Multiple effects of linolenic acid addition to pea thylakoids

The addition of linolenic acid to thylakoids produces various pH-dependent effects. We have demonstrated a binding site near the Photosystem (PS) II center with a pK_a of 6.5: when linolenic acid is unprotonated it induces in the dark a rise of the initial fluorescence level, the latter being similar to the maximum fluorescence obtained during illumination of untreated thylakoids. The comparison of the fluorescence lifetimes in the presence and absence of linolenic acid leads us to conclude that the charge stabilisation on the primary acceptor, Q, is prevented by linolenic acid. A second binding site on the protein carrying B, the secondary acceptor of PS II, has also been demonstrated for linolenic acid. It has a 3-(3,4-dichlorophenyl)-1,1-dimethylurea-type effect both in the protonated and unprotonated forms. Finally, measurements of electrophoretic mobility of the thylakoids indicate several other sites of linolenic acid inclusion with an average pK_a of 5.7. At alkaline pH the presence of unprotonated linolenic acid increases the charge density on the membrane. As a result a higher concentration of divalent cations is needed to obtain fluorescence and stacking changes than for untreated thylakoids. The presence, at acidic pH values, of the unprotonated form of linolenic acid leads to the inhibition of cation-induced fluorescence changes, probably by preventing the movement of chlorophyll-protein complexes in the membrane.