Conformational studies of dansylated enkephalins by fluorescence transfer measurements, proton nuclear magnetic resonance spectroscopy, and theoretical calculations

Dansylated on their C-terminal part, enkephalins retain all the biological properties of their parent compounds and would be useful fluorescent probes to study the enkephalin-receptor recognition process or interactions between these peptides and their degrading enzymes. Such explorations require in a first step a conformational analysis of these compounds in both aqueous and less polar solvents. This was performed by using three comparative methods: fluorescence energy transfer using the dansyl group (Dns) as an acceptor, 1H NMR, and theoretical conformational analysis. The fluorescent enkephalins, Tyr1-Gly2-Gly3-Phe4-Met 5-NH-(CH2)2-NH-Dns (1), its D-Ala2 derivative 2, and a tetrapeptide analogue, 3, lacking the Met5 residue exhibit a large energy transfer from Tyr to Dns with a strong additional quenching of the tyrosine fluorescence in H2O. Therefore, the distance r1 between the two luminophores was computed from acceptor fluorescence. The folding tendency of the three compounds evidenced by short Tyr-Dns distances (r1 = 12.4-14.9 Å) increases from H2O to trifluoroethanol (TFE) with in the case of 2 a stacking interaction between Tyr and Dns in TFE. These features are supported in both solvents by selective shielding of the proton chemical shifts in 1 and 2 as compared to their precursors without the Dns ring. All these results agree with intramolecular distances computed for a statistical model of 1 in H2O and a flexible β-turn model for 1 in TFE and 2 in both solvents. In all the solvents used, dansylated enkephalins remain very flexible and exhibit computed average distances between Tyr1 and Phe4 (10-11 Å) near those determined in biologically active Trp4-Met5-enkephalin analogues and in the highly potent morphine derivative 7α-[(R)-1-hydroxy-1-methyl-3-phenylpropyl]-6,14-endo- ethenotetrahydrooripavine [Schiller, P. W. & St. Hilaire, J. (1980) J. Med. Chem. 23, 290-294]. These features associated with a folding tendency of these fluorescent peptides allow their binding to μ receptors probably through a trans conformational process. The significant δ selectivity of 1 and 2 is in accordance with the lengthening of the peptide sequence whereas the loss of activity of 3 confirms the crucial role attributed to the simultaneous presence of a D2-amino acid and a fifth residue for the biological activity [Fournié-Zaluski, M. C. Gacel, G. Maigret, B. Premilat, S. & Roques, B. P. (1981) Mol. Pharmacol. (in press)]. © 1981 American Chemical Society.SCOPUS: ar.j