Natural diterpene β-lactone derivative as photosystem II inhibitor on spinach chloroplasts

α,7β-Dihydroxyvouacapan-17β-oic acid (1) was isolated from Pterodon polygalaeflorus Benth. Then, (1) was modified to obtain 6α-hydroxyvouacapan-7β,17β-lactone (2). Inhibition properties of both (1) and (2) were evaluated in spinach chloroplasts. Only compound (2) inhibited non-cyclic electron transport and ATP synthesis, i.e., (2) behaved as a Hill reaction inhibitor. Compound (2) did not affect photosystem I (PSI) activity but it did inhibit electron flow through PSII. Analysis of the partial PSII reactions from water to DCPIPox, water to silicomolybdate, and diphenylcarbazide to DCPIPox allowed locating inhibition sites at (i) the oxygen-evolving complex (OEC) and (ii) the redox enzymes of the electron transport chain, in the span of P680 to QA. Chlorophyll a fluorescence measurements confirmed the action site for (2).