Helicity of short E-R/K peptides

Abstract

Understanding the secondary structure of peptides is important in protein folding, enzyme function, and peptide-based drug design. Previous studies of synthetic Ala-based peptides (>12 a.a.) have demonstrated the role for charged side chain interactions involving Glu/Lys or Glu/Arg spaced three (i, i + 3) or four (i, i + 4) residues apart. The secondary structure of short peptides (<9 a.a.), however, has not been investigated. In this study, the effect of repetitive Glu/Lys or Glu/Arg side chain interactions, giving rise to E-R/K helices, on the helicity of short peptides was examined using circular dichroism. Short E-R/K-based peptides show significant helix content. Peptides containing one or more E-R interactions display greater helicity than those with similar E-K interactions. Significant helicity is achieved in Arg-based E-R/K peptides eight, six, and five amino acids long. In these short peptides, each additional i + 3 and i + 4 salt bridge has substantial contribution to fractional helix content. The E-R/K peptides exhibit a strongly linear melt curve indicative of noncooperative folding. The significant helicity of these short peptides with predictable dependence on number, position, and type of side chain interactions makes them an important consideration in peptide design.

Figure 1

Arg versus Lys in E-R/K peptides. Circular dichroism spectra of (a) EK peptides 8, 16, and 24 residues long, and short E-R/K peptides (b) 8, (c) 6, and (d) 5 residues in length. The isodichroic point at 202 nm and the minima at 208 and 222 nm are characteristic of α-helices. (e) Helix content as a function of temperature (°C) at 222 nm from circular dichroism.