Conformational exchange on the microsecond time scale in alpha-helix and beta-hairpin peptides measured by C-13 NMR transverse relaxation

C-13-NMR relaxation experiments (T-1, T-2, T-1 rho, and NOE) were performed on selectively enriched residues in two peptides, one hydrophobic staple a lpha -helix-forming peptide GFSKAELAKARAAKRGGY and one beta -hairpin-formin g peptide RGITVNGKTYGR, in water and in water/trifluoroethanol (TFE). Excha nge contributions, R-ex, to spin-spin relaxation rates for C-13(alpha) and C-13(beta) groups were derived and were ascribed to be mainly due to peptid e folding-unfolding. To evaluate the exchange time, tau (ex), from R-ex, th e chemical shift difference between folded and unfolded states, Delta delta , and the populations of these states, p(i), were determined from the tempe rature dependence of C-13 chemical shifts. For both peptides, values for ta u (ex) fell in the 1 mus to 10 mus range. Under conditions where the peptid es are most folded (water/TFE, 5 degreesC), tau (ex) values for all residue s in each respective peptide were essentially the same, supporting the pres ence of a global folding-unfolding exchange process. Rounded-up average tau (ex) values were 4 mus for the helix peptide and 9 mus for the hairpin pep tide. This 2-3-fold difference in exchange times between helix and hairpin peptides is consistent with that observed for folding-unfolding of other sm all peptides.