Induced circular dichroism of incorporated fluorescent cholesteryl esters and polar lipids as a probe of human serum low density lipoprotein structure and melting.

Two fluorescent cholesteryl esters, cholesteryl cis-parinarate and cholestatrienyl oleate, have been incorporated into human serum low density lipoprotein (LDL) by two alternative procedures. The core location of the incorporated cholesterol esters and the surface location of cis-parinaric acid and cholestatrienol are verified by fluorescence energy transfer. In these LDL preparations, the incorporated cholesteryl esters exhibit a temperature-dependent induced circular dichroic spectrum at the absorption wavelengths of the chromophore with a temperature range which is similar to the calorimetrically determined thermal transition of LDL. We also examined the induced circular dichroism which arises when cholestatrienyl oleate is dissolved in pure cholesteryl oleate and observed a temperature dependence of the sign and magnitude of the circular dichroic spectrum which correlated with the temperature range of the calorimetric phase transitions. On the basis of these studies, we conclude that below the calorimetric transition, the cholesteryl esters of low density lipoprotein have an organization which is intermediate between isotropic and cholesteric, while above the transition, the organization is more nearly isotropic. Residual environmental chirality experienced by cholesteryl ester acyl chains (cholesteryl parinarate) in the particle core and free sterol (cholestatrienol) in the particle surface could represent specific lipid-protein interactions. These results are discussed in terms of the organization of low density lipoprotein.