1H n.m.r. investigation of conformational features of cyclic enkephalinamide analogs

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Abstract
The conformational basis for the differing opioid receptor selectivities of the cyclic cystine-containing analogs, [D-Cys2, D(or L)-Cys5] enkephalinamide and the related penicillamine-containing analogs, [D-Pen2, D(or L)-Cys5] enkephalinamide (penicillamine = .beta.,.beta. dimethylcysteine) was investigated by 1H NMR in aqueous solution. Comparison of chemical shift, temperature dependence of amide proton chemical shift, and coupling constant data suggests similar overall conformations for corresponding penicillamine- and cystine-containing analogs. Differences in conformation and flexibility do appear in the carboxamide terminal region of the corresponding analogs, which may account for their selectivities for different classes of opioid receptors.