Hydration Structure of Glycine Molecules in Aqueous Alkaline Solutions

Time-of-flight (TOF) neutron diffraction measurements have been carried out on the alkaline aqueous 2 mol% glycine solution in heavy water. The isotopic substitution technique has been applied to both nitrogen and hydrogen atoms within glycine molecules in order to obtain information concerning the hydration structure around the amino- and methylene group in the glycine molecule under the high-pH condition. Results revealed that the nitrogen atom in the amino group forms a hydrogen bond of the N…D-O type with ca. one D2O molecule (rN…D = 1.97(3) Å, ∠N…D-O = 174(5)°), and simultaneously forms bonds of N-D…OD2 type with ca. two D2O molecules (rN…O = 2.92(3) Å) in the present alkaline aqueous solution. This behavior is in contrast to the hydration structure around the amino group in the glycine molecule reported for the neutral aqueous solution, in which the amino group forms the hydrogen bonds of N-D…OD2 type with ca. three D2O molecules. The first hydration shell around the methylene group in the glycine molecule involves ca. two D2O molecules per one glycine molecule. The orientational correlation between the methylene-hydrogen atoms and neighboring D2O molecules is weak.