H-bonded Complexes Research Articles

Influence of intramolecular secondary sphere hydrogen-bonding interactions on cytochrome c oxidase inspired low-spin heme-peroxo-copper complexes.

Dioxygen reduction by heme-copper oxidases is a critical biochemical process, wherein hydrogen bonding is hypothesized to participate in the critical step involving the active-site reductive cleavage of the O-O bond. Sixteen novel synthetic heme-(μ-O2 2-)-Cu(XTMPA) complexes, whose design is inspired by the cytochrome c oxidase active site structure, were generated in an attempt to form the first intramolecular H-bonded complexes. Derivatives of the "parent" ligand (XTMPA, TMPA = (tris((2-pyridyl)methyl)amine)) possessing one or two amine pendants preferentially form an H-bond with the copper-bound O-atom of the peroxide bridge. This is evidenced by a characteristic blue shift in the ligand-to-metal charge transfer (LMCT) bands observed in UV-vis spectroscopy (consistent with lowering of the peroxo π* relative to the iron orbitals) and a weakening of the O-O bond determined by resonance Raman spectroscopy (rR), with support from Density Functional Theory (DFT) calculations. Remarkably, with the TMPA-based infrastructure (versus similar heme-peroxo-copper complexes with different copper ligands), the typically undetected Cu-O stretch for these complexes was observed via rR, affording critical insights into the nature of the O-O peroxo core for the complexes studied. While amido functionalities have been shown to have greater H-bonding capabilities than their amino counterparts, in these heme-peroxo-copper complexes amido substituents distort the local geometry such that H-bonding with the peroxo core only imparts a weak electronic effect; optimal H-bonding interactions are observed by employing two amino groups on the copper ligand. The amino-substituted systems presented in this work reveal a key orientational anisotropy in H-bonding to the peroxo core for activating the O-O bond, offering critical insights into effective O-O cleavage chemistry. These findings indirectly support computational and protein structural studies suggesting the presence of an interstitial H-bonding water molecule in the CcO active site, which is critical for the desired reactivity. The results are evaluated with appropriate controls and discussed with respect to potential O2-reduction capabilities.

Stability of doubly and triply H-bonded complexes governed by acidity-basicity relationships.

We used our recently proposed acidity-basicity interplay (ABI) model (Chem. Sci., 2018, 9, 4402) and the Jorgensen secondary interactions hypothesis (JSIH) to rationalise the experimentally observed trends in the formation constants of doubly and triply H-bonded systems with -NHO[double bond, length as m-dash]C- and -NHN- interactions. Unlike the JSIH, the ABI interpretation can explain the trends in the complexation of amide/imide homo- and heterodimers as well as ADA-DAD clusters. We found that the strongest H-bonds play a very important role, a condition which offers an alternative to the well established JSIH to modulate the stability of these relevant systems.

Intermolecular dissociation energies of hydrogen-bonded 1-naphthol complexes.

We have measured the intermolecular dissociation energies D 0 of supersonically cooled 1-naphthol (1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. The naphthol OH forms non-classical H-bonds with the aromatic π-electrons of furan, thiophene, and 2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-state D 0(S 0) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran, and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the face isomers bound by dispersion; experimentally, we only observe edge isomers. We compare the calculated and experimental D 0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent, (S). By contrast, the D 0 values of the classically H-bonded complexes are larger, increase more rapidly at low (S), but saturate for large (S). The calculated D 0(S 0) values for the cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol and those of thiophene and 2,5-dimethylfuran are ∼3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D 0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD = 1.7 kJ/mol), and the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively.

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H-Bonding Liquid Observed by Transient 2D-IR Spectroscopy.

Intermolecular H-bonding dynamics around a photoexcited quadrupolar dye is directly observed using transient 2D-IR spectroscopy. Upon solvent-induced symmetry breaking, the H-bond accepting abilities of the two nitrile end-groups change drastically, and in extremely protic (superprotic) solvents, a tight H-bond complex forms at one end. The time evolution of the 2D C≡N lineshape in methanol points to rapid, 2-3 ps, spectral diffusion due to fluctuations of the H-bonding network. Similar behavior is observed in a superprotic solvent shortly after photoexcitation of the dye. However, at later times, the completely inhomogeneous band does not exhibit spectral diffusion for at least 5 ps, pointing to a glass-like environment around one side of the dye. About half of the excited dyes show this behavior attributed to the tight H-bond complex, whereas the others are loosely bound. A weak cross peak indicates partial exchange between these excited state subpopulations.

Computational exploration of the effect of molecular medium on the tautomerization of azo prodrug of 5-aminosalicylic acid

The characterization of potential tautomerization of pharmaceutical materials has significant importance. Sulfasalazine (SSZ) is a prodrug that bears 5-aminosalicylic acid and pyridylamino-sulfonyl-phenyl moieties bridged by the azo group. SSZ may be present in various tautomeric forms, where dual tautomerization may occur; namely, amide↔imide and azo↔hydrazone tautomerization through the pyridylamino-sulfonyl-phenyl and salicylic acid moieties, respectively. In this report, we describe the prospects of the effect of molecular medium on the tautomerization of SSZ using selected computational methods. Two approaches were deliberated; namely, the explicit intermolecular hydrogen bonding (HB) through complexation with dimethylformamide (DMF) and implicit solvent effects. Using the DFT/ωB97XD/6-31G+(d) calculations, we conducted geometry optimization calculations of all possible tautomers of SSZ and their corresponding HB complexes with DMF with stoichiometric ratios (SSZ:DMF) of 1:1 and 1:2. The stability of the SSZ tautomeric forms and their corresponding H-Bonded DMF complexes were examined employing the ADMP molecular dynamics approach. Obtained results demonstrate that the amide and azo tautomers are favored over imide and hydrazone in the gas phase with Etaut of 8.3 and 12.8 kcal/mol, respectively. However, these preferences were significantly affected by the implicit solvation effect of water with ΔE of 0.5 and 3.1 kcal mol−1, respectively. Obtained results demonstrate as well that DMF can bind to various sites of SSZ tautomers through intermolecular HBs with length in the range of 1.76–2.39 Å. This in turn demonstrates that intramolecular and intermolecular HB could not only play a significant role in directing the favored tautomeric forms of SSZ, but also distorting the planarity of the molecular comprising the azo and phenyl groups of the SSZ molecule. The ADMP-MD results verified that these complexes and the corresponding intra- and intermolecular HBs are stable over a timeframe of 100 femtosecond. The NBO analysis of the optimized geometries revealed that SSZ:DMF complexes can be stabilized by strong intermolecular HB, as indicated by the second perturbation energy of interaction (E(2)intr). Moreover, these results showed that the intermolecular HB of SSZ:DMF complexes has a notable effect on reducing the strength of intramolecular HB of certain tautomeric forms of SSZ and hence promotes the preference of SSZ toward a specific tautomeric form.

Stereo-preference of camphor for H-bonding with phenol, methanol and chloroform: A combined matrix isolation IR spectroscopic and quantum chemical investigation.

Camphor is known to be held in the substrate pocket of cytochrome P450cam enzyme via H-bond with a tyrosine residue of the enzyme in a unique orientation. This structural exclusivity results in regio- and stereo-specific hydroxylation of camphor by the enzyme. We have carried out a combined IR spectroscopic and quantum chemical investigation to shed light on the factors influencing the conformational exclusivity of 1R-(+)-camphor in the substrate pocket of Cytochrome P450cam, and to determine whether the selectivity is an inherent property of the substrate itself, or is imposed by the enzyme. For this purpose, complexes of camphor have been studied with three H-bond donors namely phenol, methanol and chloroform. Each of the three donors was found to form stable complexes with two distinct conformers; the one mimicking the conformation in enzyme substrate pocket was found to be more stable of the two, for all three donors. Experimentally, both conformers of the H-bonded complexes were identified separately for phenol and methanol in an argon matrix at 8 K, but not for chloroform due to very small energy barrier for interconversion of the two conformers. In room temperature solution phase spectra of camphor with all three donors, the differences in spectral attributes between the two isomeric H-bonded complexes were lost due to thermal motions.

Calculation of vibrational spectroscopic and geometrical characteristics of the [F(HF)2]- and [F(DF)2]- complexes using the second-order vibrational perturbation theory and a 6D variational method.

Vibrational spectroscopic and average geometrical parameters of the strong H-bonded complexes [F(HF)2]- and [F(DF)2]- are determined for the first time from nine-dimensional (9D) perturbative and 6D variational calculations. The frequencies and intensities for all fundamental and some combination and overtone transitions obtained by the method of second-order vibrational perturbation theory (VPT2) are reported. A two-fold decrease in the H-F (D-F) stretching band frequency and a more than ten-fold increase in the intensity of this band upon complexation are predicted. The theoretical frequencies for both isolated isotopologues are in satisfactory agreement (to better than 70 cm-1) with the scarce experimental data obtained in condensed phases. The main purpose of variational calculations is to analyze the intermode anharmonic coupling and the changes in the geometrical parameters upon vibrational excitation and H/D isotopic substitution. The equilibrium nuclear configuration and the 2D potential energy surface (PES) of [F(HF)2]- for H-F stretches are calculated in the MP2/6-311++G(3df,3pd), CCSD(T)/6-311++G(3df,3pd), CCSD(T)/aug-cc-pVTZ, and CCSD(T)/d-aug-cc-pVTZ approximations with the basis set superposition error taken into account. Anharmonic vibrational problems are solved by the variational method for 2D, 4D, and 6D systems of H-bond and H-F (D-F) stretches and in-plane bends. The VPT2 calculations and calculations of the PESs for 4D and 6D systems are performed in the MP2/6-311++G(3df,3pd) approximation. Comparison of variational anharmonic solutions for different vibrational subsystems demonstrates the influence of intermode anharmonic coupling on the mixing of wave functions and spectroscopic and geometrical characteristics. The inverse Ubbelohde effect is predicted and substantiated.

Ammonia and phosphine complexes with proton donors. Hydrogen bonding from the backside of the N(P) lone pair

Quantum chemical calculations at the MP2/aug-cc-pVTZ level of theory were carried out to study the molecular Y…HX (Y = N, P; X = O, Cl, F) complexes formed by ammonia and phosphine with H2O, HCl and HF molecules with the orientation of the XH group to the backside of the N(P) lone pair (referred to as anti-complexes). Various methods used for analysis identify the Y…HX intermolecular interaction in the anti-complexes as hydrogen bonding. The NBO analysis shows that the stabilization of the ammonia and phosphine complexes is mainly determined by the nY → σ∗XH interaction between the acceptor lone pair and anti-σ bond XH of the proton donor. The values of topological parameters at the bond critical point for the H…Y contact are typical for neutral H-bonded molecular complexes; while the bridging hydrogen and acceptor atoms demonstrate mutual penetration, and all criteria concerning the integrated properties of the H atom are satisfied. The binding energy in complexes with the anti-orientation of monomers calculated with the BSSE correction was found to vary in the range of 1–3 kcal/mol. The anti-complexes, like typical H-bonded syn-complexes, show lengthening of the covalent XH bond, red-shift in the XH stretching frequency in IR spectra, and downfield chemical shift of the bridging hydrogen atom. The inversion of the NH3(PH3) group leads to transition of the molecular system between the anti- and syn-configurations corresponding to minima on the potential energy surface.

Comparison of Redox-Dependent H-Bonding in Simple, Electroactive Ureas Containing Either Ferrocene or Phenylenediamine Redox Couples – Similar Results with Different Mechanisms

An important goal in supramolecular chemistry is the design of stimuli-responsive supramolecular systems in which binding between components can be controlled by external signals. Supramolecular chemistry research in our group focuses in particular on using electron transfer to control the strength of H-Bonding interactions. In this presentation the redox-dependent H-bonding behavior of two simple phenylureas containing either a phenylenediamine redox couple, Figure 1a, or a ferrocene redox couple, FcUHH, Figure 1b, will be compared. The urea functional group provides two good H-donors (a DD array) which can be paired with two good H-acceptors (an AA array) in the PZD guest. Oxidation of either the ferrocene or phenylenediamine would be expected to increase the acidity of the attached urea NH, increasing the H-Bonding strength to PZD. Experimentally, this should result in a negative shift in the E1/2 of the cyclic voltammetric wave upon addition of PZD, with little change in wave shape or height. This behavior is indeed observed for both ureas in 0.1 M NBu4PF6/CH2Cl2. Plots of the ΔE1/2 of FcUHH vs [PZD] are fit well using a 1:1 binding isotherm, with initial results giving estimates of the binding constants to the oxidized and reduced forms as Kred = 80 M−1 and Kox = 500 M−1. The same type of experiments have been run with the phenylenediamine urea, UHH. The data also fits well to a simple 1:1 binding isotherm and gives similar values for the binding constants, Kred = 30 M−1 and Kox = 700 M−1. This is interesting because from our previous work1 we know that the electrochemistry is quite a bit more complex for UHH than FcUHH. In the former case the reversible, apparent 1 e− cyclic voltammetric (CV) wave observed in CH2Cl2 actually corresponds to 2 e− oxidation of half of the UHH’s to a quinoidal cation accompanied by proton transfer and electro-deactivation of the other half, Figure 1a. Given this, it is somewhat surprising that any potential shift is observed upon addition of PZD, since the oxidation product can only form a single H-bond with PZD. However, the key realization was that the other product, the protonated urea, can still form a 2 H-bond complex, and that should be stronger than the one formed with the starting urea due to the increased acidity of one of the urea NH’s. From this perspective, it is not surprising that we see such similar behavior between FcUHH and UHH. In the former, oxidation creates a +1 charge on one side of the urea (ferrocenium), Figure 1b, and, in the latter, proton transfer creates +1 charge, Figure 1a. This conclusion suggests an entirely new strategy to perturb H-bonding through electron transfer. Rather than doing this directly through oxidation/reduction introduced charges, it can also be done indirectly through oxidation-reduction induced proton transfer! 1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society 2013, 135, 18930-18941. Figure 1

Novel Isophthalohydrazide-cDB24C8 cryptand derivative for the selective recognition of fluoride ion: An experimental and DFT study

A highly selective and sensitive novel Isophthalohydrazide-cDB24C8 cryptand derivative was developed for fluoride recognition at a very low concentration of 2.31 × 10−10 M. The binding was established by UV–Vis, fluorescence and 1H NMR titration. The receptor formed very strong H-bonded complex with fluoride, furnished a sharp new UV–Vis absorption peak at 280 nm which was also supported by the DFT-study. The fluorescence emission spectra showed large quenching up to 79.13% upon addition of fluoride.

Study of the H-bond interaction on binary mixtures of nitromethane with primary alcohols at different temperatures using dielectric parameters

A dielectric spectroscopic study has been carried out on binary mixtures of polar nitromethane (NM) with primary alcohols: ethanol (ETH), 1-propanol (1-PRO) and 1-butanol (1-BUT) at different temperatures. The complex dielectric spectra of pure and binary mixtures were described by Debye model. The dielectric constant (e0), relaxation time (τ), and dielectric constant at an optical frequency (e∞) of binary mixtures were determined at various temperatures. The computed excess dielectric parameters were used to interpret the intermolecular interaction between the unlike dipoles within the binary mixture system. From the experimental analysis, it is confirmed that, the mixture molecules form H-bond complex structure and dipoles are aligned in anti-parallel direction. Among all the binary systems, strong H-bond interaction can be seen in NM + 1-PRO mixture system. Rapid decrement of relaxation time for NM + 1-BUT binary system results higher amount of structural changes symmetry breaking effect. The formation of hydrogen bonding interaction between oxygen atom of nitro groups in NM and hydrogen atom of alcohols in binary mixture systems was confirmed by FTIR analysis.

Blue- and Red-Shifting Hydrogen Bonding: A Gas Phase FTIR and Ab Initio Study of RR'CO···DCCl3 and RR'S···DCCl3 Complexes.

Blue-shifting H-bonded (C-D···O) complexes between CDCl3 and CH3HCO, (CH3)2CO, and C2H5(CH3)CO, and red-shifting H-bonded (C-D···S) complexes between CDCl3 with (CH3)2S and (C2H5)2S have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. With increasing partial pressure of the components, a new band appears in the C-D stretching region of the vibrational spectra. The intensity of this band decreases with an increase in temperature at constant pressure, which provides the basis for identification of the H-bonded bands in the spectrum. The C-D stretching frequency of CDCl3 is blue-shifted by +7.1, +4, and +3.2 cm-1 upon complexation with CH3HCO, (CH3)2CO, and C2H5(CH3)CO, respectively, and red-shifted by -14 and -19.2 cm-1 upon complexation with (CH3)2S and (C2H5)2S, respectively. By using quantum chemical calculations at the MP2/6-311++G** level, we predict the geometry, electronic structural parameters, binding energy, and spectral shift of H-bonded complexes between CDCl3 and two series of compounds named RCOR' (H2CO, CH3HCO, (CH3)2CO, and C2H5(CH3)CO) and RSR' (H2S, CH3HS, (CH3)2S, and (C2H5)2S) series. The calculated and observed spectral shifts follow the same trends. With an increase in basicity of the H-bond acceptor, the C-D bond length increases, force constant decreases, and the frequency shifts to the red from the blue. The potential energy scans of the above complexes are done, which show that electrostatic attraction between electropositive D and electron-rich O/S causes bond elongation and red shift, and the electronic and nuclear repulsions lead to bond contraction and blue shifts. The dominance of the two opposing forces at the equilibrium geometry of the complex determines the nature of the shift, which changes both in magnitude and in direction with the basicity of the hydrogen-bond acceptor.

The Role of Hydrogen Bonding in Proton-Coupled Electron Transfer. It Does Not Have to be Concerted Pcet: The Case of Phenylenediamines and Pyridines in Acetonitrile

Proton transfer often accompanies electron transfer in organic redox couples due to the large changes in acidity or basicity that result from oxidation or reduction. Classically, the overall reaction is thought to occur in two separate steps, either proton transfer followed by electron transfer, PT-ET, or electron transfer followed by proton transfer, ET-PT. More recently it has become abundantly apparent that a third option is also possible in which the proton and electron move together in a single kinetic step. This is concerted proton-electron transfer, CPET, and is generally believed to occur within a H-bonded intermediate. It is straightforward to show that E° of this step will have a value in between the E° values corresponding to the oxidation of the fully protonated and fully deprotonated forms. Experimental evidence for CPET is typically the observance of a significant deuterium isotope effect for the apparent one electron transfer. However, it is important to note that the electron-proton transfer does not have to be concerted within the H-bond complex. It could also occur step-wise. If slow electron transfer occurred followed by rapid proton transfer within the H-bond complex, no significant deuterium isotope effect would be expected, yet, at the same time, the overall reversibility of the electron-proton transfer would still be aided by providing a mechanistic pathway through the H-bond complex with its intermediate E°. We believe that the second oxidation of a simple phenylenediamine, H2PD, in the presence of pyridines in acetonitrile provides a nice example of the significant role that step-wise electron-proton transfer within a H-bond intermediate can play. Without added base, the second oxidation corresponds to the reversible one electron oxidation of the radical cation, H2PD+, to the quinoidal dication, H2PD2+. CV data in the presence of one equivalent of pyridines of different basicity result in a significant negative shift in the E1/2 of the second CV wave with no loss in reversibility. Continued addition of the pyridine leads to smaller incremental shifts with still no change in reversibility. Analysis of the observed E1/2’s with one equivalent pyridine as a function of pyridine basicity indicates that, even with the weakest base, proton transfer to the pyridine occurs. This interpretation is supported by spectroelectrochemical data. Thus, the overall reaction occurring in the second CV wave is H2PD+ + B = HPD+ + HB+ + e-, where B is the pyridine base and HB+ is its conjugate acid. The shifts in E1/2 upon further additions of pyridine are nicely predicted by the Nernst equation with this as the overall reaction. However, attempts to simulate the CV data with just PT and ET steps fail at the higher concentrations of pyridine. This is because the rate of the ET step becomes exceedingly slow as the base concentration increases and the thermodynamic potential becomes increasingly removed from the E° of the actual ET step. This issue is simply solved by including in the mechanism the possibility of electron transfer occurring through the H-bond intermediate. This simple change leads to good fits to the experimental data, providing solid support for such a H-bonding intermediate being involved. However, a comparison of experiments with 10:1 cyanopyridine: H2PD run in 2% CH3OH to those run in 2% CD3OD show no significant difference in the ΔEp of the second oxidation, providing no evidence for the oxidation involving CPET. Thus, it appears that ET-PT within the H-bond complex still greatly aids the reversibility of the reaction even without CPET.

Evaluating the Roles of Proton Transfer and H-Bonding in the Electron Transfer Reactions of Organic Redox Couples in Non-Aqueous Solvents: Oxidation of Phenylenediamines in the Presence of Pyridine Bases in Acetonitrile

Oxidation or reduction of organic redox couples typically leads to large changes in acidity or basicity, withthe result that proton transfer often accompanies electron transfer, particularly in aqueous solution. In lesspolar organic solvents, H-bonding also can play an important role. While it is generally appreciated thatproton transfer will have a greater effect on the overall reaction than H-bonding, it is not always straightforward to distinguish between the two, and, despite a considerable amount of research, a complete,quantitative understanding of the relative roles that the two play in the voltammetry that is observed uponaddition of acids or bases to organic redox couples in non-aqueous solution remains elusive. This study focuses on the electrochemistry of p-tetramethylphenylenediamine, H 2 PD, in acetonitrile in thepresence of pyridine bases. In acetonitrile, this compound undergoes a reversible one electron oxidationto the radical cation, H 2 PD + , followed by a second reversible oxidation to the strongly acidic quinoidaldication, H 2 PD 2+ . With the addition of 1 equivalent of the pyridine base, no significant change in the firstoxidation is observed, but there is a large negative shift in the E 1/2 of the second oxidation with no loss inreversibility. Subsequent additions show a smaller, incremental negative shift, still with no loss inreversibility. We believe that this behavior signals proton transfer is occurring between the pyridine, pyr,and the H 2 PD 2+ , so that the overall reaction occurring in the second oxidation corresponds to H 2 PD + + pyr= HPD + + Hpyr + + e-. In this case, the observed E 1/2 should depend linearly on the pK a of the Hpyr + with aslope of −60 mV/pH unit. To test this hypothesis, the voltammetry of H 2 PD was studied with differentpyridines that cover a range of pK a values. A plot of E 1/2 of the wave observed with 1 eq pyridine vs. pKa isindeed linear with close to the predicted slope. Furthermore, it is found that the continued shift in E 1/2 ofthe second oxidation with increasing concentrations of pyridine is well-accounted for simply by applyingthe Nernst equation to the overall reaction. However, while proton transfer can explain the potentials ofthe CV waves in the presence of added pyridine, simulations of the voltammetry show that proton transferby itself cannot explain the observed reversibility of the second oxidation wave in the presence ofincreasing amounts of added pyridine. This is where H-bonding can play a role. By including H-bondingsteps, and allowing electron transfer and proton transfer to occur through the H-bond complex formedbetween H 2 PD + /pyr and HPD + /Hpyr + , the simulations nicely explain both the observed potential shifts andthe reversibility of the waves. The next question is whether the electron and proton transfer within the H-bonding complex is concertedor step wise. To test this, CV’s were run with 10:1 cyanopyridine:H2PD in 2% D 4 -methanol/acetonitrile or2% H 4 -methanol/acetonitrile. If the second oxidation involved concerted electron-proton transfer, asignificant deuterium isotope effect would be expected, causing a larger ΔE p in the deuterated solventresulting from slower electron transfer. However, no significant difference was observed. Therefore,there is no evidence that the electron-proton transfer is concerted. This result does not by any meansrule out the hypothesis that the proton-electron transfer is occurring within the H-bond complex, it merelyindicates that it is likely that the proton and electron transfer occur in a step-wise fashion within the H-bond complex, and that facilitation of proton-electron transfer by H-bonding can happen even if theprocess is not concerted.

Effects of natural organic matter with different properties on levofloxacin adsorption to goethite: Experiments and modeling

Adsorption of levofloxacin (LEV) to goethite in the pH range of 3–10, and in the absence or presence of natural organic matter (NOM) represented by nine types of humic acid (HA) and fulvic acid (FA), was studied using batch experiments. The adsorption of LEV to goethite was weak and showed a maximum around pH 5.8. Adding NOM to goethite strongly increased LEV adsorption to goethite, but hardly affected its pH dependency. The adsorption envelopes were well fitted to a linear additive model, in which LEV adsorption to goethite was simulated with the Charge Distribution Multi-Site Complexation (CD-MUSIC) model, and LEV adsorption to NOM was simulated with the Langmuir model. The fitted affinity constants (log K) for LEV adsorption to NOM were significantly and positively correlated with the SUVA (specific ultraviolet absorbance at 280 nm) values of NOM, and negatively correlated with E2/E3 (absorbance ratio at 250 nm and 365 nm) values, carboxyl contents, and the polarity of NOM. The results indicated that aromatic moieties of NOM play a key role in the interactions between LEV and NOM, and hydrophobic interactions and π-π interactions were the major mechanisms for LEV adsorption to NOM, whereas H-bond or surface complexation might not play an important role. Results show that both the concentrations and properties of NOM have a significant effect on the distribution or treatment of antibiotics in soils and waters, which will eventually affect the influence of antibiotics on microorganisms in the environmental systems.

Chloride adsorption on Fe- and Al-(hydr)oxide: estimation of Gibbs free energies

In this study, we used chemical quantum methods to analysis the adsorption of chloride on Al and Fe-(hydr)oxide clusters. Inner and outer sphere complexes were the generating complexes during the adsorption process on variably charged Al- and Fe-(hydr)oxide clusters. For the chloride adsorption on Al-(hydr)oxide, the outer sphere complexes—H-bonded—were favored for all clusters, while the adsorption modes as inner sphere complexes—BB or MM—were not favored. It was found, that the H-bonded complex on neutral clusters was the most thermodynamically favored with an adsorption energy of − 63.4 kJ/mol. For iron clusters, thermodynamic favorability was observed for both outer (− 70.5 kJ/mol) and inner monodentate (− 65.8 kJ/mol) sphere complexes. These theoretical results indicated that the thermodynamic favorability of chloride adsorption on Fe and Al-(hydr)oxide was directly related to positive surface charge.

New Insights on Hydrogen-Bond-Induced Fluorescence Quenching Mechanism of C102-Phenol Complex via Proton Coupled Electron Transfer.

The H-bonded coumarin 102 (C102)-phenol complex has been a model system usually used to understand the influence of H-bonding on photophysical processes. Zhao and Han first showed that significant H-bond strengthening occurs in the excited state and proposed the possibility of fluorescence quenching in the complex via internal conversion from a locally excited (LE) state to a low-lying charge transfer (CT) state. Later, we experimentally confirmed fluorescence quenching of C102-phenol complex in a nonpolar solvent (cyclohexane). However, we also found that the existence of the low-lying CT state is ambiguous. Here, we proposed an alternative mechanism for the fluorescence quenching in the H-bonded complex. For this, we evaluate the excited state potential energy surface considering complete H atom-transfer from phenol to C102 along the H-bonding coordinate. Surprisingly, we observed two distinct minima separated by a low-energy barrier. One minimum corresponds to the complex with shortening of H-bond consistent with that of Zhao and Han. On the other hand, the second minimum, which has even lower energy than the first minimum, is likely to be arising from the proton-coupled electron transfer (PCET) process. The nature of the lowest excited state alters from LE to CT type at the second minimum, which may account for the fluorescence quenching phenomena in the system.

Solvation of Methyl Lactate in Water: Molecular Dynamics Studies.

Methyl lactate (ML), a chiral α-hydroxy ester, has been probed widely to understand the competition between two types of intramolecular H-bonds in solvents of different polarities. Recent experimental and high-level quantum chemical studies have revealed the predominant existence of ML-water insertion complexes over addition complexes in aqueous solution. Although the stability of monohydrate insertion conformer was studied accurately, ab initio quantum chemical calculations failed to predict the most stable dihydrate conformer in analogy with the experimental spectroscopic search. Atomistic molecular dynamics simulations of aqueous solution of methyl lactate predict that the population and lifetime of different H-bonded ML-water addition complexes are dictated by their interaction energies. Although the population of dihydrate insertion complexes is higher than that of the monohydrate complexes, the lifetime of the former is smaller than the latter, which is in good agreement with the experimental result. The nature of intramolecular H-bonds within a methyl lactate molecule in aqueous solution is opposite to that in the gas phase due to the solvation process in water by intermolecular H-bonding interactions.

H-bonding tuned phase transitions of a strong microphase-separated polydimethylsiloxane-b-poly(2-vinylpyridine) block copolymer

The phase behaviors of a polydimethylsiloxane-b-poly(2-vinylpyridine) block copolymer (PDMS-b-P2VP, DV) tuned by the supramolecular self-assembly approach with the 1-pyrenebutyric acid (PBA) as additive were investigated. The PDMS-b-P2VP block copolymer (D10kV12.5k, the molecular weight of the PDMS and P2VP were 10000 and 12500 g mol−1, respectively) with volume fraction of P2VP (fP2VP) 52% exhibited hexagonally perforated layer (HPL) morphology after dried from the solution, and the nanostructure transformed to gyroid structure after thermal annealing above 140 oC, which were indicated by the small-angle X-ray diffraction (SAXS) and transmission electron microscopy (TEM) results. Through solution blending method, a series of H-bonding supramolecular complexes of PDMS-b-P2VP with PBA were fabricated. The Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) experiments demonstrated that the PBA molecules uniformly interacted with P2VP through H-bonding and the glass transition temperature of P2VP(PBA)x decreased with the increase of the mole ratio (x) of PBA/2VP. Due to the increase of the volume fraction of P2VP(PBA)x and that of the stretching extent of the P2VP backbone as the x value increased, the microphase-separated nanostructures transformed from HPL to hexagonally packed cylinder (HEX), then to body centered cubic (BCC) structure and then to disordered spheres morphologies. When the PDMS-b-P2VP(PBA)x complexes were heated above 120 oC, the H-bonding between the P2VP and PBA was broken and the PBA became compatible within both PDMS and P2VP domains and thus the nanostructure of the PDMS-b-P2VP(PBA)x complexes transformed accordingly. Therefore, a variety of the nanostructures and structural transformations were induced by the H-bonding formation and disassociation based on one block copolymer.

Fine Tuning of Microscopic Properties in Two-Component Zwitterionic-Anionic Lipid Bilayers: Determinant Role of H-Bonding

Structure, dynamics, and functioning of hydrated lipid bilayers - model cell membranes - are governed by a thin balance of intermolecular interactions between constituents of these systems. Besides the hydrophobic effect, which determines the overall bilayer skeleton, important contribution is made by H-bonds between lipids, water, and ions. This determines crucial phenomena in cell membranes: dynamic clustering, hydration, fine tuning of microscopic physico-chemical properties, which permit fast adaptation of membranes to external agents (e.g., proteins). Characteristics of H-bonds (strength, spatial location, etc.) dramatically depend on local polarity properties of water-lipid environment. Here, we calculated free energies of H-bonded complexes between lipids and water in explicit solvents of different polarity (water, methanol, chloroform) mimicking membrane environment at different depth. The strongest H-bonds were observed in nonpolar environment, although the overall bilayer organization imposes serious limitations on the distribution of various types of H-bonds over hydrophobic/hydrophilic regions (corresponding to dielectric media with low and high permeability). This creates a delicate balance, which determines a unique H-bonding pattern for each particular lipid bilayer. This was confirmed via atomistic molecular dynamics (MD) of several hydrated lipid bilayers. Understanding of the factors regulating H-bonding propensities in such systems is indispensable for rational design of new membrane-like materials with predefined properties. One example - an artificial lipid with engineered hydroxyl group - is studied via MD simulations. It is shown that such lipids can induce significant changes of key characteristics of model membranes. This opens new avenues in goal-oriented design of artificial membranes with engineered properties. Acknowledgements: Russian Science Foundation (14-50-00131), Russian Foundation for Basic Research (16-04-00578), RAS MCB Program, Supercomputer Center “Polytechnical” (St. Petersburg Polytechnic University), Joint Supercomputer Center of RAS (Moscow).