Experimental Evidence of Differences in the Hydrogen-Bonded Structure of Concentrated Aqueous Solutions Involving Dl- and l-Alanine Molecules

Abstract

Abstract Time-of-Flight (TOF) and steady state reactor neutron diffraction measurements have been carried out for aqueous 2.5 mol% Dl- and l-alanine solutions in order to investigate the differences in intermolecular hydrogen-bonded structure between solutions involving amino acid molecules with different optical activities. The observed difference functions, Δi(Q) and Δiinter(Q), between observed scattering cross sections for Dl- and l-alanine solutions with a D content of 96.1% exhibit the first peak located at Q = 2 Å−1 followed by oscillatory features extending up to the higher-Q region. The difference distribution function, Δg(r), obtained from the Fourier transform of Δi(Q), clearly indicates negative peaks at r = 2 and 2.5 Å and a positive one at r = 3.5 Å. Partial structure factors, aHH(Q), aXH(Q), and aXX(Q) (X: O, N, C, HM, and HM′, where HM and HM′ denote methyl and methine hydrogen atoms within the alanine molecule, respectively) for 2.5 mol% Dl-alanine solutions are successfully determined from combined analyses of intermolecular interference terms observed for solutions with 96.1, 66.0, and 35.9% exchangeable deuterium content. The nearest neighbor O···Hex and Hex···Hex (Hex: exchangeable hydrogen atom) distances are determined from the least squares fit of the observed partial structure factors, 1.90(1) and 2.48(1) Å, respectively. These values correspond to the positions of negative peaks observed in the present Δg(r) function. The least squares fitting analysis of the observed Δi(Q) revealed that the difference in the coordination number of the nearest neighbor O···Hex and Hex···Hex interactions between the Dl- and l-alanine solutions are −0.031(5) and −0.072(5), respectively. It is concluded that the intermolecular hydrogen bonds among solvent water molecules in the Dl-alanine solution are ca. 2% weaker than those in the l-alanine solution.