Conformational features responsible for binding of cyclic analogues of enkephalin to opioid receptors

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Abstract
Low-energy peptide backbone conformers were found by means of energy calculation for several cyclic analogues of enkephalin in an attempt to assess models for receptor-bound conformations for opioid receptors of the .mu.- and .delta.-types. They included [D-Cys2, L-Cys5]- and [D-Cys2, D-Cys5]-enkephalinamides showing moderate preference for .mu.-receptors, the .delta.-selective compounds [D-Pen2, L-Pen5] and [D-Pen2, D-Pen5]-enkephalins and Tyr-D-Lys-Gly-Phe-analogue possessing very high affinity to receptors of the .mu.-type. The low-energy conformers obtained for these analogues were in good agreement with the results of calculations by other authors and with experimental evidence. All of the analogues contain a Phe residue in position 4 of the peptide chain which facilitates the eventual search for geometrical similarity between the low-energy backbone conformers of different analogues in question.