Structural and interaction properties of beta-Lactoglobulin as studied by FTIR spectroscopy

New aspects of beta-Lactoglobulin (BLG) structure which have not previously been found by FT-IR spectroscopy are presented. The conformational changes of BLG and its heat-induced denaturation have been studied at low concentr ation and different pHs. First, it was found that the spectra of BLG in sol ution are concentration-dependent. Below 1%, they reveal one component in t he 1620-1635 cm(-1) region while above this concentration, two components a re present. These changes are related to the modifications of the quaternar y structure of BLG, i.e. to the monomeric or dimeric forms. As shown at hig h temperature (85 degrees C), this concentration (1%) represents the thresh old of protein aggregation. Second, the thermal behaviour of BLG at pH 7.4 and 4.4 is compared. The results suggest that the denaturation process and the intermolecular interactions in aggregates are different and read to two types of aggregate. These observations are in agreement with the formation of two gel microstructures near neutral pH and near the pi of BLG as obser ved by Stading & Hermasson (1990). Finally, interactions between BLG and tw o zwitterionic phospholipids have been investigated. Dipalmitoylphosphatidy lcholine (DPPC) is unaffected in the presence of BLG, suggesting that no in teraction occurs. In contrast, BLG increases the lipid chain conformational disorder of milk sphinglomyelin (SM) as a consequence of hydrophobic inter actions of BLG with SM. Since this effect occurs at and above the gel-to-li quid-crystalline phase transition, it is suggested that membrane fluidity p lays an important role in these interactions.