Phytochemical and biochemical characterization of epimeric photogedunin derivatives.: Their different sites of interaction on the redox electron transport carrier of Spinacea oleracea L. chloroplasts

Bioactivity-directed fractionation of the dichloromethane extract of the heartwood of Cedrela salvadorensis L. resulted in the isolation of two epimeric photogedunins and in the acquirement of two bioactive epimeric photagedunin acetates 3 (beta) and 4 (alpha) at the C-23 position. Compounds 3 and 4 inhibited photophosphorylation, H+-uptake, and noncyclic electron flow from water to methyl viologen (basal, phosphorylating, and uncoupled). Inhibiton of all photosynthetic activities require 3 min of preincubation in the light before inhibition is observed. The results indicate that compounds 3 and 4 behave as Hill reaction inhibitors. alpha-Photogedunin acetate 3 inhibits PS II electron flow from H2O to DCBQ without affecting PS I electron transport from reduced DCPIP to MV and PSII from water to SiMo. Therefore, its target is localized at the Q(B) level. beta-Photogedunin acetate 4 inhibits PS I electron flow from reduced DCPIP to MV, PMSred to MV, and TMQH(2) to MV without affecting PS II electron flow, interacting at the b(6)f level and in the span of P-700 to F-x.