Biradicals as ESR Probes of Conformations in Model β-Turn Peptides

Electron spin-spin exchange interaction in biradicals has been examined for its potential usefulness in the conformational analysis of peptides in solution. Three peptides with high propensity to adopt β-turn conformations in solution, Boc-Cys-Pro-Xxx-Cys-NHMe (Xxx = Leu, 1; Xxx = Aib, 2; Xxx = Tyr, 3), have been modified into biradicals by spin labeling the thiols groups. Analysis of electron spin resonance spectra of these peptides in it variety of solvents and at different temperatures suggests that the population of folded conformations follows the order 2 >1 >3.