Nuclear magnetic resonance studies of peptides and their interactions with receptors.

Peptides are flexible The experimental parameters measured in n.m.r. studies, such as nuclear Overhauser effects (NOEs), coupling constants and chemical shifts, are all time averaged [ 11. NOEs are averaged by any process occurring faster than the cross-relaxation rate [2], which is usually in the range 0.1-50 s-'. Coupling constants are averaged by processes that occur faster than the coupling (up to 20 Hz for three-bond coupling or 200 Hz for one-bond coupling). Chemical-shift averaging (a process that leads to exchange between two sites with different chemical shifts) leads to coalescence of exchanging signals when the exchange rate is of the same order as the chemical shift difference, and is undetectable when the exchange rate is about one order of magnitude faster [ 31. Thus, if the exchanging signals are 1 ppm apart at 500 MHz, exchange causes line broadening at an exchange rate of about lo4 s ' or slower. Thus, any exchange process that occurs at a rate greater than lo4 s' is undetectable using any easily measured n.m.r. parameter. A rate of lo4 s-' corresponds to an activation energy of 12 kcal molI (50 kJ molI ) at room temperature, using simple Arrhenius activation theory. This implies that any system that involves activation energy barriers of less than 12 kcal mol-' will not be detectable as exchanging. Rotations around single bonds have a