The Infrared Spectrum of Solid l-Alanine: Influence of pH-Induced Structural Changes

The influence of the pH on the infrared spectrum of l-alanine has been analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The amino acid was precipitated from aqueous solutions and dried at 36.5 °C, in order to stabilize cationic l-alanine or alaninium [CH3CH(NH3+)COOH] at pH 1, the zwitterionic form [CH3CH(NH3+)COO−] at pH 6, and anionic l-alanine or alaninate [CH3CH(NH2)COO−] at pH 13. New insight on the specific inter and intramolecular interactions in the different forms of l-alanine was reached by a novel methodological approach: an infrared technique not used before to analyze solid amino acid samples (DRIFTS), in combination with a detailed analysis based on spectral deconvolution. The frequency ranges of interest include the carbonyl/carboxyl stretching and amine deformation modes and the OH/NH stretching modes. It was shown that intermolecular hydrogen bonds between the NH3+ and COO− groups are predominant in the zwitterionic form, whereas in cationic l-alanine, H bonds between the COOH groups are responsible for the formation of dimers. In anionic l-alanine, only strong electrostatic interactions between the COO− groups and Na+ ions are proposed, evidencing the relevant role of the counterion.