Low Temperature Electronic Absorption Spectra of Oxidized and Reduced Spinach Ferredoxins. Evidence for Nonequivalent Iron(III) Sites.

The electronic absorption spectra of oxidized and reduced spinach ferredoxins have been measured between 1200 and 600 nm at low temperature in D(2)O/ethylene glycol glasses. Relatively weak absorption bands are observed at 720, 820, and 920 nm in oxidized ferredoxin, and at 652, 820, and 920 nm in reduced ferredoxin. The spectral results show that the two Fe(III) centers in oxidized ferredoxin are not equivalent, and that the 820- and 920-nm bands are associated with the nonreducible site. Assignment of the reducible site as tetrahedral Fe(III) is indicated. The 720-nm (13.9 kcm(-1)) band in oxidized ferredoxin is attributed to an intensity-enhanced (6)A(1) --> (4)T(1)d-d transition, whereas the 652-nm (15.3 kcm(-1)) feature of reduced ferredoxin could be due either to (5)E --> (3)T(1) in tetrahedral Fe(II)S(4) or an Fe(II) --> Fe(III) intervalence excitation.