Low-temperature solid-state FTIR study of glycine, sarcosine and N,N-dimethylglycine: observation of neutral forms of simple α-amino acids in the solid state

Abstract

Neutral forms of glycine and their N-methylated derivatives, sarcosine (N-methylglycine) and N,N-dimethylglycine were, for the first time, observed in the solid state pure compounds. The substances were sublimated under high vacuum, quickly deposited onto a cold CsI substrate at 9 K and examined using FTIR spectroscopy within the temperature range 9–300 K. For all the compounds studied, the spectra obtained at 9 K after deposition revealed the presence of both the neutral and zwitterionic amino acid forms. The identification of the neutral forms and their vibrational signatures could be unequivocally established by taking into consideration the available data for the matrix-isolated neutral monomeric and aggregated amino acids as well as solid state data for acid and alkaline salts of glycine. Upon temperature increase, neutral forms convert non-reversibly to the zwitterionic forms, as noticed by the systematic increase in the relative intensities of the infrared bands ascribable to the latter species. Observation of aggregates of neutral amino acids in the solid phase, together with the fact that the amount of the neutral forms observed reduces with increasing both the temperatures of the cold substrate and of the vapour prior to deposition, enabled to conclude that the observed (neutral form)→zwitterion conversion occurs in the solid state, to the therein most stable zwitterionic species, and that intermolecular interactions in this phase should play an important role in the mechanism of proton transfer.