EFFECT OF PLANT MONOFUNCTIONAL PENTACYCLIC TRITERPENES ON THE DYNAMICAND STRUCTURAL-PROPERTIES OF DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYERS

Fluorescence lifetime and rotational behaviour of 1,6-diphenyl-1,3-5-h exatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5 hexat riene (TMA-DPH) were used to study the influence of several monofuncti onal pentacyclic triterpenes on the structural and dynamic properties of dipalmitoylphosphatidylcholine (DPPC) bilayers. The plant monoalcoh ols alpha-amiryn, lupeol and taraxerol, the semisynthetic (H-18-alpha) -alpha-amiryn, and the ketone taraxerone were studied in comparison wi th cholesterol. Among them, only alpha-amiryn is incorporated at as hi gh levels as cholesterol into DPPC bilayers. At 50 mol%, it results in the phase transition vanishing. Pentacyclic monoalcohols are more pot ent than cholesterol to decrease the DPPC gel phase order, but less po tent than it to increase the liquid-crystalline state order. They also produce more packing defects than cholesterol in DPPC bilayers. These effects correlate with the absence of planarity in the pentacyclic sk eleton, which would obstruct the 'all trans' packing of acyl chains in the gel state and Van der Waals interactions in the liquid-crystallin e state. Taraxerone has little effect on DPPC bilayers, probably due t o its difficulty to form hydrogen bonds. (C) 1997 Elsevier Science Ire land Ltd.