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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 36; pp. 8280 - 8287
• Main Authors: Garcia, Ana R, de Barros, Ricardo Brito, Lourenço, João P, Ilharco, Laura M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.09.2008
• Subjects: A: Kinetics, Spectroscopy; Alanine - chemistry; Aldehydes - chemistry; Amines - chemistry; Anions - chemistry; Carboxylic Acids - chemistry; Cations - chemistry; Chemical Precipitation; Dimerization; Hydrogen Bonding; Hydrogen-Ion Concentration; Ketones - chemistry; Neutron Diffraction - methods; Sodium - chemistry; Solutions - chemistry; Spectroscopy, Fourier Transform Infrared - methods; Static Electricity; Temperature; Water - chemistry; X-Ray Diffraction - methods
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp802170n

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.