Conformational Exchange on the Microsecond Time Scale in α-Helix and β-Hairpin Peptides Measured by 13C NMR Transverse Relaxation†

13C-NMR relaxation experiments (T1, T2, T1ρ, and NOE) were performed on selectively enriched residues in two peptides, one hydrophobic staple α-helix-forming peptide GFSKAELAKARAAKRGGY and one β-hairpin-forming peptide RGITVNGKTYGR, in water and in water/trifluoroethanol (TFE). Exchange contributions, Rex, to spin−spin relaxation rates for 13Cα and 13Cβ groups were derived and were ascribed to be mainly due to peptide folding−unfolding. To evaluate the exchange time, τex, from Rex, the chemical shift difference between folded and unfolded states, Δδ, and the populations of these states, pi, were determined from the temperature dependence of 13C chemical shifts. For both peptides, values for τex fell in the 1 μs to 10 μs range. Under conditions where the peptides are most folded (water/TFE, 5 °C), τex values for all residues in each respective peptide were essentially the same, supporting the presence of a global folding−unfolding exchange process. Rounded-up average τex values were 4 μs for the helix pept...