Thermodynamics of α-lactalbumin-dl-α-dipalmitoylphosphatidylcholine interactions and effect of the antioxidant nicotinamide on these interactions

Differential scanning calorimetry has been used to understand the thermodynamics of the interactions of dl-α-dipalmitoylphosphatidylcholine (DPPC) with α-lactalbumin and the effect of the antioxidant nicotinamide on these interactions. Nicotinamide decreases the thermal transition temperature of both the lipid and the protein at high concentrations. The thermal unfolding transitions of the protein were two state and calorimetrically reversible. There was no significant change in the shape and thermodynamic parameters accompanying the lipid endotherms, suggesting that nicotinamide did not penetrate the lipid bilayer. The thermal unfoldings of α-lactalbumin in the presence of DPPC as cosolute also adhered to two-state reversible mechanism. The changes in the thermodynamic parameters accompanying the thermal transitions were small, indicating no significant interaction of α-lactalbumin with DPPC. The changes in the thermodynamic parameters indicate that the lipid bilayer organization, as well as the partitioning of the extrinsic protein α-lactalbumin into the bilayer, is not affected in the entire studied concentration range of the lipid. It is observed that the presence of increasing concentration of nicotinamide (as high as 1.0 mol dm−3) in the lipid-protein mixture does not affect its partitioning into the lipid bilayer, although nicotinamide preferentially interacts with α-lactalbumin. The change in the effect of nicotinamide on lipid transition temperature in the mixture and literature report suggests that nicotinamide may be forming a hydrogen-bonded complex with the protein through its amide functionality. The surface tension data of aqueous nicotinamide in combination with the thermal denaturation results of protein in presence of nicotinamide confirmed that surface tension effect does not have any significant contribution to the effect of nicotinamide on protein.