Changes In Hydrogen Bond Strength Research Articles

Substituent derivatives of benzothiazole-based fluorescence probes for hydrazine with conspicuous luminescence properties: A theoretical study

In the present work, four probe molecules for detecting hydrazine have been designed based on the 2-(4-Acetoxy-3-benzothiazole-2-yl-phenyl)-4-methyl-thiazole- 5-carboxylic acid ethyl ester (HP1) to investigate the influence of the amino and cyano groups on the excited-state intramolecular proton transfer (ESIPT) behavior and photophysical properties. The changes in hydrogen bond strength indicate that the intramolecular hydrogen bond of all probe products is enhanced upon photoexcitation. Frontier molecular orbitals (FMOs) and natural bond orbital (NBO) reveal the driving force of ESIPT. In addition, the potential energy curves and transition state theory explain the reason for the single fluorescence phenomenon in the experiment. The simulated absorption and fluorescence spectra of HP1 and its product (HPP1) are completely consistent with the experimental results, which also verify the viewpoint. Meanwhile the cyano derivative HPP4 exhibits a larger Stokes-shift (201 nm) than that of HPP1 (145 nm) and has the same low energy barrier as HPP1. These excellent properties allow HPP4 to be a fluorescent probe with superior performance than the original molecule. In conclusion, this work can provide a theoretical basis for the design and synthesis of more sensitive fluorescent probes for the detection of hydrazine.

Mix-and-inject XFEL crystallography reveals gated conformational dynamics during enzyme catalysis

How changes in enzyme structure and dynamics facilitate passage along the reaction coordinate is a fundamental unanswered question. Here, we use time-resolved mix-and-inject serial crystallography (MISC) at an X-ray free electron laser (XFEL), ambient-temperature X-ray crystallography, computer simulations, and enzyme kinetics to characterize how covalent catalysis modulates isocyanide hydratase (ICH) conformational dynamics throughout its catalytic cycle. We visualize this previously hypothetical reaction mechanism, directly observing formation of a thioimidate covalent intermediate in ICH microcrystals during catalysis. ICH exhibits a concerted helical displacement upon active-site cysteine modification that is gated by changes in hydrogen bond strength between the cysteine thiolate and the backbone amide of the highly strained Ile152 residue. These catalysis-activated motions permit water entry into the ICH active site for intermediate hydrolysis. Mutations at a Gly residue (Gly150) that modulate helical mobility reduce ICH catalytic turnover and alter its pre-steady-state kinetic behavior, establishing that helical mobility is important for ICH catalytic efficiency. These results demonstrate that MISC can capture otherwise elusive aspects of enzyme mechanism and dynamics in microcrystalline samples, resolving long-standing questions about the connection between nonequilibrium protein motions and enzyme catalysis.

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d(6) metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ~750 ns in dichloromethane to ~3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ~50 to ~2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET.

A Single‐Molecule Perspective on the Role of Solvent Hydrogen Bonds in Protein Folding and Chemical Reactions

We present an array of force spectroscopy experiments that aim to identify the role of solvent hydrogen bonds in protein folding and chemical reactions at the single-molecule level. In our experiments we control the strength of hydrogen bonds in the solvent environment by substituting water (H(2)O) with deuterium oxide (D(2)O). Using a combination of force protocols, we demonstrate that protein unfolding, protein collapse, protein folding and a chemical reaction are affected in different ways by substituting H(2)O with D(2)O. We find that D(2)O molecules form an integral part of the unfolding transition structure of the immunoglobulin module of human cardiac titin, I27. Strikingly, we find that D(2)O is a worse solvent than H(2)O for the protein I27, in direct contrast with the behaviour of simple hydrocarbons. We measure the effect of substituting H(2)O with D(2)O on the force dependent rate of reduction of a disulphide bond engineered within a single protein. Altogether, these experiments provide new information on the nature of the underlying interactions in protein folding and chemical reactions and demonstrate the power of single-molecule techniques to identify the changes induced by a small change in hydrogen bond strength.

The strength of hydrogen bonding to metal-bound ligands can contribute to changes in the redox behaviour of metal centres

A series of nine tripodal tetradentate ligands based on tris(pyridyl-2-methyl)amine TPA with hydrogen bond donors R in one, two and three of the pyridine 6-positions (R = NH2 amino, L(Am-1,2,3); NHCH2(t)Bu neopentylamino, L(Np-1,2,3); NHCO(t)Bu pivaloylamido, L(Piv-1,2,3)) and TPA are used to investigate the effect of different hydrogen bonding microenvironments on electrochemical properties of their LCuCl complexes. The hydrogen bond donors are rigidly preorganised and suitably oriented for intramolecular N-H...Cl-Cu hydrogen bonds. Cyclic voltammetry studies show that the reduction potential of the Cu(II)/Cu(I) couple as a function of the ligand follows the order TPA < L(Am-n) < or approximately L(Np-n) < L(Piv-n), and that the magnitude of the effect increases with the number of hydrogen bonding groups. These trends could be explained in terms of the steric and electronic effects exerted by these groups stabilising the Cu(I) oxidation state. In fact, the X-ray structure of the air-stable [(L(Piv-3))Cu(I)Cl] complex is reported and shows elongated Cu-N and Cu-Cl bonds, presumably due to the combination of steric and electron withdrawing effects exerted by the three pivaloylamido groups. We reasoned that the strength of hydrogen bonding in the Cu(I) and Cu(II) oxidation states could differ and therefore contribute also to the aforementioned redox changes; this hypothesis is tested using IR and NMR spectroscopy. IR studies of the [(L(Piv-1,2,3))Cu(I)Cl] and [(L(Piv-1,2,3))Cu(II)Cl]+ complexes in acetonitrile show that the intramolecular N-H...Cl-Cu hydrogen bonding weakens in the order L(Piv-1) > L(Piv-2) > L(Piv-3), and that it is stronger in the Cu(I) complexes. The 1H NMR spectra of the [(L(Piv1,2,3))Cu(I)Cl] complexes are in complete agreement with the IR data, and reveal that the stability of the Cu(I) complexes to oxidation in air increases in the order L(Piv-1) < L(Piv-2) << L(Piv-3). The hydrogen bonds in the Cu(I) complexes are stronger because of the higher electron density on the Cl ligand, when compared to the Cu(II) complexes. This is consistent with ab initio MP2 calculations performed on the complexes [(L(Piv-3))Cu(I)Cl] and [(L(Piv-3))Cu(II)Cl]+. Thus, the electron density of a metal-bound ligand acting as hydrogen bond acceptor is revealed as the major factor in determining the strength of the hydrogen bonds formed. From the IR data the energies of the N-H...Cl-Cu hydrogen bonds is estimated, as is the contribution of changes in hydrogen bond strength with the oxidation state of the copper centre and number of interactions to stabilising the Cu(I) state. Some of the implications of this result in dioxygen activation chemistry are discussed.

Enthalpy relaxation and free volume changes in aged styrene copolymers containing a hydrogen bonding co-monomer

Enthalpy relaxation of polystyrene (PS) and four modified polystyrene copolymers containing co-monomers capable of forming hydrogen bonds of different strengths is described. Values of enthalpy lost (Δ H(Ta, ta)) were calculated from experimental data plotted against log (ta) and modelled using the Cowie-Ferguson (CF) semi-empirical model. This gives a set of values for three adjustable parameters, Δ H∞(Ta), log (tc) and β . Each of the parameters defines the relaxation process, which was found to be sensitive to changes in hydrogen bond strength. The introduction of hydrogen bonding causes a slower relaxation compared with PS, with a greater overall enthalpy lost measured for the all the copolymers except the styrene-co–4-hexafluoro-2-hydrox isopropyl styrene (SHFHS). Interestingly, the free volume of this copolymer measured using Positron Annihilation Lifetime Spectroscopy (PALS) was greater than that of PS. Furthermore, the SHFHS copolymer had the lowest change in heat capacity (Δ Cp) of any of the systems on passing through the glass transition, Tg. All experiments indicate that the enthalpy lost by the fully relaxed glass (Δ H∞(Ta)) is less than the theoretical amount possible on reaching the state defined by the equilibrium liquid enthalpy line (Δ Hmax(Ta)). The results are discussed with reference to the strength of interaction and free volume.

NMR Evidence of Weak Continuous Transitions in Water and Aqueous Electrolyte Solutions

Nuclear magnetic resonance chemical shifts are used to examine the perturbations in water structure that occur with concentration changes in aqueous KF, KCl, and LiOH solutions. Changes in the slope of ion chemical shifts as a function of solute concentration can be explained by changes in water structure. The equilibrium shift in water structure occurs as a result of changes in the hydrogen bond strength. The changes in hydrogen bond strength are a result of changes in electrolyte concentration and electron delocalization throughout the liquid. The location of the changes in slope with concentration is temperature dependent. A correlation of the changes in slope of chemical shifts to minima in specific heat capacity suggests the occurrence of a weak continuous transition in the solution structure at the critical concentration corresponding to the specific heat capacity minimum. By extrapolation the experiments reported here imply that there is a weak continuous transition associated with the heat capacity minimum for pure water. There must also be a structural relaxation time in the liquid associated with this transition. The results of these experiments provide confirmation for the model of aqueous solutions we recently proposed in which the solution is composed of regions of pure water and regions of liquid crystalline electrolyte hydrates. The subphase composed of structurally perturbed water is the part of the system that participates in the weak continuous phase transition that is evidenced by the NMR chemical shifts. In complete agreement with earlier Raman experiments it appears that the entire solution is a single electronic whole with exquisite electronic delocalization between the water and liquid crystalline subphases so that the ionic nuclei experience the electronic effects of the transition in the water subphase.

Alterations in chemical shifts and exchange broadening upon peptide boronic acid inhibitor binding to alpha-lytic protease.

alpha-Lytic protease, a bacterial serine protease of 198 amino acids (19 800 Da), has been used as a model system for studies of catalytic mechanism, structure-function relationships, and more recently for studies of pro region-assisted protein folding. We have assigned the backbones of the enzyme alone, and of its complex with the tetrahedral transition state mimic N-tert-butyloxycarbonyl-Ala-Pro-boro Val, using double- and triple-resonance 3D NMR spectroscopy on uniformly 15N- and 13C/15N-labeled protein. Changes in backbone chemical shifts between the uncomplexed and inhibited form of the protein are correlated with distance from the inhibitor, the displacement of backbone nitrogens, and change in hydrogen bond strength upon inhibitor binding (derived from previously solved crystal structures). A comparison of the solution secondary structure of the uninhibited enzyme with that of the X-ray structure reveals no significant differences. Significant line broadening, indicating intermediate chemical exchange, was observed in many of the active site amides (including three broadened to invisibility), and in a majority of cases the broadening was reversed upon addition of the inhibitor. Implications and possible mechanisms of this line broadening are discussed.

The change in hydrogen bond strength accompanying charge rearrangement: implications for enzymatic catalysis.

The equilibrium for formation of the intramolecular hydrogen bond (KHB) in a series of substituted salicylate monoanions was investigated as a function of delta pKa, the difference between the pKa values of the hydrogen bond donor and acceptor, in both water and dimethyl sulfoxide. The dependence of log KHB upon delta pKa is linear in both solvents, but is steeper in dimethyl sulfoxide (slope = 0.73) than in water (slope = 0.05). Thus, hydrogen bond strength can undergo substantially larger increases in nonaqueous media than aqueous solutions as the charge density on the donor or acceptor atom increases. These results support a general mechanism for enzymatic catalysis, in which hydrogen bonding to a substrate is strengthened as charge rearranges in going from the ground state to the transition state; the strengthening of the hydrogen bond would be greater in a nonaqueous enzymatic active site than in water, thus providing a rate enhancement for an enzymatic reaction relative to the solution reaction. We suggest that binding energy of an enzyme is used to fix the substrate in the low-dielectric active site, where the strengthening of the hydrogen bond in the course of a reaction is increased.

ZEKE spectroscopy of hydrogen-bonded phenol complexes

The recently-developed technique of zero-kinetic-energy photoelectron spectroscopy (ZEKE Spectroscopy) has been applied to a series of hydrogen-bonded complexes: phenol-water, phenol-methanol and phenol-ethanol. These resonant (1+1′) ZEKE spectra easily resolve intermolecular vibrational frequencies as well as a number of intramolecular (phenol-localized) ones. Accurate ionization energies are derived for these species and by comparison with the ionization energy of phenol, the change in hydrogen-bond strength between the neutral and corresponding ionic state for each complex is derived. By exciting the molecule through different intermediate vibrational levels of the S 1 state, the Franck-Condon (FC) activity in the ionization step is changed leading to the observation of all six intermolecular modes for each complex.

Vibrational spectra of perovskite-type layer compounds. Bis(ethylammonium) tetrachlorocadmate and propylenediammonium tetrachlorocadmate

The infrared spectra of the perovskite-type layer compounds (CH 3CH 2NH 3) 2CdCl 4 and (NH 3CH 2CH 2CH 2NH 3)CdCl 4 have been measured at temperatures between ambient and 77 K. The first-order phase transition at 216 K in the former salt was observed and appeared to be associated with changes in hydrogen-bond strength and molecular motions. It is shown, in conjunction with the results of earlier studies, that vibrational spectra of alkylammonium perovskite-layer compounds are determined by factor-group symmetry whilst those of the alkylenediammonium salts are simply dependent upon site-group symmetry.

The infrared spectrum of the methylammonium ion in bis(methylammonium)tetrachlorocadmate(II)

The IR spectrum of polycrystalline (CH 3NH 3) 2CdCl 4 has been recorded at temperatures between 300 and 90 K. The phase transition reported to take place at 173 K was observed and appears to involve a significant change in hydrogen-bond strength. Factor-goup splitting, affecting both parallel and perpendicular vibrational modes, was found to occur in the low-temperature spectra; its presence was demonstrated by the use of isotopically dilute CD 3NH 3 + ions. The formally IR-active torsional mode was not observed.