Photochemical dimerization of parinaric acid in lipid bilayers.

Parinaric acid (9,11,13,15-octadecatetraenoic acid), a conjugated tetraene fatty acid, undergoes a second-order photochemical reaction in phospholipid bilayers. The reaction results in the loss of the characteristic absorption of this chromophore and the development of new absorption demonstrating the presence of a triene chromophore. The progress of this reaction is easily monitored by measurement of the decrease in the fluorescence intensity from a uniformly illuminated sample. The reaction rate measured in this way is sensitive to the thermal phase transition of the bilayer and to the presence of cholesterol. The relationship of the second-order rate constant to the lipid diffusion coefficient is discussed. This relationship differs from that previously used for the analysis of similar photochemical processes.