration on regulating factors for proton transfer along hydrogen-bonded water chains.

Proton transfer along a single-file hydrogen-bonded water chain is elucidated with a special emphasis on the investigation of chain length, side water, and solvent effects, as well as the temperature and pressure dependences. The number of water molecules in the chain varies from one to nine. The proton can be transported to the acceptor fragment through the single-file hydrogen-bonded water wire which contains at most five water molecules. If the number of water molecule is more than five, the proton is trapped by the chain in the hydroxyl-centered H(7)O(3) (+) state. The farthest water molecule involved in the formation of H(7)O(3) (+) is the fifth one away from the donor fragment. These phenomena reappear in the molecular dynamics simulations. The energy of the system is reduced along with the proton conduction. The proton transfer mechanism can be altered by excess proton. The augmentation of the solvent dielectric constant weakens the stability of the system, but favors the proton transfer. NMR spin-spin coupling constants can be used as a criterion in judging whether the proton is transferred or not. The enhancement of temperature increases the thermal motion of the molecule, augments the internal energy of the system, and favors the proton transfer. The lengthening of the water wire increases the entropy of the system, concomitantly, the temperature dependence of the Gibbs free energy increases. The most favorable condition for the proton transfer along the H-bonded water wire is the four-water contained chain with side water attached near to the acceptor fragment in polar solvent under higher temperature.     

Energy transfer along a sequence controlled hybrid polymer

The present work provides an ideal model for intra‐chain energy transfer study in conjugated polymer through shielding the polymer backbone by using bulky polyhedral oligomeric silsesquioxanes (POSS). POSS provides a circumference shielding of the polymer backbone to prevent closed packing of the polymer chains, allowing the intra‐chain energy transfer dominating in large concentration range. Bi‐functional POSS (B‐POSS) is specially designed to separate donor (fluorene) and acceptor (benzothiadiazole) within the polymer chain. The dynamics of energy transfer in poly(fluorene‐POSS‐alt‐POSS‐benzothiodiazole) (PTBtTbOFl₃) is studied by steady state as well as time resolved fluorescence spectroscopy at different donor/acceptor ratios. Results reveal that POSS can effectively shield inter‐chains energy transfer of the polymers, suggesting it is an effective model for energy transfer study with less inter‐chains effects. PTBtTbOFl₃ works as a chemosensors is also reported in the detection of explosive derivatives. These results provide insights for optimizing nanostructured materials for use in optoelectronic devices. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1225–1233     

Studies on solid-state proton transfer along hydrogen bond of pyrazolone-ring

Evidence from the time-dependent UV-vis reflection spectra studies indicates the compound 1-phenyl-3-methyl-4-(4-methylbenzal)-5-pyrazolone 4-ethylthiosemicarbazone (PM4MBP-ETSC) undergoes a solid-state photochromism. The reaction rate constant was studied by the first-order kinetics curves. X-ray single crystal structural analysis shows that the pyrazolone-ring stabilizes in the keto form. The conclusion can be made that its photochromism in crystalline is associated with a photoinduced proton transfer reaction (inter- and intra-molecular hydrogen transfer) along hydrogen bond leading to a colored tautomer as the compound crystallizes in H-bonded supramolecular configuration.     

Transmission mode ion/ion proton transfer reactions in a linear ion trap

A new method is described for effecting ion/ion proton transfer reactions that involves storage of analyte ions while oppositely charged ions are transmitted through the stored ion population. In this approach, the products are captured and stored in the linear ion trap for subsequent mass analysis. Charge reduction of multiply charged protein ions is used as an example to illustrate the analytical usefulness of this method. In another variation of the transmission mode ion/ion reaction approach, two charge inversion experiments, implemented by passing analyte ions through a population of multiply charged reagent ions in a LIT, are also demonstrated. A pulsed dual ion source approach coupled with a hybrid triple quadrupole/linear ion trap instrument was used to demonstrate these two methods. The results for ion/ion reactions implemented using these so-called "transmission mode" experiments were comparable to those acquired using the more conventional mutual storage mode, both in terms of efficiency and information content of the spectra. An advantage of transmission mode experiments compared with mutual storage mode experiments is that they do not require any specialized measures to be taken to enable the simultaneous storage of oppositely charged ions.     

Histidinylated linear PEI: a new efficient non-toxic polymer for gene transfer

A series of linear polyethylenimine (lPEI) substituted with histidine residue (His-lPEI) was synthesized using the Michael reaction in order to provide new highly efficient vectors for gene therapy applications (up to 95% of transfected cells) with remarkable low cytotoxicity compared to lPEI-based polyplexes.     

Bimodal proton transfer in acid-base reactions in water

We investigate one of the fundamental reactions in solutions, the neutralization of an acid by a base. We use a photoacid, 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS; pyranine), which upon photoexcitation reacts with acetate under transfer of a deuteron (solvent: deuterated water). We analyze in detail the resulting bimodal reaction dynamics between the photoacid and the base, the first report on which was recently published. We have ascribed the bimodal proton-transfer dynamics to contributions from preformed hydrogen bonding complexes and from initially uncomplexed acid and base. We report on the observation of an additional (6 ps)(-1) contribution to the reaction rate constant. As before, we analyze the slower part of the reaction within the framework of the diffusion model and the fastest part by a static, sub-150 fs reaction rate. Adding the second static term considerably improves the overall modeling of the experimental results. It also allows to connect experimentally the diffusion controlled bimolecular reaction models as defined by Eigen-Weller and by Collins-Kimball. Our findings are in agreement with a three-stage mechanism for liquid phase intermolecular proton transfer: mutual diffusion of acid and base to form a "loose" encounter complex, followed by reorganization of the solvent shells and by "tightening" of the acid-base encounter complex. These rearrangements last a few picoseconds and enable a prompt proton transfer along the reaction coordinate, which occurs faster than our time resolution of 150 fs. Alternative models for the explanation of the slower "on-contact" reaction time of the loose encounter complex in terms of proton transmission through a von Grotthuss mechanism are also discussed.     

Solvent effect on the excited-state proton transfer of 7-hydroxyquinoline along a hydrogen-bonded ethanol dimer

We have studied the solvent effect on structures and potential energy surfaces along proton transfer in the ground and the excited states of 7-hydroxyquinoline interacting with an ethanol dimer using ab initio calculations. The proton transfer is forbidden in the ground state not only in vacuum but also in solvents of n-heptane, ethanol, and dimethyl sulfoxide. In the excited state, although the proton transfer is forbidden in vacuum, it is possible in solvent due to its greatly reduced barrier (～10 kcal mol(-1)) and highly stabilized product. It has also been found from the calculations that the proton-transfer barrier in the excited state decreases as the dielectric constant of a solvent increases. Our calculations are consistent with experimental results that the proton transfer does not take place in the ground state and that the excited-state proton-transfer rate increases as the solvent polarity increases. Our calculated absorption and emission properties are in excellent agreement with experimental results. Projection factors (reflecting geometrical change from the ground state to the excited state) and reorganization energies for several low frequency vibrations in connection with the excited-state proton transfer are discussed as well.     

Tunneling in the proton transfer between two water molecules

SCF CNDO calculations were performed for the species H₅O⁺₂ at several positions of the intervening proton and at interoxygen distances of 2.65, 2.70 and 2.75 Å. The energy profile was fitted to a potential energy function containing a quadratic term plus a gaussian. The eigenvalues and eigenvectors were obtained by using the variational method with the eigenfunctions of the parabolic potential as basis set. The results indicate that at 2.65 Å the top of the barrier is below the first energy level and that at 2.75 Å the first two energy levels are below the top of the barrier with the splitting of the symmetric-antisymmetric pair of 0.00132 au indicating that tunneling occurs at a frequency of 10¹⁴ reciprocal seconds.     

On the influence of semirigid environments on proton transfer along molecular chains

The dynamics of proton transfer along ammonia chains (chemical composition N(x)H(+)(3x+1), x=2, 4, and 6) in a constraining environment is investigated by ab initio molecular dynamics simulations. A carbon nanotube of defined length and diameter is used as an idealized constraining environment such that the ammonia chain is forced to maintain its quasilinear geometry. It is found that, although the energetics of proton transport shows considerable energetic barriers, proton translocation along the wire is possible at finite temperature for all chain lengths studied. The proton transport involves rotational reorientation of the proton-carrying ammonia molecule. High level ab initio calculations (MP2/aug-cc-pVTZ) yield barriers for internal rotation of 9.1 kcal/mol for NH(4) (+)-NH(3) and 11.7 kcal/mol for OH(3) (+)-OH(2), respectively. The infrared spectrum calculated from the dipole-dipole autocorrelation function shows distinct spectral features in the regions (2000-3000 cm(-1)) where the NHN proton transfer mode is expected to absorb. Assigning moderate opposite total charges between 0.002 and 0.2e to the carbon atoms at the end caps of the nanotube leads to a considerable speedup of the proton transfer.     

Excited-state hydrogen-atom transfer along solvent wires: water molecules stop the transfer

Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ.(NH3)n.(H2O)m, n + m = 3, using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is approximately 200 cm(-1) for 7HQ.(NH3)3, approximately 350 cm(-1) for both isomers of the 7HQ.(NH3)2.H2O cluster, and approximately 600 cm(-1) for 7HQ.NH3.(H2O)2 but increases to approximately 2000 cm(-1) for the pure 7HQ.(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ fluorescence.     

Excited-state double proton transfer of 7-azaindole in water nanopools

The excited-state proton-transfer dynamics of 7-azaindole occurring in the water nanopools of reverse micelles has been investigated by measuring time-resolved fluorescence spectra and kinetics, as well as static absorption and emission spectra, with varying water content and isotope. 7-Azaindole molecules are found to exist in the bound-water regions of reverse micelles. The rate constant and the kinetic isotope effect of proton transfer are smaller than those in bulk water although both increase with the size of the water nanopool. The retardation of proton transfer in the bound regions is attributed to the increased free energy of prerequisite solvation to form a cyclically H-bonded 1:1 7-azaindole/water complex.     

Proton-coupled electron transfer from tryptophan: a concerted mechanism with water as proton acceptor

The mechanism of proton-coupled electron transfer (PCET) from tyrosine in enzymes and synthetic model complexes is under intense discussion, in particular the pH dependence of the PCET rate with water as proton acceptor. Here we report on the intramolecular oxidation kinetics of tryptophan derivatives linked to [Ru(bpy)(3)](2+) units with water as proton acceptor, using laser flash-quench methods. It is shown that tryptophan oxidation can proceed not only via a stepwise electron-proton transfer (ETPT) mechanism that naturally shows a pH-independent rate, but also via another mechanism with a pH-dependent rate and higher kinetic isotope effect that is assigned to concerted electron-proton transfer (CEP). This is in contrast to current theoretical models, which predict that CEP from tryptophan with water as proton acceptor can never compete with ETPT because of the energetically unfavorable PT part (pK(a)(Trp(?)H(+)) = 4.7 ? pK(a)(H(3)O(+)) ≈ -1.5). The moderate pH dependence we observe for CEP cannot be explained by first-order reactions with OH(-) or the buffers and is similar to what has been demonstrated for intramolecular PCET in [Ru(bpy)(3)](3+)-tyrosine complexes (Sjo?din, M.; et al. J. Am. Chem. Soc.2000, 122, 3932. Irebo, T.; et al. J. Am. Chem. Soc.2007, 129, 15462). Our results suggest that CEP with water as the proton acceptor proves a general feature of amino acid oxidation, and provide further experimental support for understanding of the PCET process in detail.     

Intramolecular proton transfer

[structure: see text] Reversal of the normal kinetic protonation stereochemistry results as a consequence of intramolecular delivery.     

The mechanism of proton transfer between adjacent sites exposed to water

The surface of a protein, or a membrane, is spotted with a multitude of proton-binding sites, some of which are only a few angstroms apart. When a proton is released from one site, it propagates through the water by a random walk under the bias of the local electrostatic potential determined by the distribution of the charges on the protein. Some of the released protons disperse into the bulk, but during the first few nanoseconds, the released protons can be trapped by encounter with nearby acceptor sites. This process resembles a scenario which corresponds with the time-dependent Debye-Smoluchowski equation. In the present study, we investigated the mechanism of proton transfer between sites that are only a few angstroms apart, using as a model the proton exchange between sites on a small molecule, fluorescein, having two, spectrally distinguishable, proton-binding sites. The first site is the oxyanion on the chromophore ring structure. The second site is the carboxylate moiety on the benzene ring of the molecule. Through our experiments, we were able to reconstruct the state of protonation at each site and the velocity of proton transfer between them. The fluorescein was protonated by a few nanosecond long proton pulse under specific conditions that ensured that the dye molecules would be protonated only by a single proton. The dynamics of the protonation of the chromophore were measured under varying initial conditions (temperature, ionic strength, and different solvents (H(2)O or D(2)O)), and the velocity of the proton transfer between the two sites was extracted from the overall global analysis of the signals. The dynamics of the proton transfer between the two proton-binding sites of the fluorescein indicated that the efficiency of the site-to-site proton transfer is very sensitive to the presence of the screening electrolyte and has a very high kinetic isotope effect (KIE = 55). These two parameters clearly distinguish the mechanism from proton diffusion in bulk water. The activation energy of the reaction (E(a) = 11 kcal mol(-1)) is also significantly higher than the activation energy for proton dissociation in bulk water (E(a) approximately 2.5 kcal mol(-1)). These observations are discussed with respect to the effect of the solute on the water molecules located within the solvation layer.     

Intermolecular proton transfer as a multistage process

An expression for the rate constant of a cyclic process of pseudounimolecular type with an arbitrary number of stages is used to show that intermolecular proton transitions may be considered as a sequential multistage process.