文摘
Sulfite oxidase from Arabidopsis thaliana has been reduced at pH = 6 with sulfite labeled with33S (nuclear spin I = 3/2), followed by reoxidation by ferricyanide to generate the Mo(V) state of the activecenter. To obtain information about the hyperfine interaction (hfi) of 33S with Mo(V), continuous-wave electronparamagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) experiments havebeen performed. The interpretation of the EPR and ESEEM spectra was facilitated by a theoretical analysisof the nuclear transition frequencies expected for the situation of the nuclear quadrupole interaction beingmuch stronger than the Zeeman and hyperfine interactions. The isotropic hfi constant of 33S determined inthese experiments was about 3 MHz, which demonstrates the presence of coordinated sulfate in the sulfite-reduced low-pH form of the plant enzyme.