On the location of β‐turns

Abstract β‐Turns are a common feature of cyclic peptides, but judging from recent x‐ray and solution studies of cyclic hexapeptides it is not always possible to predict in advance the type of turn and the position of the turns in the sequence. Two or more backbone conformations containing β‐turns may be of comparable energy and in rapid solvent‐ and temperature‐dependent equilibrium in solution. The use of differential relaxation effects produced by a nitroxyl radical to locate β‐turns with only minor perturbation of such equilibria is noted. Examination of the effect of a nitroxyl on the N‐H resonances of the decapeptide hormone luteinizing hormone releasing hormone supports a dominant conformation with a β‐turn at Gly6‐Leu7. Although this turn is probably part of the biologically active conformation, it is not obvious in the more active [D‐Ala6] analog.