THEORETICAL ANALYSIS OF THE STRUCTURE AND VIBRATIONAL SPECTR OF ASPARAGINE AND GLUTAMIC AMINO ACIDS IN WATER AT DIFFERENT pH
Performed has been the calculation of the spectra of CU dicarboxylic aspartic and glutamic amino acids in aqueous solution at different pH, the structure of which may be in the form of cation (charge = 1), anion (charge = -2) and zwitterionic-ion with zero and negative unit charge. To account for intermolecular interactions of water molecules with molecules of aspartic and glutamic amino acids were used in the model reaction field to take into account the influence of van der Waals forces. Theoretical CD spectra of Asp and Glu at low and high pH values are in good agreement with the experimental spectra, confirming the existence of dicarboxylic amino acids in the form of protonated ions СООНСН2СНNН3+COOH, COOH(CH2)2СНNН3+COOH and anions COO-СН2СНNН2СОО<sup>-</sup>, COO-(CH2)2СНNН2СОО - for Asp and Glu, respectively. Made the interpretation of the vibrational spectra of CU Asp and Glu in the field of 1800-750 cm<sup>-1</sup> based on the analysis and comparison of intensities of CU lines and frequencies of normal vibrations of the theoretical and experimental spectra. It is shown that in the region of pH = 5…8 CD spectra of Asp and Glu are a superposition of the spectra of two structures of zwitterionic ion forms with zero and negative unit charge. For Asp, the contribution of these structures at pH = 7 is 65 and 35 %, respectively, while for Glu the experimental spectrum measured at pH = 7.5, is a superposition of spectra of the same structures with approximately equal percentages.