Solid L-α-alanine: spectroscopic properties and theoretical calculations

A spectroscopic study of L-α-alanine in different environments is presented, with special emphasis on the neutral to zwitterion transformation of this amino acid. Spectra of room temperature solids as deposited from the vapour and in KBr pellets are compared and discussed revealing some discrepancies. An assignment is proposed based on theoretical calculations of the solid structure. Vapour deposits at 25 K are prepared both of pure alanine and of mixtures with polar (H2O) and non-polar (CO2, CH4) components. The spectra of the solids contain IR bands which can be individually assigned to the neutral and to the zwitterion, and whose relative intensity variation can be used to follow the neutral to zwitterion transformation. The assignment of the spectrum of the neutral species is proposed, again with help from theoretical calculations of the single neutral molecule. The neutral/zwitterion ratio in deposits at 25 K varies between ~60 % for pure alanine and H2O mixtures and ~90 % for non-polar matrices. This ratio drops when the solid is heated until the ionic species only remains at 200 K. The conversion process depends on the environment surrounding the alanine molecules.