BIRADICALS AS ESR PROBES OF CONFORMATIONS IN MODEL BETA-TURN PEPTIDES

Electron spin-spin exchange interaction in biradicals has been examine d for its potential usefulness in the conformational analysis of pepti des in solution. Three peptides with high propensity to adopt beta-tur n conformations in solution, Boc-Cys-Pro-Xxx-Cys-NHMe (Xxx = Leu, 1; X xx = Aib, 2; Xxx = Tyr, 3), have been modified into biradicals by spin labeling the thiols groups. Analysis of electron spin resonance spect ra of these peptides in it variety of solvents and at different temper atures suggests that the population of folded conformations follows th e order 2 > 1 > 3. (C) 1995 Academic Press, Inc.