270-MHz 1H nuclear-magnetic-resonance study of met-enkephalin in solvent mixtures. Conformational transition from dimethylsulphoxide to water.

1H spectra at 270 MHz of zwitterionic Met-enkephalin pentapeptide (Tyr-Gly-Gly-Phe-Met) in (C2H3)2SO/water mixtures are reported and discussed in terms of solvent-induced conformational transitions. The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around chi 1 and chi 2 suggests that the conformational properties of Met-enkephalin in the two solvents are quite different. In aqueous solution, the preferred structure, characterized by the absence of intramolecularly hydrogen-bonded NH groups and head-to-tail interactions, very likely is an equilibrium of unfolded conformations with approximately equal energy. In (C2H3)2SO, the preferred structure is folded, with the Met-5 NH intramolecularly bonded and the Gly-3 NH protected from the solvent, while the Gly-2 and Phe-4 amide protons are solvent exposed. A conformational transition of Met-enkephalin from the intramolecularly bonded to the unbonded one takes place at about 40 mol-% water in (C2H3)2SO, involving the Met-5 NH proton and the Tyr-Gly-Gly fragment. The Phe-4 and Met-5 phi angles do not change appreciably, which suggest that an inversion at the Gly-3 residue of the folded form, responsible for the conformational transition, does not affect the C-terminal moiety. At about 70 mol-% water in (C2H3)2SO a change in the solvent mixture properties affects the chi 1 rotamer populations and the ring dynamics of the aromatic side chains. The line broadening of the Tyr-1 delta and epsilon proton resonances indicates a specific interaction of the N-terminal ring with the solvent.