Interaction of native and aggregated albumin with DMPC bilayers

The study of protein-lipid interaction offers interesting insights into the mutual alterations determined in the formation of the supramolecular complex. It gains even more interest, not only in basic research but also in biomedical and biomaterial applications, when protein aggregation and fibril formation are involved. In this study, the reciprocal influence of human serum albumin (HSA), in both the native and the thermally aggregated state, and dimyristoylphosphatidylcholine (DMPC) bilayers is investigated by combining UV-Vis scattering, attenuated total reflection Fourier transform infrared (ATR-FTIR), and spin-label electron paramagnetic resonance (EPR) spectroscopies. Temperature-dependent optical density at fixed wavelength reveals the pre- and the main phase transitions in DMPC bilayers as well as the onset of protein aggregation at T_agg ≈ 70 °C. In native protein/lipid complexes, the protein adsorption on the membrane surfaces suppresses the pre-transition and downshifts the temperature of the main phase transitions of DMPC, whereas the presence of DMPC increases T_agg without affecting the thermal profile. Kinetics experiments reveal that lipid bilayers reduce the thermally-induced aggregation of the protein. ATR-FTIR data indicate that albumin weakens the hydrogen bonding network at the carbonyl groups of the membrane. Conversely, lipid bilayers in any physical state do not alter the structural features of both native and aggregated HSA. In protein/lipid complexes, spin-label EPR of the lipid component reveals that the proteins reduce the packing density of the first chain segments and stabilize the fluid state, the effect being more evident for the native protein.