Structure and orientation study of fusion peptide FP23 of gp41 from HIV-1 alone or inserted into various lipid membrane models (mono-, bi- and multibi-layers) by FT-IR spectroscopies and Brewster angle microscopy

In the present work, we study the structure and the orientation of the 23 N-terminal peptide of the HIV-1 gp 41 protein (AVGIGALFLGFLGAAGSTMGARS) called FP23. The behaviour of FP23 was investigated alone at the air/water interface and inserted into various lipid model systems: in monolayer or multibilayers of a DOPC/cholesterol/DOPE/DOPG (6/5/3/2) and in a DMPC bilayer. PMIRRAS and polarized ATR spectroscopy coupled with Brewster angle microscopy and spectral simulations were used to precisely determine the structure and the orientation of the peptide in its environment as well as the lipid perturbations induced by the FP23 insertion. The infra-red results show the structural polymorphism of the FP23 and its ability to transit quasi irreversibly from an α-helix to antiparallel β-sheets. At the air/water interface, the transition is induced by compression of the peptide alone and is modulated by compression and lipid to peptide ratio (Ri) when FP23 is inserted into a lipid monolayer. In multibilayers and in a single bilayer, there is coexistence in quasi equal proportions of α-helix and antiparallel β-sheets of FP23 at low peptide content (Ri = 100, 200) while antiparallel β-sheets are predominant at high FP23 concentration (Ri = 50). In (multi)bilayer systems, evaluation of dichroic ratios and sprectral simulations show that both the α-helix and the antiparallel β-sheets are tilted at diluted FP23 concentrations (tilt angle of α-helix with respect to the normal of the interface = 36.5 ± 3.0° for FP23 in multibilayers of DOPC/Chol/DOPE/DOPG at Ri = 200 and 39.0 ± 5.0° in a single bilayer of DMPC at Ri = 100 and tilt angle of the β-sheets = 36.0 ± 2.0° for the β-sheets in multibilayers and 30.0 ± 2.0° in the lipid bilayer). In parallel, the FP23 induces an increase of the lipid chain disorder which shows both by an increase of the methylene stretching frequencies and an increase of the average C-C-C angle of the acyl chains. At high FP23 content (Ri = 50), the antiparallel β-sheets induce a complete disorganization of the lipid chains in (multi)bilayers.