title: The infrared spectrum of solid L-alanine: influence of pH-induced structural changes.
abstract: 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 degrees 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.
authors.affiliation: Centro de Química-Física Molecular, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal.
authors.forename: Ana R
authors.initials: AR
authors.lastname: Garcia
authors.forename: Ricardo Brito
authors.initials: RB
authors.lastname: de Barros
authors.forename: João P
authors.initials: JP
authors.lastname: Lourenço
authors.forename: Laura M
authors.initials: LM
authors.lastname: Ilharco
mesh_terms: D000409:Alanine / Q000737:chemistry*; D000447:Aldehydes / Q000737:chemistry; D000588:Amines / Q000737:chemistry; D000838:Anions / Q000737:chemistry; D002264:Carboxylic Acids / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D011232:Chemical Precipitation; D019281:Dimerization; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007659:Ketones / Q000737:chemistry; D033363:Neutron Diffraction / Q000379:methods; D012964:Sodium / Q000737:chemistry; D012996:Solutions / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*; D055672:Static Electricity; D013696:Temperature; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D000447:Aldehydes; D000588:Amines; D000838:Anions; D002264:Carboxylic Acids; D002412:Cations; D007659:Ketones; D012996:Solutions; D014867:Water; D012964:Sodium; D000409:Alanine