Experimental attempt to simulate receptor site environment. A 500-MHz 1H nuclear magnetic resonance study of enkephalin amides.

The amides of Leu/sup 5/-enkephalin, Met/sup 5/-enkephalin, and three analogues, D-Ala/sup 2/, Leu /sup 5/-enkephalin, (AcO)Tyr/sup 1/, Met/sup 5/-enkephalin, and (AcO)Tyr/sup 1/,D-Ala/sup 2/, Met/sup 5/-enkephalin, have been studied by means of /sup 1/H NMR spectroscopy in two different solvent systems: Me/sub 2/SO-d/sub 6/ and CDC1/sub 3/. In the latter solvent the peptides were dissolved as complexes with 18-crown-6-ether, a coronand that binds strongly to the NH/sub 3//sup +/ groups. The crown ether complexation and the apolar solvent were used to simulate the anionic subsite of the receptor and they hydrophobic environment of the receptor cavity, respectively. The very unusual amide proton chemical shifts and their temperature coefficients suggest the presence of folded conformations in CDCl/sub 3/ for all peptides, consistent with several models of opioid receptors and with the crystal structure of Leu/sup 5/-enkephalin. The differences among the proposed cyclic conformations of the five peptides may be correlated, in part, with their different biological activity. All peptides in Me/sub 2/SO-d/sub 6/ are characterized by complex mixtures of extended fully solvated conformations.