Synthesis, opiate receptor affinity, and conformational parameters of [4-tryptophan]enkephalin analogs

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Abstract
A series of analogs of the opioid peptide enkephalin with Tp substituted for phenylalanine in position 4 was synthesized by the solid-phase method. The [Trp4]enkephalin analogs and the corresponding [Phe4]enkephalin analogs displayed nearly parallel affinities in the opiate receptor binding assay in rats throughout the series. In a conformational study fluorescence parameters were measured and intramolecular Tyr-Trp distances were estimated on the basis of resonance energy transfer experiments. No gross conformational differences were observed between analogs with widely differing opiate receptor affinity; however, small but significant changes in the intramolecular distance between the phenol ring and the indole moiety and/or in their relative orientation became apparent in some compounds. Identical intramolecular distances of 9.3 .+-. 0.2 .ANG. between the 2 aromatic rings were obtained with [Trp4,Met5]enkephalin, [Trp4,Leu5]enkephalin, and the N-terminal tetrapeptide comprised in the latter 2 analogs, indicating the existence of folded conformations in 2 .times. 10-5 M aqueous solution and demonstrating conformational analogy between these 3 peptides. The conformational parameters were discussed in relation to the affinities and the putative opiate receptor topography.