Tricolorin A, a potent natural uncoupler and inhibitor of photosystem II acceptor side of spinach chloroplasts

Tricolorin A, (11 S )‐11‐hydroxyhexadecanoic acid 11‐ O ‐ α ‐ l ‐ rhamnopyranosyl‐(1?3)‐ O ‐ α ‐ l ‐{2‐ O ‐(2 S ‐methylbutanoyl)‐4‐ O ‐(2 S ‐methylbutanoyl)}‐rhamnopyranosil‐(1?2)‐ O ‐ β ‐ d ‐glucopyranosil‐(1?2)‐ β ‐fucopyranoside‐(1,3′′‐lactone), the major phytogrowth inhibitor isolated from Ipomoea tricolor Cav. (Convolvulaceae) was found to be a potent uncoupler (U 50 =0.33 μ M ) of photophosphorylation in spinach chloroplasts. Tricolorin A inhibited H + ‐uptake and adenosine 5′‐triphosphate (ATP) synthesis, and stimulated basal and phosphorylating electron flows. Using a combination of two well‐known fluorescent ΔpH probes, 9‐aminoacridine and 9‐amino‐6‐chloro‐2‐methoxyacridine, the uncoupling behavior of tricolorin A was also demonstrated for submitochondrial particles. Polarographic data showed that high concentrations (20 μ M ) of tricolorin A inhibited photosystem II (PSII) electron flow at the level of plastoquinone B (Q B ). Chlorophyll (Chl) a fluorescence analysis showed that tricolorin A induced accumulation of Q A − and strongly decreased the electron transport capacity, suggesting that the target of this molecule was located at the Q B level. The macrocyclic lactone‐type structure of this allelopathic agent proved to be an important structural requirement for uncoupling activity since its hydrolysis caused loss of the inhibitory potential.