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The infrared spectrum of solid L-alanine: influence of pH-induced structural changes
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Resumo(s)
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 COOgroups and Na+ ions are proposed, evidencing the relevant role of the counterion.
Descrição
Palavras-chave
L-alanine Structural changes FTIR
Contexto Educativo
Citação
Garcia, A. R.; de Barros, R. B.; Lourenço, J. P.; Ilharco, L. The infrared spectrum of solid L-alanine: influence of pH-induced structural changes, J. Phys. Chem. A, 112, 8280-8287, 2008.