Molecular basis for semiquinone stabilization in respiratory enzymes: a pulsed EPR study of the menasemiquinone binding mode in E. coli nitrate reductase A

Abstract

(PQA/PQA) differential spectra in hydrated (r=76%) and dehydrated (r=11%) RC films over the 40001000 cm. The spectra differ significantly in the 3750-3550 cm range, the band attributed to weakly hydrogen bonded water molecules [5] being strongly reduced in the dried film. Dehydration also affects the 1800-1200 cm range, which includes contributions from P, the quinones and the peptide. Optical absorption measurements performed under the same photoexcitation regime reveal a slow (t~5 s) kinetic component of PQA recombination which disappears in the dehydrated sample, indicating at low r a destabilization of the charge separated state. As a whole the data suggest a correlation between the hydration shell dynamics and the conformational RC dynamics which stabilize the charge separated state.