Circular dichroism and ultraviolet resonance Raman indicate little Arg-Glu side chain α-helix peptide stabilization

Electrostatic interactions between side chains can control the conformation and folding of peptides and proteins. We used circular dichroism (CD) and ultraviolet (UV) resonance Raman spectroscopy (UVRR) to examine the impact of side chain charge on the conformations of two 21 residue mainly polyala peptides with a few Arg and Glu residues. We expected that attractions between Arg-10 and Glu-14 side chains would stabilize the α-helix conformation compared to a peptide with an Arg-14. Surprisingly, CD suggests that the peptide with the Glu-14 is less helical. In contrast, the UVRR show that these two peptides have similar α-helix content. We conclude that the peptide with Glu-14 has the same net α-helix content as the peptide with the Arg but has two α-helices of shorter length. Thus, side chain interactions between Arg-10 and Glu-14 have a minor impact on α-helix stability. The thermal melting of these two peptides is similar. However the Glu-14 peptide pH induced melting forms type III turn structures that form α-helix-turn-α-helix conformations.