On the Orientation of Photosystem II Inhibitors in the QB-Binding Niche: Acridones, Xanthones and Quinones

Abstract The orientation of acridones, xanthones, 1,4-benzo-and naphthoquinones within the photosystem II Q B herbicide-binding niche was studied by means of mild trypsination and by estimation of pI 50 -values in Chlamydomonas reinhardtii D 1 mutants (Val 219 > lie, Ala 251 > Val, Phe 255 > Tyr, Ser 264 > Ala, Asn 266 > Thr, and Leu 275 > Phe). As judged from the R/S-values (ratios of I 50 -values resistant versus susceptible type) close to 1 in all mutants, the acridones and xanthones do not have strong interactions with the parent amino acids. Contrary, the quinones exhibit extreme low R/S-values down to 0.003 (for 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone; DBMIB) in the Ser 264 mutant. This extreme negative cross resistance or supersensitivity indicates that the quinones do not form a hydrogen bond to the serine hydroxyl group.