Chemical Shifts Of Hydrogen-bonded Protons Research Articles

Hydrogen Bond Coupling in the Ketosteroid Isomerase Active Site

Hydrogen bond networks are key elements of biological structure and function. Nevertheless, their structural properties are challenging to assess within complex macromolecules. Hydrogen-bonded protons are not observed in the vast majority of protein X-ray structures, and static crystallographic models provide limited information regarding the dynamical coupling within hydrogen bond networks. We have brought together 1.1-1.3 A resolution X-ray crystallography, (1)H NMR, site-directed mutagenesis, and deuterium isotope effects on the geometry and chemical shifts of hydrogen-bonded protons to probe the conformational coupling of hydrogen bonds donated by Y16 and D103 in the oxyanion hole of bacterial ketosteroid isomerase. Our results suggest a robust physical coupling of the equilibrium structures of these two hydrogen bonds such that a lengthening of one hydrogen bond by as little as 0.01 A results in a shortening of the neighbor by a similar magnitude. Furthermore, the structural rearrangements detected by NMR in response to mutations within the active site hydrogen bond network can be explained on the basis of the observed coupling. The results herein elucidate fundamental structural properties of hydrogen bonds within the idiosyncratic environment of an enzyme active site and provide a foundation for future experimental and computational explorations of the role of coupled motions within hydrogen bond networks.

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O–O or N–N distances. Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O–H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue.

Interaction of 2,6-dimethylpyridine with chloroacetic acid in some aprotic solvents: problem of stoichiometry

Infrared and 1H NMR spectra are reported for mixtures of chloroacetic acid and 2,6-dimethylpyridine with various acid-base ratios (2:1 and from 1:1 to 1:15) in benzene, dichloromethane, and acetonitrile. The pyridine ring stretching vibration bands demonstrate that two species, A H⋯B and A −⋯H B +, are present in the investigated solutions. In benzene and dichloromethane some uncomplexed acid is observed in 2:1 and 1:1 mixtures. In acetonitrile a complex between the acid and acetonitrile is formed even when there is a 1:10 mixture of acid and 2,6-dimethylpyridine. Both the chemical shift of hydrogen-bonded protons and the continuous absorption in IR spectra are affected when an excess of base is added to the equimolar mixture of 2,6-dimethylpyridine with chloroacetic acid. This is caused by the presence of the 2:1 complex [B + H(A −⋯HA)] in the equimolar mixture. The excess of base shifts complexation to the 1:1 acid-base complex. The continuous absorption due to strong hydrogen bonds was separated numerically from that of the skeletal ones due to the 2,6-dimethylpyridine and acid. The observed increase of the extent of proton transfer and the continuous absorption due to changes in solvent polarity is explained by the increasing interaction of the dipole of the hydrogen bond with the reaction field from the environment.

Interaction of 2,4,6-trimethylpyridine with some halogenocarboxylic acids in benzene and dichloromethane. Problem of stoicheiometry

The practical molal osmotic coefficients of 2,4,6-trimethylpyridine complexes with dichloroacetic and 2,2-dichloropropionic acid in benzene solutions over the concentration range 0.05–0.5 mol kg–1 were determined by vapour pressure osmometry (v.p.o.). The non-ideal behaviour of the investigated systems is interpreted on the basis of the stepwise aggregation model and the association constants are derived. 1H N.m.r. and i.r. spectra are reported for 2,4,6-trimethylpridine with dichloroacetic, 2,2-dichloropropionic, and trifluoroacetic acids with various acid-base ratios (2:1, 1.33:1, and from 1:1 to 1:10). Both the chemical shift of hydrogen-bonded protons and the continuous absorption in i.r. spectra are affected when an excess of base is added to the equimolar mixture of 2,4,6-trimethylpyridine with dichloroacetic and 2,2-dichloropropionic acids. The results can be rationalized in terms of formation of the following complexes: B ⋯ HA, B+ H ⋯ A–, (BHA)n, and B+H(A–⋯ HA). For trifluoroacetic acid the B+H(A–⋯ HA) complex is formed only when there is an excess of acid, while for dichlorocetic and 2,2-dichloropropionic acids the equilibrium can be completely shifted to the 1:1 complex only when there is a three-fold excess of base.

Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine N-oxide trifluoroacetates and trichloroacetates in chloroform. Correlation of OH stretching frequencies and proton chemical shifts with pyridine N-oxide pK a values

The i.r. and 1H n.m.r. spectra are reported for the trifluoro-and trichloro-acetates of 12 substituted pyridine N-oxides in dry chloroform. If the centre of gravity of the OH stretching absorption (H)and the chemical shift of hydrogen-bonded protons (δ) are plotted against the pKa value of the N-oxides, the points fall on two intersecting straight lines. The correlations reflect changes in the hydrogen-bond strength. Plots of H and δ of trifluoroacetates against the corresponding values for the trichloroacetates are used to show that the proton-donor properties of the acids are comparable. A direct plot of δversusH gives a straight line consistent with data for the monomeric acids. This correlation is discussed with respect to the difference between hydrogen bonds in molecular and ion-pair complexes.

Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine trifluoroacetates and their deuteriated analogues in dichloromethane

I.r. and 1H n.m.r. spectra are reported for the trifluoroacetates of 12 substituted pyridines in dry dichloromethane. The carbonyl–carboxylate region shows that the weakest and strongest pyridines form, respectively, hydrogen-bonded complexes (B ⋯ HA ) and hydrogen-bonded ion pairs (B+H ⋯ A–). The intermediate-strength pyridines form a mixture of both species. Dichloromethane, as a more polar solvent than benzene, slightly shifts the equilibrium between these species to the ion-pair form. If the centres of gravity (H or D) and the chemical shifts of hydrogen-bonded protons (δ) are plotted versusΔpKa, the points fall on two intersecting straight lines. The correlations reflect changes in the hydrogen bond strength and the equilibrium between the molecular complex and the ion pair with ΔpKa. The centres of gravity in dichloromethane are compared with those in benzene and discussed with respect to variations of hydrogen bond strength. A plot of δversusH gives again two intersecting straight lines, which reflects the effect of a positive charge caused by the proton transfer on the chemical shift. The effect of the solvent on the correlation of δversusH is examined and discussed. The isotope frequency ratio (H/D) varies from 1.07 to 1.21.

Infrared and 1H nuclear magnetic resonance studies of hydrogen bonds in some pyridine N-oxide trifluoroacetates and their deuteriated analogues in dichloromethane

The i.r. and 1H n.m.r. spectra of 11 substituted pyridine N-oxides with CF3COOH and CF3COOD were examined in dry dichloromethane. All the investigated complexes show a typical carbonyl band. The variation of ν(CO) frequency with ΔpKa, its shape and band-width, and changes on deuteraition confirm the assumption that the structure of complexes with strong OHO hydrogen bonding is altered gradually with the rise of the interaction energy, from the molecular A-H … B, to the ionic, A–… H-B+. If the centre of gravity (H or D) and the chemical shift of hydrogen-bonded protons (δ) are plotted against ΔpKa, points fall on two intersecting straight lines. These correlations are used to compare the hydrogen bond strength in complexes of pyridines and their N-oxides. A discrepancy between the i.r. and 1H n.m.r. data was found and discussed with respect to the contribution of charge to the shielding of strongly hydrogen-bonded protons. A plot of H/Dagainst Hshows a characteristic shape with a minimum, similar to that found by Novak. The variations of H/D with proton acceptor properties of N-oxides are consistent with the ‘quasi-symmetrical’ structure A … H … B postulated in the literature.

Infrared investigation of hydrogen bonding and proton transfer of some pyridine trifluoroacetates in benzene

I.r. spectra are reported for the trifluoroacetates of ten substituted pyridines in dry benzene. The carbonyl–carboxylate region shows that the weakest and strongest pyridines form, respectively, hydrogen-bonded complexes (B ⋯ HA) and hydrogen-bonded ion pairs (B+H ⋯ A–). The intermediate-strength pyridines form a mixture of both species. A plat of the gravity centre (vH) of vAH complex absorption against ΔpKa consists of two intersecting straight lines. This correlation is considered in the light of the previously reported similar relationship between the chemical shift of hydrogen-bonded protons (δ) and ΔpKa. If δ is plotted versus vH, again two intersecting straight lines are found. The correlation reflects the effect of a positive charge caused by the proton transfer on the chemical shift. All these results confirm the long-held assumption that no general correlation exists for hydrogen-bonded complexes and hydrogen-bonded ion-pairs.