The interaction of alpha-tocopherol and homologues with shorter hydrocarbon chains with phospholipid bilayer dispersions. A fluorescence probe study

Abstract

The intrinsic fluorescence of tocopherol homologues with hydrocarbon chains ranging from 1 carbon (C1) to 16 carbons (alpha-tocopherol, C16) and their ability to quench the fluorescence of 9-anthroyloxy derivatives of fatty acids with the fluorophore located at different positions in the hydrophobic domain of phospholipid bilayers has been used to model the interaction of tocopherol with lipid bilayer membranes. All the tocopherol homologues used, C1, C6, C11 and alpha-tocopherol, showed a similar fluorescence emission intensity at 325 nm in cyclohexane but were almost completely self-quenched by aggregation in water. Fluorescence was restored when dispersions of dimyristoylglycerophosphocholine were added but the maximum intensity was lower with the longer-chain homologues. Full intensity was observed in all homologues on addition of the detergent Triton X-100. Studies using 9-anthracenecarboxylic acid and 9-anthracenecarboxymethyl ester, 6-(9-anthroyloxy)stearic acid and 16-(9-anthroyloxy)palmitelaidic acid showed that the tocopherol homologues partitioned into the hydrophobic domain of phospholipid dispersions composed of dimyristoylglycerophosphocholine at 40 degrees C and dioleoylglycerophosphocholine at 40 degrees C. The 9-anthroyloxy fatty acids and 1,6-diphenyl-1,3,5-hexatriene were quenched by all the homologues and Stern-Volmer plots of the concentration dependence of the quenching indicated that this was predominantly via dynamic processes. No fluorescence energy transfer was observed between diphenylhexatriene and tocopherols but efficient transfer was recorded to the 9-anthroyloxy fatty acid probes. The results are consistent with a model in which the chromanol nucleus of tocopherol is oriented towards the lipid-water interface of the phospholipid bilayer. As the phytol chain length increases there is an increasing tendency for the chromanol nucleus to reside in the hydrophobic interior of the structure. alpha-Tocopherol appears to form clusters within the phospholipid dispersion which are not fluorescent and do not quench the fluorescence of the different fluorescent probes nor transfer fluorescence energy to them. It is suggested that the monomeric form is responsible for the vitamin effects of tocopherol and the aggregated form acts as a reservoir that does not markedly perturb bilayer stability.