Laser-Raman investigation of lysozyme-phospholipid interactions

The interaction of lysozyme with mixed 1,2-dipalmitoyl-L-phosphatidic acid/1,2-dimyristoyl-L-phosphatidylcholine liposomes was investigated by laser Raman spectroscopy. Substantial changes were observed i the spectra of both the lipid and protein in the mixed liposomes over the range 10-62 degrees C. At temperatures below 27 degrees C, interaction with lipid appears to slightly increase the amount of helical structure in lysozyme at the expense of random conformation. At temperatures above 30 degrees C, considerable beta-sheet is irreversibly formed. Onset of beta-formation appears to coincide with the formation of disordered lipid side-chains in the acidic component of the lipid. At all temperatures, the O-P-O diester stretching mode at 782 cm-1 is much more intense in the lipid/protein mixture than in lipid alone. It is observed that the dimyristoyl phosphatidylcholine chain-disorder transition is lowered by 3 degrees C, while that of the phosphatidic acid is lowered by 12 degrees C, yet the post-transition conformation contains a significantly higher proportion of trans-segments in the presence of lysozyme. These results are interpreted in terms of: (1) a polar interaction between acidic phospholipid and lysozyme at temperatures below either chain-disorder transition, in which lysozyme is essentially excluded from the hydrophobic portion of the lipid and (2) an interaction at higher temperatures which involves the lipid side-chains of dipalmitoyl phosphatidic acid in the disordered state and is manifested by a substantial conformational change.