Raman fingerprint of the interaction of K + with the COO − group of zwitterionic alanine

The interaction of K + with the zwitterionic form of alanine (ZAla) is investigated using Raman spectroscopy and density functional theory calculations. The Raman spectra of an aqueous solution of Ala and its mixture with KOH at different molar concentrations [ZAla +  x KOH, x  = 1–5 M] have been recorded in the spectral region 400–1800 cm −1 . The wavenumber position of the band at ~529 cm −1 shows a red shift of 14 cm −1 , while the Raman band at ~634 cm −1 shows a blue shift of 10 cm −1 with the increasing x from 1 to 5 M. The intensity ratio I 634 / I 529 is increased with increasing x , and it could be because of the increase in concentration of the [ZAla + K + ] complex in the solution. The new Raman band appeared at ~1079 cm −1 in the Raman spectra of [ZAla +  x KOH, x  = 1–5] complex. To determine the most probable site for the interaction of K + with ZAla, the structures of ZAla and the [ZAla + K + ] were optimized at B3LYP/6‐311++G(d,p) level of theory. The electrostatic potential calculation carried out for ZAla reveals that the maximum density of electron is lying over COO − , and therefore, COO − would be the most probable site for the interaction of K + with ZAla. The theoretically calculated Raman spectra of ZAla, [ZAla + K + ] and the [ZAla −  + K + ] are in good agreement with experimentally observed Raman spectra. Thus, the Raman bands at ~529, 634, and 1079 cm −1 may be used as the Raman fingerprint for the interaction of K + with COO − of the ZAla and ZAla − . Copyright © 2015 John Wiley & Sons, Ltd.