Electrostatically induced pK_a shifts in oligopeptides: the upshot of neighboring side chains

pK_a values of homorepeat hexapeptides with a 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) chromophore attached at the peptide C termini, through an asparagine derivative (Dbo), namely His₆-Dbo (H₆), Lys₆-Dbo (K₆), and Arg₆-Dbo (R₆), were determined by a novel fluorescence-based method. The fluorescence lifetime of Dbo in the peptides (τ) was measured as a function of pH. The side chains collide with Dbo intramolecularly and quench it efficiently only when they are deprotonated (i.e., pH ≥ side chain pK_a). The pK_a values of the H₆, K₆, and R₆ peptides, attributable to side chain ionization, were found to be depressed compared to the pK_a values of the His, Lys, and Arg residues in their free amino acid forms. We further looked into the structural changes of the peptides by molecular dynamics (MD) simulations; the peptides were structurally more expanded when their side chains are protonated. The structural expansion of the peptides reflects an electrostatic repulsion between the protonated side chain residues, which also accounts for the observed decrease in pK_a values, which corresponds to a facilitated deprotonation, assisted by electrostatic repulsion.