A fluorescence and CD study on the interaction of synthetic lipophilic hepatitis B virus preS(120-145) peptide analogues with phospholipid vesicles.

The interaction of the immunogenic peptide of human hepatitis B virus (HBV) preS(120-145), including B and T epitopes, with phospholipid vesicles has been studied by fluorescence techniques and CD. In addition, interaction of three lipopeptides derived from preS(120-145) containing stearoyl, cholanoyl, and tripalmitoyl-S-glyceryl-cysteine (Pam3C) SS moieties with dipalmitoylphosphatidylcholine (DPPC) has been investigated by polarization fluorescence spectroscopy. Fluorescence experiments showed an increase in fluorescence intensity and a blue shift of the maximum emission wavelength upon interaction of preS(120-145) with DPPC vesicles below the transition temperature (Tc), indicating that the tryptophan moiety enters a more hydrophobic environment. Moreover, fluorescence polarization experiments showed that the peptide decreased the membrane fluidity at the hydrophobic core, increasing the Tc of the lipid and decreasing the amplitude of the change of fluorescence polarization associated with the cooperative melting of 1,6-diphenyl-1,3,5-hexatriene labeled vesicles. The absence of leakage of vesicle-entrapped carboxyfluorescein indicates that the peptide did not promote vesicle lysis. Besides, the three lipopeptides derived from preS(120-145) showed a more pronounced rigidifying effect at the hydrophobic core of the bilayer, with a significative increase in the Tc. Stearoyl- and cholanoyl-preS(120-145) restricted the motion of lipids also at the polar surface, whereas Pam3CSS-preS(120-145) did not alter the polar head group order. Finally, CD studies in 2,2,2-trifluoroethanol or in presence of vesicles suggested that the bound peptide adopted amphiphilic alpha-helical and beta-sheet structures, with an important contribution of the beta-turn. It is concluded that preS(120-145) can interact with the lipid membrane through the formation of an amphipathic structure combination of beta-sheet and alpha-helix aligned parallel to the membrane surface, involving the N-terminal residues, and penetrating only a short distance into the hydrophobic core. The C-terminal part, with a combination of beta-turn and beta-sheet structure, remains at the outer part of the bilayer, being potentially accessible to immunocompetent cells. Furthermore, coupling of an hydrophobic moiety to the N-terminal part of the peptide favors anchoring to the membrane, probably facilitating interaction of the peptide with the immunoglobulin receptor. These results are in agreement with the induction of immune response by preS(120-145) and with the enhanced immunogenicity found in general for lipid-conjugated immunopeptides.