Proton magnetic resonance and magnetic susceptibility characterization of ferredoxin I from Bacillus polymyxa.

Magnetic susceptibility and proton magnetic resonance spectra are reported for the oxidized and reduced forms of the iron-sulfur protein Bacillus polymyxa ferredoxin I. The magnetic susceptibility of the oxidized form indicates antiferromagnetic exchange coupling between component iron atoms that is quantitatively similar to that observed for the clostridial ferredoxins and for the (C(2)H(5))(4)N [Fe(4)S(4)(SCH(2)C(6)H(5))(4)] analog. Contact-shifted resonances observed in the proton-magnetic-resonance spectra of oxidized and reduced forms of the B. polymyxa protein can be correlated with the contact-shifted resonances of corresponding redox forms of the clostridial ferredoxins. Characteristics of the contact-shifted resonances observed in partially reduced B. polymyxa ferredoxin I are compatible with a "slow" rate of electron exchange between redox forms, which suggests that the "fast" electron exchange earlier observed in the eight-iron clostridial ferredoxins may derive from an intramolecular component.