Hydration Structure of Alanine Molecule in Concentrated Aqueous Solutions

Time-of-Flight (TOF) neutron diffraction measurements have been carried out on aqueous 3 mol% d,l-alanine solutions. The isotopic substitution technique has been applied to both nitrogen and methyl-hydrogen atoms within the alanine molecule in order to obtain information concerning the hydration structure around the amino- and the methyl groups within the alanine molecule in the aqueous solution. There exist, on the average, 2.4(1) water molecules that are hydrogen bonded to the amino group with an intermolecular distance of rNO = 2.88(2) Å. The tilt angle between the N–D···O axis and the molecular plane of the D2O molecule has been determined to be 60.4(5)°. The first hydration shell of the methyl group involves ca. two water molecules per one alanine molecule. The nearest neighbor HM(methyl)–OW(water) and HM(methyl)–HW(water) distances have been determined to be 2.58(1) and 2.99(2) Å, respectively.