Time-dependent fluorescence intensity and depolarization of diphenylhexatriene in micellar complexes of apolipoprotein C-I and dimyristoylglycerophosphocholine

Abstract

The lipophilic fluorescent probe diphenylhexatriene was used to probe the lipid order and dynamics in apolipoprotein C-I . dimyristoylglycerophosphocholine (Myr2Gro-P-Cho) complexes. These complexes contain on the average 25 mol Myr2Gro-P-Cho/mol of apolipoprotein C-I, have a molecular weight around 200 000, and appear as discoidal, stacked particles by negative-stain electron microscopy. Steady-state fluorescence polarization of diphenylhexatriene as a function of temperature gives a broadened and shifted phase transition for Myr2Gro-P-Cho from the gel to liquid-crystalline state, with a mid-point around 27 degrees C. Time-dependent fluorescence intensity and anisotropy measurements of the diphenylhexatriene probe at 15 degrees C and 35 degrees C give fluorescence decay curves which can best be fit by two exponential functions, in each case. The fluorescence lifetimes and their fractional amplitudes approach the corresponding parameters in Myr2Gro-P-Cho vesicles and suggest insignificant effects of the protein on the microenvironment and conformations of the probe. The rotational correlation times and their fractional anisotropies indicate similar local motions of the probe in complexes and in vesicles, but reveal a significant ordering effect of the protein at both temperatures. The overall complex rotation at 15 degrees C has a correlation time of 136 +/- 13 ns, consistent with the size (approximately equal to 200 kDa) and shape (disc approximately equal to 5 x 15 nm) of the particle.