Fluorescence study on the conformation of a cyclic enkephalin analog in aqueous solution

Conformational parameters of the potent cyclic enkephalin analogs [D-Cys2, D-Cys5]enkephalinamide and [D-Cys2, Trp4, D-Cys5]enkephalinamide were determined by steady-state fluorescence measurements. The very low tyrosine fluorescence quantum yield observed with [D-Cys2, D-Cys5]enkephalinamide reflects a close intramolecular interaction between the phenol ring of Tyr1 and the disulfide bridge. Evaluation of resonance energy transfer between Tyr1 (donor) and Trp4 (acceptor) of [D-Cys2, Trp4, D-Cys5]enkephalinamide on the basis of Förster's equation resulted in an average intramolecular distance of 9.7 +/- 0.2 A between the two aromatic rings. This mean distance is nearly identical with the average Tyr1 - Trp4 distance (9.5 +/- 0.3 A) previously determined with the linear analog [D-Ala2, Trp4, Met5]enkephalin.