Intermolecular hydrogen-bonded structure in aqueous solutions involving amino acid molecules with different optical activities

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Neutron and X-ray diffraction measurements were carried out on aqueous 2.5 mol% DL- and L-alanine solutions in D_2O in order to investigate the difference in intermolecular hydrogen-bonded structure between these solutions involving amino acid molecules with different optical activities. The difference distribution functions Δg(r) derived from the Fourier transform of the difference neutron intermolecular interference function Δi(Q) between DL- and L-alanine solutions exhibited negative peaks at r = 2 and 2.5 Å, these positions correspond to O-H and H-H intermolecular hydrogen-bonded distances, respectively. Quantitative analysis was carried out by means of the least squares fitting analysis of observed Δi(Q). Intermolecular distances and difference coordination numbers obtained were r(O-H)=1.99(3)Å, r(H-H)=2.54Å, Δn(O-H)=-0.031(5), and Δn(H-H)=-0.072(5), respectively. The present values of Δn(O-H) and Δn(H-H) indicate that the intermolecular hydrogen bonds between water molecules in t … More he L-alanine solution are ca.2% stronger than those in the DL-alanine solution. The molecular dynamics simulation was adopted for 2.5 mol% and 1 mol% DL- and L-alanine heavy water solutions. Difference in partial distribution functions for water-water interaction, g_<HH>(r), g_<OH>(r), and g_<OO>(r) does not observed between 1 mol% DL- and L-alanine solutions, implying that the difference in the intermolecular hydrogen-bonded structure between DL- and L-alanine solutions is not present in the diluted condition. On the other hand, the partial distribution functions describing water-water interaction, g_<HH>(r), g_<OH>(r), and g_<OO>(r), obtained for 2.5 mol% DL- and L-alanine solutions indicated that the intermolecular hydrogen bonds in L-alanine solution are ca.1% stronger than that in the DL-alanine solutions. Time-of-flight neutron diffraction measurements on ^<14>N/^<15>N isotopically substituted aqueous 3 mol% DL- and L-alanine heavy water solutions were carried out in order to investigate the difference in hydrogen-bonds between the amino group within the alanine molecule and the nearest neighbor water molecules. The data analysis is in progress. Less