Serum lipoprotein structure: resonance energy transfer localization of fluorescent lipid probes

The location of several fluorescent chromophores in lipoproteins has been determined by using resonance energy transfer. The primary acceptor is 5-(N-hexadecanoylamino)fluorescein whose chromophore is shown to reside at the lipoprotein surface at pH 7.4. Polar donors include cis-parinaric acid (cis,trans,trans,cis-9,11,13,15-octadecatetraenoic acid), trans-parinaric acid (all-trans-9,11,13,15-octadecatetraenoic acid), and 16-(9-anthroyloxy)palmitic acid; nonpolar donors are parinaric acid methyl ester, parinaric acid cholesteryl ester, and 1,6-diphenyl-1,3,5-hexatriene. The polar donors transfer more efficiently than the nonpolar donors in several classes of lipoprotein particles. The data are analyzed by a simple mathematical model from which it is concluded that the polar donors are localized in the putative lipoprotein surface monolayer; the possibility that nonpolar donors are partitioned between the surface and core of lipoproteins is considered.