H-bonding Properties Research Articles

Molecular recognition probes of solvation thermodynamics in solvent mixtures

High-throughput UV-Vis experiments using four molecular recognition-based probes, made by the combination of two hydrogen bond acceptors, tri-n-butylphosphine oxide and N,N'-bis(2-ethylhexyl)acetamide, and two hydrogen bond donors, 4-phenylazophenol and 4-nitrophenol, were performed. The association constants for the 1 : 1 H-bond interaction involved in each probe system were measured in mixtures of a polar and non-polar solvent, di-n-hexyl ether and n-octane, respectively. Similar behaviour was observed for all four systems. When the concentration of the polar solvent was low, the association constant was identical to that observed in pure n-octane. However, once the concentration of the polar solvent exceeded a threshold, the association constant decreased linearly with the concentration of di-n-hexyl ether. Selective solvation in mixtures can be understood based on the competition between the multiple competing equilibria in the system. In this case, solvation thermodynamics are dominated by competition of the ether for solvation of H-bond donors. For the more polar solute, 4-nitrophenol, the selective solvation starts at lower concentrations of the polar solvent compared with the less polar solute, 4-phenylazophenol. Thus the speciation and hence the properties of systems containing multiple solutes and multiple solvents can be estimated from the H-bond properties and the concentrations of the individual functional groups.

Aromaticity balance, π-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline: the quantum theory of atoms in molecules (QTAIM) approach

Topological analysis based on DFT calculations regarding proton transfer reaction in salicylideneaniline (SA) was performed to scrutinize possible changes in the intramolecular H-bond, π-electron delocalization and aromaticity levels of certain fragments. Quantum chemical calculations and natural bond orbital (NBO) analyses were carried out over a tautomeric ensemble whose members correspond to the molecules at different stages in tautomeric interconversion of SA. The elaboration of intramolecular hydrogen bonding in terms of the relevant topological parameters and the interpretation of certain dependencies regarding its strength were examined. The results show that delocalization index (DI) between donor and acceptor atom δ(O,N) is a useful topological parameter for describing H-bond strength, which is influenced by π-delocalization level within quasiaromatic chelate ring, indicating its resonance-assisted character. NBO analyses reveal that lone-pair (LP) population on N center also affects the strength of intramolecular H-bond in SA. Furthermore, π-electron transfer accompanying intramolecular proton migration in SA is brought into being through formally vacant non-Lewis type LP* orbital on the tautomeric proton. As a result of this, tautomeric protons in molecular entities near TS have hypovalent character due to the lack of electron population in the bonding orbital relative to that in LP* orbital. While H-bonds in the tautomeric ensemble of SA are predominantly partial covalent, molecular entities close to transition state have the strongest covalent H-bonds. The most important result is also that there are linear correlations between the orders of bonds (hydroxyl and amine) involving intramolecular H-bond and electron density values at the relevant BCPs due to partially covalent character of these bonds, contrary to exponential behavior as for purely covalent bonds. Quasiaromatic chelate ring formation is established not only to compel a reduced aromaticity of salicylidene ring but also to decrease in LP-population on N.

An alignment independent 3D QSAR study of the antiproliferative activity of 1,2,4,5-tetraoxanes

An alignment-free 3D QSAR study on antiproliferative activity of the thirty-three 1,2,4,5-tetraoxane derivatives toward two human dedifferentiated cell lines was reported. GRIND methodology, where descriptors are derived from GRID molecular interaction fields (MIF), were used. It was found that pharmacophoric pattern attributed to the most potent derivatives include amido NH of the primary or secondary amide, and the acetoxy fragments at positions 7 and 12 of steroid core which are, along with the tetraoxane ring, common for all studied compounds. Independently, simple multiple regression model obtained by using the whole-molecular properties, confirmed that the hydrophobicity and the H-bond donor properties are the main parameters influencing potency of compounds toward human cervix carcinoma (HeLa) and human malignant melanoma (FemX) cell lines. Corollary, similar structural motifs are found to be important for the potency toward both examined cell lines.

Abstract 756: Optimization of tetrahydroisoquinoline-based microtubule disruptors as anticancer agents

Abstract We outline the discovery and optimisation of new microtubule disruptors with in vivo anti-tumor activity. Translation of the SAR from a steroidal series of microtubule disruptors led us to identify a series of tetrahydroisoquinoline (THIQ) based systems which exhibit a similar activity profile. Of this new series, 2-(3′,4′,5′-Trimethoxybenzyl)-7-methoxy-6-O-sulfamoyl THIQ 1 proved especially potent in both in vitro (GI50 [DU-145] 297 nM) and in vivo experiments. Herein, we describe the results of optimisation at C-6 and C-7 of the THIQ core and assessment of other polymethoxylated N-benzyl systems. Variations at the N-2 and C-6 positions were achieved by alkylation, esterification and etherification. Friedel Crafts acylation of C-7 and functional group interconversion allowed access to various C-7 alkyl and alkoxy derivatives. The various dimethoxybenzyl compounds proved similar in activity to the lead compound 2 (GI50 2.1 μM) in the N-mono-methoxybenzyl series while, apart from the 3′,4′,5′-trimethoxybenzyl compound 1, only 2′,4′,5′-trimethoxybenzyl substitution delivered sub-micromolar activity. Investigations of the effect of C-6 substitution proved more fruitful. In contrast to the SAR observed for 2 where the sulfamate group is essential for activity, the 6-OH, with the 6-O-acyl and 6-O-mesyl derivatives of 1 displayed similar or improved activity to the parent compound (GI50s range from 650 to 220 nM). The 6-O-methyl derivative, in contrast, proved completely inactive, highlighting the importance of a H-bond donor directly attached to C-6 or a H-bond acceptor projecting further out from this position. The most pronounced improvement in activity was obtained from exploration of C-7 substitution. In the 3′,4′,5′-trimethoxybenzyl series isosteric replacement of methoxy with ethyl delivered a 7-fold improvement in activity (3 GI50 41 nM). Intriguingly, the corresponding phenol proved significantly active suggesting different binding modes operate for the phenol and sulfamate derivatives since the H-bond acceptor properties of the C-7 substituent of the former are clearly important. Incorporation of a C-7 ethoxy group meanwhile proved detrimental for both sulfamate and phenol derivatives. The same transformations were made to 2, though no improvement in activity was obtained. In order to establish the potential of these compounds as anti-tumor agents their activity in the RPMI-8226 multiple myeloma xenograft model was assessed. The >75% inhibition of tumor growth observed (3 p.o. 40 mg/kg, 28d) in this preliminary study augers well for the development of this class of anti-cancer agents. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 756.

5D-QSAR for spirocyclic σ 1 receptor ligands by Quasar receptor surface modeling

Based on a contiguous and structurally as well as biologically diverse set of 87 σ 1 ligands, a 5D-QSAR study was conducted in which a quasi-atomistic receptor surface modeling approach (program package Quasar) was applied. The superposition of the ligands was performed with the tool Pharmacophore Elucidation (MOE-package), which takes all conformations of the ligands into account. This procedure led to four pharmacophoric structural elements with aromatic, hydrophobic, cationic and H-bond acceptor properties. Using the aligned structures a 3D-model of the ligand binding site of the σ 1 receptor was obtained, whose general features are in good agreement with previous assumptions on the receptor structure, but revealed some novel insights since it represents the receptor surface in more detail. Thus, e.g., our model indicates the presence of an H-bond acceptor moiety in the binding site as counterpart to the ligands’ cationic ammonium center, rather than a negatively charged carboxylate group. The presented QSAR model is statistically valid and represents the biological data of all tested compounds, including a test set of 21 ligands not used in the modeling process, with very good to excellent accuracy [ q 2 (training set, n = 66; leave 1/3 out) = 0.84, p 2 (test set, n = 21) = 0.64]. Moreover, the binding affinities of 13 further spirocyclic σ 1 ligands were predicted with reasonable accuracy (mean deviation in p K i ≈ 0.8). Thus, in addition to novel insights into the requirements for binding of spirocyclic piperidines to the σ 1 receptor, the presented model can be used successfully in the rational design of new σ 1 ligands.

Hydrogen bond models and theories: The dual hydrogen bond model and its consequences

The H-bond can be reinterpreted starting from the dual H-bond model, for which any D–H···:A bond is not a bond donated by D–H to :A but rather consists of two bonds formed by the central proton with two adjacent acceptors. Analogously, the H-bond energy, E HB, is not the D–H···:A dissociation energy but the smaller of two bond-dissociation energies, D 0(D–H) and D 0(H–A), by which −D: and :A are competitively bound to the proton. If one is stronger, the other is weaker, and weak the overall H-bond will be. Strong bonds occur when Δ D 0 = D 0(D–H) − D 0(H–A) = 0 or, in terms of affinity for the proton ( pa), when Δ pa = pa(D −) − pa(A) = 0. H-bond properties are then function of two variables, pa(D −) and pa(A), or better of their linear combinations, Σ pa = pa(D −) + pa(A) and Δ pa = pa(D −) − pa(A), having respective meanings of mean donor/acceptor electronegativity and of energy difference, Δ rE , between tautomeric D–H···:A and −D:···H–A + forms. Two cases are studied. The case: ‘Σ pa variable for Δ pa = 0’ leads to quantitative relationships between D/A electronegativity and maximum energy, E HB,MAX, achievable for each D/A electronegativity class, EC(D,A). The case: ‘Δ pa variable for Σ pa = constant’ leads to formulate three different but inter-consistent H-bond theories which are separately discussed. The last one, which is called ‘p K a equalization principle’ and where Δ pa values are empirically estimated from the acid–base dissociation constants in water as Δp K a = p K AH(D–H) − p K BH+(A–H +), is shown to be a powerful method of large applicability for predicting the H-bond strengths from thermodynamic parameters.

Effect of hydrogen bond cooperativity on the behavior of water

Four scenarios have been proposed for the low-temperature phase behavior of liquid water, each predicting different thermodynamics. The physical mechanism that leads to each is debated. Moreover, it is still unclear which of the scenarios best describes water, because there is no definitive experimental test. Here we address both open issues within the framework of a microscopic cell model by performing a study combining mean-field calculations and Monte Carlo simulations. We show that a common physical mechanism underlies each of the four scenarios, and that two key physical quantities determine which of the four scenarios describes water: (i) the strength of the directional component of the hydrogen bond and (ii) the strength of the cooperative component of the hydrogen bond. The four scenarios may be mapped in the space of these two quantities. We argue that our conclusions are model independent. Using estimates from experimental data for H-bond properties the model predicts that the low-temperature phase diagram of water exhibits a liquid-liquid critical point at positive pressure.

Solvent and structural effects on the photo-reduction of cobalt(III) complexes in aquo-organic solvent media possessing different H-bonding properties

The photo-reduction of a series of cobalt(III) complexes of the type cis-β-[Co(ampy) 2 (4-R-Py)Cl]Cl 2 where ampy = 2-aminomethylpyridine and R = H, Me, Et, t-Bu, COMe, and CN has been studied in propan-2-ol/water and 1,4-dioxane/water medium. The photo-reduction quantum yield (Φ Co II ) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of these reactions. Increase in the percentage of organic co-solvent in both the medium increased the photo-reduction quantum yield. Correlation of Φ Co II with macroscopic solvent parameters viz. relative permittivity indicated that the reactivity is influenced by both specific and non-specific solute-solvent interactions. Kamlet-Taft solvatochromic comparison method was used to separate and quantify these effects. H-bonding property of the mixture was found to dominate the solvation and consequently the magnitude of the quantum yield.

Concerted proton–electron transfers in the oxidation of phenols

The oxidation of phenols is an emblematic example where the mechanisms of proton-coupled electron transfers could be investigated in depth thanks to non-destructive electrochemical techniques such as cyclic voltammetry. A concerted proton-electron transfer could then be shown to be the prevailing pathway in the oxidation of amino-phenols mimicking the tyrosine-histidine couple in Photosystem II. The theoretical model developed on this occasion leads to the introduction of two main parameters characterizing reorganization of heavy atoms in the reactant and in the solvent on the one hand and proton tunneling on the other. When water used as the solvent is at the same time the proton acceptor, the concerted pathway also predominates. It is characterized by a remarkably large standard rate constant both in electrochemistry and in the oxidation by homogenous reactants. Another aspect of the importance of H-bonding in concerted proton-electron transfer is provided by H-bond relays that efficiently mediate the electron transfer-triggered transport of protons between two sites over large distances thanks to the displacement of two protons concerted with electron transfer. Intermediary protonation of the relay is avoided by fine tuning of its H-bond acceptor and donor properties.

Hydrogen bonding properties of non-polar solvents

A combination of high-throughput NMR titration experiments, UV-Vis absorption titrations and data collected from the literature on 1:1 H-bonded complexes has been used to characterise the H-bond properties of non-polar organic solvents: alkanes, perfluorocarbons, aromatic and halogenated organic solvents. The results are analysed in the context of the electrostatic solvent competition model, which assumes that solvent effects on intermolecular interactions can be interpreted based on the exchange of specific functional group contacts, with minimal involvement of the bulk solvent. For solvents where the H-bond parameters have been measured as solutes in carbon tetrachloride solution, the H-bond parameters measured here for the same compounds as solvents are practically identical, i.e. solute and solvent H-bond parameters are directly interchangable. For the very non-polar solvents, alkanes and perfluorocarbons, the experimental H-bond parameters are significantly larger than expected based on calculated molecular electrostatic potential surfaces. This suggests an increase in the relative importance of van der Waals interactions when electrostatic effects are weak, but there is no detectable difference between the solvation properties of cyclic and linear alkanes, which have different van der Waals interaction properties.

Heterogeneity in the Nitroxide Micro-Environment: Polarity and Proticity Effects in Spin-Labeled Proteins Studied by Multi-Frequency EPR

This study aims to investigate the g xx heterogeneity of the g-tensor commonly observed in high-field electron paramagnetic resonance (EPR) spectra of nitroxide spin-labeled sites in proteins. This heterogeneity is addressed in terms of spin-label populations characterized by different polarity and H-bonding properties of the nitroxide micro-environment. The g xx value for each population is determined from the fit of continuous-wave high-field spectra obtained at 95, 275 and 360 GHz with a series of nitroxide spin-labels covalently attached to different sites in both membrane and water-soluble proteins. The spin-labeled proteins investigated include sensory rhodopsin II and its cognate transducer molecule (HtrII) from Natronomonas pharaonis both in micelles and membranes, bacteriorhodopsin from Halobacterium salinarum in native purple membrane lipid bilayers and water-soluble colicin A from Escherichia coli. To avoid contributions to the g xx spectral features of the nitroxide label due to nuclear quadrupole interactions arising from 14N nuclei, and to simplify the nitrogen hyperfine pattern, methanethiosulfonate spin labels, containing the 15N isotope (I = 1/2) in some experiments, were employed. A consistent analysis of all multi-frequency EPR spectra revealed three distinct g xx values, g i xx , for each investigated position of the labeled proteins. In contrast, distinctly different nitrogen hyperfine splittings A zz of the nitroxides in the various labeled proteins could not be resolved, but rather an average hyperfine splitting \(\bar{A}_{{zz}}\) was obtained. The g i xx values as well as the fractions of the different nitroxide populations were found to be correlated with the average hyperfine constant \(\bar{A}_{{zz}},\) a parameter which likewise is known to be sensitive to the local polarity of the spin-label micro-environment. Plotting the different g i xx values obtained for each EPR spectrum versus \(\bar{A}_{{zz}}\) of the labeled proteins reveals new interesting aspects of the nitroxide label micro-environment in terms of polarity and H-bonding propensity (proticity). Linear approximations of the different regions of the plot g i xx versus \(\bar{A}_{{zz}}\) are presented and compared with theoretical and experimental data available from the literature.

Differential Pulse Voltammetric Studies of Some Mixed-ligand Co(III) Complexes in Aquo-Organic Solvent Media Possessing Different Hydrogen Bonding Properties

The electro-reduction of a series of Co(III) complexes of the type cis-β -[Co(trien)(4-R-Py)Cl]Cl2, where trien = triethylenetetramine and R = H, Me, Et, t-Bu, COMe, and CN, has been studied in propan-2-ol/water and 1,4-dioxane/water binary mixtures. The redox potential (E1/2) data were correlated with solvent and structural parameters with an aim to shed some light on the mechanism of these reactions. Correlation of E1/2 with macroscopic solvent parameters indicated that the reactivity is influenced by both specific and non-specific solute-solvent interactions. The reduction of Co(III) to Co(II) in these complexes was observed to become increasingly easier with an increase in the dipolarity/ polarizability of the medium as evidenced by the Kamlet-Taft correlation. The difference in the relative lenience of the reduction has been explained using the difference in H-bonding properties of the medium.

Effect of dimethyl sulfoxide on the phase behavior of model stratum corneum lipid mixtures

Dimethyl sulfoxide (DMSO), an efficient transdermal enhancer, is proposed to alter the skin barrier by, at least partially, disturbing the lipid phase of the stratum corneum (SC). We have investigated, using differential scanning calorimetry and vibrational microspectroscopy, the effect of DMSO on the phase behavior of a lipid mixture formed by N-palmitoyl- d- erythro-sphingosine, deuterated palmitic acid, and cholesterol, mimicking the SC lipid phase. Our results reveal that DMSO favors the disordering of the lipid acyl chains. Moreover, the effect of DMSO is strongly concentration dependent and this dependence is reminiscent of that describing the DMSO transdermal enhancement. DMSO-induced fluidification affects primarily the fatty acid in the mixture. Therefore, it is proposed that the molecular mechanism of the transdermal transport enhancement caused by DMSO is associated with its H-bonding properties; its presence alters the interfacial H-bond network involving the fatty acid molecules and consequently the cohesive lipid packing.

Intramolecular hydrogen bonding as a determinant of the inhibitory potency of N-unsubstituted imidazole derivatives towards mammalian hemoproteins

Enzymes involved in the mammalian microsomal metabolism of drugs are, in numerous cases, inhibited by compounds bearing an imidazolyl scaffold. However, the inhibition potency is highly dependent upon the accessibility of the imidazolyl nitrogen lone pair. In order to highlight some structural parameters of inhibitors that control this phenomenon, a series of compounds containing a nitrogen unsubstituted imidazolyl moiety with varying degrees of nitrogen lone pair accessibility was tested on human and rat hepatic cytochromes P450 and microperoxidase 8, an enzymatically active peptide derived from cytochrome c. In each case, we have shown that the accessibility of the imidazole lone pair determined the extent of inhibition. Nitrogen accessibility was tuned not only by varying the steric hindrance in the vicinity of the imidazolyl ring but also by modifying its surrounding hydrogen bonding network. Compounds in which there exists intramolecular hydrogen bonding between the imidazole moiety and an H-bond acceptor, such as an appropriately positioned amide carbonyl group, demonstrated enhanced inhibitory effects. Conversely, imidazole moieties which are in proximity to H-bond donors, such as an amide NH group, displayed reduced potency. This trend was observed in cyclo-peptide derivatives in which the intramolecular H-bond network was adjusted through the modification of the stereochemistry of a dehydrohistidine residue. It was observed that (Z)-isomers weakly bind heme, whereas (E)-isomers demonstrated higher degrees of metal binding. Therefore, enzymatic inhibition of heme-containing proteins by compounds bearing a dehydrohistidine motif seems to be closely related to its stereochemistry and hydrogen binding propensity. At neutral pH, these differences in binding affinities can be confidently attributed to the ambident H-bond properties of imidazole nitrogen atoms. This structure-activity relationship may be of use for the design of novel imidazolyl compounds as new P450 inhibitors or drug candidates.

Effects of Polymer Additives on the Crystallization of Hydrates: A Molecular-Level Modulation

The purpose of this study is to elucidate the effects of polymer additives on the growth behavior and final internal structure of hydrate crystals. The polymers used were poly(ethylene) glycol (PEG), ethyl(hydroxyethyl) cellulose (EHEC) and hydroxylpropyl methylcellulose (HPMC). Rheological characterization of the polymer solutions was carried out. Nitrofurantoin and carbamazepine were crystallized separately from a mixture of acetone and polymer/water solution through evaporative crystallization. The crystallization process and final crystals were characterized by light microscopy, XRPD and FT-IR spectroscopy. PEG showed a minor viscosity increase, but polymer network associations were found in the EHEC and HPMC solutions. With no additive, nitrofurantoin crystallized as a mixture of plate-like (monohydrate I) and needle-like (monohydrate II) crystals. All polymers affected the crystalline phase, and HPMC and EHEC also changed the crystal morphology from needle-like to dendrite-like. H-Bonding was found to exist between these polymers (O-H group) and nitrofurantoin monohydrate (CO== group), while this was not the case with carbamazepine hydrate. Additives were able to modify the crystal phase and morphology due to H-bonding and polymer network properties. We envisage that the improved understanding of additives' role provided by this study can be utilized when designing control strategies for the crystallization process.

Effects of C-17 heterocyclic substituents on the anticancer activity of 2-ethylestra-1,3,5(10)-triene-3-O-sulfamates: synthesis, in vitro evaluation and computational modelling

The potent activity of 2-substituted estra-1,3,5(10)-triene-3-O-sulfamates against the proliferation of cancer cells in vitro and tumours in vivo highlights the therapeutic potential of such compounds. Optimal activity is derived from a combination of a 2-XMe group (where X = CH(2), O or S), a 3-O-sulfamate group in the steroidal A-ring and a H-bond acceptor around C-17 of the D-ring. Herein, we describe the synthesis and anti-proliferative activities of a series of novel 2-substituted estra-1,3,5(10)-triene-3-O-sulfamates bearing heterocyclic substituents (oxazole, tetrazole, triazole) tethered to C-17. In vitro evaluation of these molecules revealed that high anti-proliferative activity in breast and prostate cancer cells lines (GI(50) of 340-850 nM) could be retained when the heterocyclic substituent possesses H-bond acceptor properties. A good correlation between the calculated electron density of the heterocyclic ring and anti-proliferative activity was observed. Docking of the most active compounds into their putative site of action, the colchicine binding site of tubulin, suggests that they bind through a different mode to the previously described bis-sulfamate derivatives and 1 and 2, which possess similar in vitro activity.

Metallothioneins with unusual residues: Histidines as modulators of zinc affinity and reactivity

For many years, paradigms regarding metallothioneins comprised the exclusive metal coordination by thiolates from cysteine residues and the absence of aromatic residues. As more sequence and in vitro data on metallothioneins, in particular from non-vertebrate organisms, has become available, both the occurrence of and metal coordination by histidine residues in metallothioneins is emerging as a more frequent feature than expected. We discuss the general implications of histidines versus cysteines in zinc binding sites, and review some recent results from literature and our own lab. We conclude that histidines can stabilise metallothionein clusters by reducing the overall charge, offering the ability to help with structural organisation by supplying H-bond donor and acceptor properties, reducing the likelihood for disulfide bond formation, whilst maintaining a high affinity towards metal ions, in particular the borderline zinc ion.

Characterisation and structure of two Cu(II) mononuclear compounds with 7-azaindole as a ligand that display interesting H-bond properties

Two Cu(II) compounds with the ligand 7-azaindole (abbreviated as Haza) have been synthesized and fully characterized by spectroscopy, EPR and X-ray structure analysis. The first compound, Cu 2Cl 4(Haza) 6 ( 1), consists of two different Cu(II) sites, i. e. [Cu(Haza) 2Cl 3] + and [Cu(Haza) 3Cl] −, both with a distorted square-pyramidal geometry for the Cu(II) ion. A hydrogen bond network exists between N–Cl atoms in each unit and also between the two units. The second compound, [Cu(Haza) 4F](BF 4)(H 2O) ( 2), in which the fluoride anion is generated from decomposition of BF 4 − anions, has also a distorted square-pyramidal geometry. The structure is stabilized by hydrogen bonds between the N–H groups and the F ions. The water is at a disordered lattice position. Further characterization has been performed using IR, LF and EPR spectroscopy.

Infrared and Raman spectroscopic characterization of the hydrogen-bonding network in l-serine crystal

The IR spectra (4000–400 cm −1) of neat and isotopically substituted (ND/OD ≤ 10% D and ≅30% D) polycrystalline l-serine (α-amino-β-hydroxypropionic acid; HO–CH 2–CH(NH 3) +–COO −) were recorded in the temperature range 300–10 K and assigned. The isotopic-doping/low-temperature methodology, which allows for decoupling of individual proton vibrational modes from the crystal bulk vibrations, was used for estimating the lengths and energies of the different H-bonds present in l-serine crystal. To this end, the frequency shifts observed in both the NH/OH stretching and out-of-plane bending spectral regions (relatively to reference values for these vibrations in non-hydrogen-bonded l-serine molecules) were used, together with previously developed empirical correlations between these spectral parameters and the H-bond properties. In addition, the room-temperature Raman spectrum (4000–150 cm −1) of a single crystal of neat l-serine was also recorded and interpreted. A systematic comparison was made between the spectroscopic data obtained currently for l-serine and previously for dl-serine, revealing that the vibrational spectra of the two crystals reflect well the different characteristics of their hydrogen-bond networks, and also correlate accurately with the different susceptibility of the two crystals to pressure-induced strain.

Modulation of the axial water hydrogen-bonding properties by chemical modification of the substrate in resting state, substrate-bound heme oxygenase from Neisseria meningitidis; coupling to the distal H-bond network via ordered water molecules.

The hydrogen bonding of ligated water in ferric, high-spin, resting-state substrate complexes of heme oxygenase from Neisseria meningitidis has been systematically perturbed by variable electron-withdrawing substituents on the hemin periphery. The pattern of 1H NMR-detected dipolar shifts due to the paramagnetic anisotropy is strongly conserved among the four complexes, with the magnitude of dipolar shifts or anisotropy increasing in the order of substituent formyl < vinyl < methyl. The magnetic anisotropy is axial and oriented by the axial Fe-His23 bond, and while individual anisotropies have uncertainties of approximately 5%, the relative values of deltachi (and the zero-field splitting constant, D proportional, variant deltachi(ax)) are defined to 1%. The unique changes in the axial field strength implied by the variable zero-field splitting are in accord with expectations for the axial water serving as a stronger H-bond donor in the order of hemin substituents formyl > vinyl > methyl. These results establish the axial anisotropy (and D) as a sensitive probe of the H-bonding properties of a ligated water in resting-state, substrate complexes of heme oxygenase. Correction of observed labile proton chemical shifts for paramagnetic influences indicates that Gln49 and His53, some approximately 10 angstroms from the iron, sense the change in the ligated water H-bonding to the three nonligated ordered water molecules that link the two side chains to the iron ligand. The present results augur well for detecting and characterizing changes in distal water H-bonding upon mutagenesis of residues in the distal network of ordered water molecules and strong H-bonds.