DEUTERIUM NMR RELAXATION STUDIES OF PEPTIDE-LIPID INTERACTIONS

A unique model membrane system composed of a synthetic amphiphilic peptide (Lys2-Gly-Leu16-Lys2-Ala-amide) and a specifically labeled phospholipid (1,2-[7,7-H-2(2)]dipalmitoyl-sn-glycero-3-phosphocholine) has been studied by H-2 NMR, using inversion recovery, quadrupolar echo, and modified Jeener-Broekaert sequences, from 213 to 333 K, at molar peptide concentrations of 0,2,4, and 6%. Analysis of the experiments, employing a density matrix treatment based on the stochastic Liouville equation, revealed information about the dynamic organization of the lipid in the model membrane system, whose phase behavior has been determined previously [Huschilt et al. (1985) Biochemistry 24, 1377-1386]. The dynamic organization is described in terms of segmental and molecular order parameters and in terms of correlation times corresponding to both internal and overall lipid motions. In the liquid crystalline phase, the molecular order parameter, S(ZZ), was observed to decrease slightly upon addition of peptide while the conformational order parameter corresponding to the seventh segment, S(Z'Z'), did not change for any concentration of peptide. In general, the gauche-trans isomerization rate in the middle of the chain was not observed to change upon peptide addition, whereas the whole body reorientational correlation times (tau(R)parallel-to and tau(R)perpendicular-to) increased by nearly an order of magnitude. The anisotropy ratio (tau(R)perpendicular-to/tau(R)parallel-to) decreased with peptide added. An additional motion which involves a jump about the axis of the sn-2 chain is also observed to be slowed down significantly in the presence of peptide. Assuming T2 is dominated by order director fluctuations in the liquid crystalline phase, we conclude that peptide decreases the effective membrane elastic constant. In the gel state, the jumping process of the sn-2 chain is observed to slow down slightly while the whole body correlation times (tau(R)parallel-to and tau(R)perpendicular-to) decrease with peptide added. In one sense, the peptide serves to decrease the differences between the fluid and gel states.