Circular Dichroism Study of Lipid-Lipid and Lipid-Protein Interaction

Abstract

The circular dichroism (CD) active phospholipid having azobenzene moiety in a close proximity to the chiral, carbon, [1], was used for the study of lipid-lipid and lipid-protein interaction in liposome system.
The large CD enhan cement of the chiral bilayer arised from the interaction of spatially fixed chromophores even in the fluid state (50°C) and this CD enhancement was remarkably reduced by the presence of a small amount of cis-azobenzene chromophore (Fig.1). After irradiation of the liposome solution at 5°C, the amount of three isomers of phospholipid, trans, trans-, trans, cis- and cis, cis- were investigated by HPLC. Observed cis; cis- content was much larger than that calculated from trans, cis- to trans, trans- ratio (Table 1), suggesting that one cis-azobenzene chromophore changes the conformation of trans-azobenzene chromophores to make their isomerization easy and this effect is preferential on trans-azobenzene in chromophore the same molecule.
Delipidated bacteriorhodop sin purified from purple membrane of H. halobium was reconstituted with the CD active phospholipid [1], The observed CD spectra in the 450-700 nm region characteristic of bacteriorhodopsin showed the temperature dependence characterized by midpoint at ca.45°C and this spectral change showed the disaggregation of bacteriorhodopsin trimer to monomer. The CD spectra in the 250-400 nm region characteristic of the azo chromophore of phospholipid exhibited a remarkable temperature dependence synchronized with the disaggregation of bacteriorhodopsin, suggesting that a large proportion of the phospholipid is present as boundary lipid (Figs.2, 3). This paper is dedicated to professor I w ao Tabushi who deceased on Mar.22, 1987, with my great respect for him.