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

pKa 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 pKa). The pKa values of the H6, K6, and R6 peptides, attributable to side chain ionization, were found to be depressed compared to the pKa 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 pKa values, which corresponds to a facilitated deprotonation, assisted by electrostatic repulsion.