Perceptible isotopic effect in 3D-framework of α-glycine at low temperatures

Glycine, the most fundamental amino acid, albeit studied for many decades, has kept researchers captivated with interesting structural variations relevant to important biological, astrophysical and technological applications. We report here a noticeable effect of deuteration on the three dimensional hydrogen bonding network of α-glycine using low temperature infrared absorption studies in a wide spectral range, corroborated with Raman scattering studies. These systematic studies in the range 300-4.2 K have demonstrated a relatively compact assembly of glycine molecules in the three dimensional bilayered structure of hydrogenated glycine (gly-h) at low temperatures. This is inferred from a remarkable temperature effect in the weak intra-bilayer hydrogen bond ~ along the b-axis, which strengthens upon cooling. A pronounced increase in the intensity of NH₃ torsional and NH stretching modes has been observed. This is accompanied with a large rate of stiffening and softening respectively of these modes upon cooling and a change in slope across 210 K and 80 K. In contrast, the D---O hydrogen bond lengths in fully deuterated isotope (gly-d), as estimated using empirical correlation, show that the weak intra-bilayer hydrogen bond is not strengthened upon cooling down to 180 K, whereas the stronger intra-layer hydrogen bonds in the ac-plane become further strong. The ND₃ torsional vibrations show no temperature effect. This implies a relatively stable two dimensional layered structure formed by strongly hydrogen bonded glycine sheets in the ac-plane. Below 180 K, similar qualitative trends have been obtained for the hydrogen bond lengths in the two isotopes. In addition, temperature induced variation of the characteristic "indicator" band of zwitterionic gly-h and gly-d has also been reported.