H-1 NMR relaxation investigation of herbicides interacting with photosystem II preparations from Spinacia oleracea L.

The interaction of three common herbicides, paraquat, acifluorfen and alach lor, with spinach chloroplast photosystem II (PS II) was investigated by me asuring LH nuclear magnetic resonance spin-lattice relaxation rates, transi ent nuclear Overhauser effect (NOE) and NOE spectroscopy (NOESY) spectra. B inding to PS II was detected by (i) the enhancement of single-selective rel axation rates and (ii) the decrease in the optimal mixing time providing ma ximal cross-peak intensity in NOESY spectra. Titration of relaxation enhanc ements was used to calculate the dissociation constants (K-d) from the boun d state for paraquat (K-d = 292 +/- 71 mu M-1) and acifluorfen (K-d = 311 /- 58 mu M-1). A similar K-d was apparent for alachlor. Double-selective re laxation rates allowed the isolation of dipolar relaxation terms between se lected proton pairs wherefrom dynamic features of the bound state were eval uated. In all cases the motional correlation time of bound herbicide (tau(c ) = 0.1-0.4 ns at 300 K) was found two orders of magnitude slower than in t he free-solution state. In the case of alachlor the E and Z isomers were ob served to bind differently to PS II and a change in conformation could be h ypothesized.