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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.

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Reversible binding of 5-(dimethylamino)-naphthalene-1-sulfonic acid (DNS) to human and bovine serum albumin has been monitored by changes in fluorescence intensity, wavelength maxima, and polarization. DNS has only one major binding site (ka = 5 x 10(6)) and one minor site (ka = 3 x 10(5)) on these proteins. The probe is competitively displaced from its high-affinity site by medium chain fatty acids and by N-acetyl-L-tryptophan. The binding site for DNS is highly hydrophobic and is considerably less polar than the hydrocarbon region of lipid bilayers. Resonance energy transfer indicates that the binding site is located 20.7 +/- 2 A from the single tryptophan residue of human serum albumin.

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