Infrared spectrum analysis of the dissociated states of simple amino acids

•Systematic subtraction of solution spectrum to create solvated amino acid spectrum.•pH effects on solvated amino acid spectra.•Peak deconvolution to interpret apparent peak shift with increasing pH.•pKa value evaluation from deconvoluted absorption-peak areas at selected pH.

In this work, we present detailed analyses of the dissociation of dilute aqueous solutions of glycine and of lysine over the range 1 < pH < 12. Using appropriate spectrum subtraction methods, we obtained ATR-IR spectra of the solvated species as a function of pH. Discernible changes in the ionic species were identified in the absorption region between 1800 and 1100 cm−1. By applying peak deconvolution techniques to the spectra, we correctly interpret the apparent peak shift from 1615 to 1600 cm−1 as being due to the receding NH3+ asymmetric deformation alongside the appearing COO− asymmetric stretching. The effect of aqueous solution environment was also investigated in terms of 10 and 100 mmol/L NaCl. Salt solution spectra at each pH were also subtracted from each solution phase spectrum. Analysis of the deconvoluted peak areas due to CO and COO− at pH ranges < 4.5 and those due to NH2NH2 and NH3+ for pH > 8 resulted in consistent pKa values for the amino acids.

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