Exploring the peptide 310-helix ⇆ α–helix equilibrium with double label electron spin resonance

Over the last several years we have used spin labeling as a means for exploring the structure of helical peptides. Two nitroxide labels are engineered into a peptide sequence and distances are ranked with electron spin resonance (ESR). We have found that there is a significant amount of 3₁₀–helix in 16–residue model peptides containing only L –amino acids. This review covers several facets of the methodology including spin labeling strategy, interpretation of ESR spectra and the influence of molecular dynamics on the spectral line shapes. Also covered are recent findings of a length–dependent 3_l0-helix → α-helix transition and the role of Arg⁺ in the stabilization of specific helix structures. © 1994 John Wiley & Sons, Inc.