title: Herbicide effect on the hydrogen-bonding interaction of the primary quinone electron acceptor QA in photosystem II as studied by Fourier transform infrared spectroscopy.
abstract: The redox potential of Q(A) in photosystem II (PSII) is known to be lower by approximately 100 mV in the presence of phenolic herbicides compared with the presence of DCMU-type herbicides. In this study, the structural basis underlying the herbicide effects on the Q(A) redox potential was studied using Fourier transform infrared (FTIR) spectroscopy. Light-induced Q(A)(-)/Q(A) FTIR difference spectra of Mn-depleted PSII membranes in the presence of DCMU, atrazine, terbutryn, and bromacil showed a strong CO stretching peak of Q(A)(-) at 1,479 cm(-1), while binding of phenolic herbicides, bromoxynil and ioxynil, induced a small but clear downshift by approximately 1 cm(-1). The CO peak positions and the small frequency difference were reproduced in the S(2)Q(A)(-)/S(1)Q(A) spectra of oxygen-evolving PSII membranes with DCMU and bromoxynil. The relationship of the CO frequency with herbicide species correlated well with that of the peak temperatures of thermoluminescence due to S(2)Q(A)(-) recombination. Density functional theory calculations of model hydrogen-bonded complexes of plastoquinone radical anion showed that the small shift of the CO frequency is consistent with a change in the hydrogen-bond structure most likely as a change in its strength. The Q(A)(-)/Q(A) spectra in the presence of bromoxynil, and ioxynil, which bear a nitrile group in the phenolic ring, also showed CN stretching bands around 2,210 cm(-1). Comparison with the CN frequencies of bromoxynil in solutions suggested that the phenolic herbicides take a phenotate anion form in the Q(B) pocket. It was proposed that interaction of the phenolic C-O(-) with D1-His215 changes the strength of the hydrogen bond between the CO of Q(A) with D2-His214 via the iron-histidine bridge, causing the decrease in the Q(A) redox potential.
authors.affiliation: Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Takano
authors.forename: Ryouta
authors.initials: R
authors.lastname: Takahashi
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Suzuki
authors.forename: Takumi
authors.initials: T
authors.lastname: Noguchi
mesh_terms: D006540:Herbicides / Q000494:pharmacology*; D006860:Hydrogen Bonding; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000187:drug effects; D045332:Photosystem II Protein Complex / Q000187:drug effects*; D010971:Plastoquinone / Q000378:metabolism*; D017550:Spectroscopy, Fourier Transform Infrared; D018724:Spinacia oleracea
chemical_list: D006540:Herbicides; D045332:Photosystem II Protein Complex; D010971:Plastoquinone