Orientational Adaptations Leading to Plausible Phase Transitions in l-Leucine at Low Temperatures: Revealed by...

Hydrogen bonding is essential for the stability of amino acids. A change in the geometry and conformation of hydrogen bonds in such molecular systems, for example, under varying thermodynamic conditions of temperature/pressure, may lead to subtle or drastic phase transitions. We demonstrate here the mechanism of temperature-induced phase transitions in the polycrystalline solid sample of l-leucine [(CH₃)₂-C(₄)H-C(₃)H₂-C(₂)H(C(₁)OO^-)(NH₃⁺)], an "essential" amino acid, using in situ Fourier transform infrared spectroscopy in the temperature range 300-4.3 K. Unambiguous spectral signatures of preferred microstructural changes have been reported, which are linked to phase transitions at ∼150 and ∼240 K. The transition at 150 K is found to be associated with a sudden change in reorientation dynamics of the torsional vibrations of the (C₃C₄) group. In contrast, the transition at 240 K is associated with the conformational distortions in the NH₃ group, which causes strengthening of the hydrogen bonds in the ac-plane forming two-dimensional sheets, well separated from each other in the b-direction. These findings pave the way toward settling the long-standing debate on the temperature-induced behavior of l-leucine as well as harnessing its physicochemical properties.