Influence of Glu/Arg, Asp/Arg, and Glu/Lys Salt Bridges on a-Helical Stability and Folding Kinetics

Using a combination of ultraviolet circular dichroism, temperature-jump transient-infrared spectroscopy, and molecular dynamics simulations, we investigate the effect of salt bridges between different types of charged amino-acid residue pairs on α-helix folding. We determine the stability and the folding and unfolding rates of 12 alanine-based α-helical peptides, each of which has a nearly identical composition containing three pairs of positively and negatively charged residues (either Glu⁻/Arg⁺, Asp⁻/Arg⁺, or Glu⁻/Lys⁺). Within each set of peptides, the distance and order of the oppositely charged residues in the peptide sequence differ, such that they have different capabilities of forming salt bridges. Our results indicate that stabilizing salt bridges (in which the interacting residues are spaced and ordered such that they favor helix formation) speed up α-helix formation by up to 50% and slow down the unfolding of the α-helix, whereas salt bridges with an unfavorable geometry have the opposite effect. Comparing the peptides with different types of charge pairs, we observe that salt bridges between side chains of Glu⁻ and Arg⁺ are most favorable for the speed of folding, probably because of the larger conformational space of the salt-bridging Glu⁻/Arg⁺ rotamer pairs compared to Asp⁻/Arg⁺ and Glu⁻/Lys⁺. We speculate that the observed impact of salt bridges on the folding kinetics might explain why some proteins contain salt bridges that do not stabilize the final, folded conformation.