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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