Int. J. Peptide zyxwvutsr Protein Res. zyxwvuts 29, zyxwvutsr 1987, zyxwvutsr 282-288 H-n.m.r. investigation of conformational features of cyclic, penicillamine-containing enkephalin analogs HENRY I. MOSBERG College of Pharmacy, University ofMichigan, Ann Arbor, MI, USA zyx Received 9 June, accepted for publication 2 July 1986 Conformational features of a series of cyclic, penicillamine-containing enkephalin analogs, all of which display selectivity for the delta opioid receptor, were studied by zyxwvut 'H n.m.r. in aqueous solution. Comparison of chemical shifts, coupling constants, and temperature dependence of amide proton chemical shifts suggests different conformational features among the analogs, some of which can be related to the different primary sequences of these peptides. The observation that some of the analogs display disparate individual conformational features while exhibiting similar opioid potency and receptor selectivity suggests that such analogs may share a similar overall topography or at the least maintain the same relative orientations of key portions of the molecule. Key words: conformation;enkephalins; 'H n.m.r.; opioids; peptides Many endogenous peptide hormones and neurotransmitters are relatively small, flexible molecules that can utilize their inherent flexi- bility to interact with different subclasses of receptors which mediate different physio- logical events and which presumably place different conformational requirements upon the peptide ligand. In order to elucidate the molecular mechanism of action of a particular peptide hormone or neurotransmitter it is necessary to unravel the distinct actions mediated by the individual receptor subclasses and to determine the bioactive conformation of the peptide ligand at each of these receptors. A particularly useful approach toward these ends is the design and synthesis of analogs of the native peptide into which conformational restrictions are incorporated. One benefit of this approach is that the proper choice of conformational restriction can result in an analog able to assume the conformational 282 requirements for interaction with one subclass of receptor but not other subclasses. Such highly receptor selective analogs can then be used to determine the physiological actions mediated by distinct receptor subclasses. An additional benefit of conformationally restricted analogs is that while the conformational analysis of flexible peptides is hampered by dynamic averaging of conformation dependent spectro- scopic parameters, leading to the determination of an average solution conformation of dubious physical or biological significance, more rigid analogs do not share this liability. Thus a conformationally restricted analog can be expected to assume a more well defined solution conformation and allow a more reliable extra- polation to the active, receptor bound con- formation. For conformationally restricted, receptor selective analogs this allows the determination of the bioactive conformation at specific receptor subclasses.