Hydrophilic and hydrophobic interactions in concentrated aqueous imidazole solutions: a neutron diffraction and total X-ray scattering study

Abstract

The intermolecular interactions in concentrated (5 M) aqueous imidazole solutions have been investigated by combining neutron diffraction with isotopic substitution, total X-ray scattering and empirical potential structure refinement (EPSR) simulations using a box containing 5530 water and 500 imidazole molecules. The structural model with the best fit was used to generate radial distribution functions and spatial density functions. The local volume surrounding imidazole molecules is dominated by water, due to strong hydrogen-bonding between the nitrogen moieties of imidazole and water molecules; within a radius of 6.4 Å from the central imidazole molecule there are, on average, 17 water and only 3 imidazole molecules. Even though imidazole interacts with water it appears to disrupt hydrogen bonding in the surrounding water network only minimally. Hydrogen-bonding between imidazole molecules is negligible. The most probable positions of imidazole nearest-neighbours are above and below the plane of the aromatic ring. At low distances (up to B3.5-3.8 Å) these nearest neighbours were found to prefer parallel orientation of the molecular planes, indicating hydrophobic (p-p) stacking. At longer distances (up to B5 Å), imidazole neighbours assume both parallel and edgeto-face orientations. Overall, hydrated imidazole molecules are the most probable structural motif in aqueous solutions, with very few direct imidazole-imidazole interactions.

The Molecular Basis for Properties of Binary Solvent Systems: Synchrotron X-ray Pair Distribution Function Analysis of the Acetone-Water System

“…This is also observed in the fact that features in g(r) corresponding to C2a-O1 and C2a-Ot occur at a short distance (~3.89 Å) compared to that of C2a-O1a intermolecular-interactions (~4.6 Å). These observations are similar to our findings in the solution structure of imidazole, where individual units are hydrated and the secondary interactions are between these hydrated units, 15 unlike the structure proposed by Max et. al using factor analysis of infrared spectroscopy.…”

“…17 Even though organic systems are characterised by relatively weak X-ray scattering cross sections, the data quality is sufficient that one can even envisage variable temperature and other in-situ studies. Our previous studies 15 and others in the literature 18 , have validated the structures obtained from XPDF against those modelled from NPDF, thereby overcoming the limitations of XPDF in getting information to a high Q value compared to NPDF.…”

“…Especially when combined with NPDF of the corresponding deuterated systems and with laboratory based X-ray PDF measurements it provides precise insight into the local interactions in binary systems of one given concentration. 15 A drawback of this in-depth analysis is the need for access to a spallation neutron source and the isotopically tagged (usually deuterated) systems. More importantly, acquiring a complete set of such complementary experimental data represents at minimum one day of measurement work.…”

Physicochemical Properties of a Bi-aromatic Heterocyclic-Azo/BSA Hybrid System at the Air–Water Interface

“…In contrast, one benzene ring fused within the imidazole (i.e., benzimidazole) decreases the solubility in hydrophilic solvent compare to the imidazole derivatives. 23 − 25 Essentially, one of these N-heterocyclic azo-imidazole compounds can exhibit many biological 26 − 30 and physical properties. 31 , 32 …”

Local electronic structure of histidine in aqueous solution

“…27 The imidazole motif prevalent in histidine has also been implicated in nucleation and crystallization processes in concentrated aqueous media, however neutron diffraction and X-ray scattering studies suggest that it is solvation which drives assembly and not π-stacking of the imidazole pairs. 28 Several X-ray absorption, 29,30 X-ray emission, 31 and resonant inelastic X-ray scattering (RIXS) 32,33 studies have been conducted on glycine, proline, cysteine, and lysine to investigate the change of their electronic structures engendered by varying the pH of solutions. The above mentioned techniques provide a view of the bulk solution, while X-ray photoelectron spectroscopy (XPS), which can provide direct information on electronic structure of the surface and interface, brings an extra layer of sensitivity to the measurements.…”