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 isotopi c substitution technique has been applied to both nitrogen and hydrogen ato ms within glycine molecules in order to obtain information concerning the h ydration structure around the amino- and methylene group in the glycine mol ecule 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 (r(N ... D) = 1.97(3) Angstrom, angle N ... D-O = 174(5)degre es), and simultaneously forms bonds of N-D ... OD2 type with ca. two D2O mo lecules (r(N ... O) = 2.92(3) Angstrom) in the present alkaline aqueous sol ution. This behavior is in contrast to the hydration structure around the a mino group in the glycine molecule reported for the neutral aqueous solutio n, in which the amino group forms the hydrogen bonds of N-D ... OD2 type wi th ca. three D2O molecules. The first hydration shell around the methylene group in the glycine molecule involves ca. two D2O molecules per one glycin e molecule. The orientational correlation between the methylene-hydrogen at oms and neighboring D2O molecules is weak.