Interaction of opioid peptides with model membranes. A carbon-13 nuclear magnetic study of enkephalin binding to phosphatidylserine.

The binding of enkephalin to phosphatidylserine has been studied, by using 13C NMR, as a model for interactions with components of biological receptors. Chemical shifts, line widths and spin-lattice relaxation times were measured for peptides enriched to 90% in 13C. The pKa values of the terminal amino and carboxyl groups were determined from the pH dependence of the 13C chemical shifts. Interaction of (2-[2-13C]glycine) methionine-enkephalin, (3-[2-13C]-glycine)methionine-enkaphalin, and (3-[2-13C]glycine)methionine-enkephalinamide with phosphatidylserine (PS) was studied as a function of pH. Salt and morphine antagonism to binding was manifest. Binding was shown to be pH dependent, exhibiting a maximum under slightly acidic conditions. Whereas the -NH3+ group of enkephalin is essential for binding, the data suggest that neither the tyrosyl hydroxyl group nor the COO- group is involved. Binding affects the 13C spin-lattice relaxation times most strongly; the chemical shifts and line widths of the 13C-enriched material show little perturbation in the presence of PS. The internal flexibility of the peptides is decreased, on binding to model membranes, by 1 order of magnitude. Dissociation constants have been measured as 4 X 10(-1) M and 2.6 X 10(-3) M for enkephalin and enkephalinamide, at pH 6.3 and 6.4, respectively.