High-pressure effects on beta-lactoglobulin interactions with ligands studied by fluorescence

The effects of pressure (0.1 MPa to 400 MPa) on intrinsic fluorescence of beta-lactoglobulin and on its binding of retinol and cis-parinaric acid have been studied at neutral and acid pHs. In neutral pH, fluorescence emission spectra of beta-lactoglobulin tryptophanes are characterized by an irreversible 14 nm red-shift indicating pressure-induced folding changes. The intensity of the fluorescence of retinol in beta-lactoglobulin-retinol complex is enhanced by a pressure increase up to 150 MPa. It decreases at higher pressures and disappears altogether at 300 MPa. beta-Lactoglobulin-retinol complex does not reassociate after decompression at neutral pH. At acid pH condition, the fluorescence quenching by pressure of beta-lactoglobulin tryptophans is coupled with a 2 nm spectral shift and is fully reversible demonstrating almost complete restoration of globulin folding. The evolution of retinol fluorescence in beta-lactoglobulin-retinol complex is also entirely reversible between 0.1 MPa and 400 MPa and the complex never dissociates in the studied pressure range. beta-lactoglobulin-cis-parinaric acid complexes at neutral and acid pH values dissociate irreversibly at 200 MPa and 350 MPa, respectively.