Some insights about 1,6-diphenyl-1,3,5-hexatriene-lipid supramolecular assemblies by steady-state fluorescence measurements

1,6-Diphenyl-1,3,5-hexatriene (DPH) is the most widely proposed molecular probe for the post-column fluorescence derivatization of lipids after liquid chromatography separation. This kind of detection consists of a supramolecular combination of DPH and eluted lipids. The detection is optimally performed in a mainly aqueous environment (over 80% v/v) because the weak fluorescence of DPH in water is drastically enhanced upon formation of supramolecular assemblies with lipids. In the present study, and in order to obtain better spectroscopic insights into the nature of these supramolecular assemblies, two different lipids were tested, 1,2,3-tridodecanoylglycerol (LLL) as a model triglyceride (nonpolar lipid) and dimyristoylphosphatidylcholine (DMPC) as a model phosphatidyicholine (charged amphiphilic lipid). Stoichiometry and association constants were determined on the basis of the variation of fluorescence intensity in the presence of various concentrations of lipids. LLL60-DPH2 and DMPC200-DPH2 complexes were identified with association constants as high as K-2 = (5.8 +/- 0.5) X 10(13) M-2 and (17.3 +/- 2.0) X 10(13) M-2 for LLL and DMPC, respectively. The fluorescence intensity of DPH in the presence of LLL is greater than in the presence of DMPC. An attempt to characterize the insertion mode of DPH in the lipidic supramolecular assemblies is also made.