Interaction of beta-lactoglobulin with phospholipid bilayers: a molecular level elucidation as revealed by infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy has been used to study, at a molecular level, the interactions between beta-lactoglobulin (BLG), the mo st abundant globular protein in milk, and some lipids (sphingomyelin, SM; d imyristoylphosphatidylcholine, DMPC; dipalmytoylphosphatidylcholine, DPPC; dimyristoylphosphatidylserine-sodium salt, DMPS; dipalmitoylphosphatidylser ine-sodium salt, DPPS) constituting the milk fat globule membrane (MFGM). T he interactions were monitored with respect to alteration in the secondary structure of BLG, as registered by the amide I' band, and phospholipid conf ormation, as revealed by the acyl chain and carbonyl bands. The results sho w that neither the conformation nor the thermotropism of neutral bilayers c ontaining DMPC or DPPC is affected by BLG. Reciprocally, the secondary stru cture and thermal behaviour of pure BLG remain the same in the presence of PC. These results suggest that no interaction occurs between PC and BLG, in agreement with previous studies. However, it is found that BLG interacts w ith neutral bilayers constituted by milk SM lipids, increasing gauche confo rmers and thus conformational disorder of the lipid acyl chains. This pertu rbing effect has been attributed to a partial penetration of BLG into the h ydrophobic core of the bilayer, which allows hydrophobic interactions betwe en BLG and SM. Moreover, the fact that SM possesses the same headgroup of P C implies that the head group does not prevent the occurrence of BLG-lipid interactions and other lipid regions can control the binding of BLG to lipi ds. Furthermore, BLG was found to interact electrostatically with charged b ilayers containing PS, leading to a rigidification of the lipid hydrocarbon chains and a dehydration of the interfacial region. This last effect sugge sts that the protein limits the accessibility of water molecules to the int erfacial region of the phospholipids by its presence at the membrane surfac e. (C) 2000 Elsevier Science B.V. All rights reserved.