Low Voltage Voltammetry Probes Proton Dissociation Equilibria of Amino Acids and Peptides br

Platinum-catalyzed electrochemical reduction of dissociable protons at low potentials was used to investigate proton dissociation equilibria of freely diffusing and peptide-incorporated charged amino acids. We first demonstrate with fivecharged essential amino acids and their analogs that the electrochemically induceddeprotonation of each amino acid occurs at distinct formal reduction potential. Moreover, the observed direct reduction for all the charged species, excludingarginine, occurs at low potentials suitable for investigation under aqueous conditions(-0.4 to-0.9 V vs Ag/AgCl). The direct proton reduction was resolved viadeconvolution of the observed differential pulse voltammogram (DPV) frombackground hydronium reduction and water electrolysis. A linear correlation wasfound between the formal reduction potentials and the pKavalues of the dissociableprotons hosted by various molecular moieties in the amino acids and their analogsand further verified with tripeptides. DPV of poly(L-lysine) decamer (Lys10)distinctively resolved the pKavalues of the amino groups in the side chains and N-terminus, at a resolution not possible by conventional acid-base titration. This work demonstrates selective electrochemical titration of dissociable protons in charged amino acids in the free state and as residues in biomolecules, as well as the utility of DPV to indirectly interrogate local electrostatic environments that are essential to the stability and function of biomolecules.