Photochemical generation of the dichloroacetate radical by reduced ferredoxin

Abstract

Solutions of reduced [2Fe–2S] ferredoxin from the cyanobacterium Spirulina platensis, are found to be sensitive to visible light in the frozen state at 77 K, if trichloroacetate is also present. An EPR-detectable radical is generated, which is stable for several hours at 77 K. The process is dependent on the presence of the protein and a reductant (dithionite). The shape of the radical spectrum is the same when the solvent is D₂O. It has been identified as the dichloroacetate radical anion, ˙CCl₂CO₂^–. The ferredoxin becomes partially oxidized in the photochemical process, in parallel with the appearance of the radical. On thawing the samples, the EPR spectrum of the reduced ferredoxin increases as the protein is partially re-reduced by dithionite. This is an unusual situation in which photochemically-induced electron transfer can be observed from a metal site within a protein.