Conformational characteristics of receptor-selective opioid peptides. 1H n.m.r. and c.d. spectroscopic studies of δ-kephalin and [Val4]morphiceptin

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Abstract
An investigation on the conformations of highly receptor-selective opioid peptides was carried out to gain further understanding of the structure-activity relationship of endogenous enkephalins. The preferred conformations of a highly .mu.-selective [Val4]morphiceptin and a highly .delta.-selective .delta.-kephalin have been probed by 1H n.m.r. solvent-, concentration- and temperature-dependences of amide protons to take the folded conformations stablized by an intramolecular hydrogen bond and the anti-parallely extended dimeric structures respectively. Their possible stereo-conformations were proposed, based on the analyses of the vicinal coupling constants (JHNC.alpha.H). The conformational difference between the .mu.- and .delta.-selective opioid peptides was further ascertained by the c.d. measurements. The c.d. spectra of the .mu.-selective peptides show negative bands in the range of 210-230 nm, while those of the .delta.-selective ones show the opposite positive bands. A correlation between c.d. spectra and receptor-selectivity was possible.