The temperature-dependent single-crystal Raman spectroscopy of a model dipeptide: l-Alanyl-l-alanine

• Temperature dependent polarized Raman spectra of the l-alanyl-l-alanine.
• The crystal of l-alanyl-l-alanine undergoes a structural phase transition.
• The phase transition is observed between 80 and 60 K.

A single-crystal of peptide l-alanyl-l-alanine (C6H12N2O3) was studied by Raman spectroscopy at low-temperature, and a tentative assignment of the normal modes was given. Evidence of a second order structural phase transition was found through Raman spectroscopy between the temperatures of 80 K and 60 K. Group theory considerations suggest that the transition leads the sample from the tetragonal to a monoclinic structure. Additionally, our study suggests that the mechanism for the structural phase transition is governed by the occupation of non-equivalent C1 local symmetry sites by the CH3 molecular groups. Analysis based on group theory suggests l-alanyl-l-alanine presents C2 symmetry at low temperatures.

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