7-carboxyindole derivatives as photosystem II inhibitors

Four 7-carboxyindole derivatives (1-4) were evaluated as photosynthesis inhibitors by chlorophyll a (Chl a) fluorescence assays using the OJIP method. [1] Based on the parameters obtained in the OJIP test, it is possible to understand the photosynthetic apparatus, since they are based on the four main photosynthesis events: energy absorption, energy trap, electron transport, final electron receptors and the performance index (PI(abs)) parameters. [2].
The N,2,3-trimethyl-1H-indole-7-carboxamide (3) demonstrated the best result by blocking the electron flow. The ABS, TR0, ET0, and DI0 parameters by RC were reduced by 38, 38, 52 and 37% respectively, at 200 µM (Figure 1 - A). The energy absorption reduction by the antenna complex directly influences the electron transport and system energy dissipation, causing the photosynthesis Z-scheme does not occur properly. The decrease of PSI0 and PHI(E0) parameters by 30 and 35%, respectively, corroborates the PSII quantum efficiency reduction, which suggests a damage in electron excitation and transfer by the electron transport chain after QA-.
Molecular docking was carried out by using the crystallographic structure of the reaction centre of PSII (D1) from spinach (PDB ID: 3JCU). Results suggested that compound 3 are bonded via oxygen and hydrogen. The carbonyl group of amide group interacts with NH of the His215 with a distance bond of 1.888 Å. Furthermore, hydrophobic interactions were found with Met214, Leu218 and Leu271. The interaction of 3 with these residues might lead to an inhibition of PSII (D1). In addition, the orientation of 3 is the same of most of the photosystem II inhibitors, which prefer the position near the loop made by Ser264 and Phe265 (Figure 1 - B).