title: Circular dichroism and magnetic circular dichroism of Azotobacter vinelandii ferredoxin I.
abstract: Room temperature circular dichroism (CD) and low temperature magnetic circular dichroism (MCD) spectra of air-oxidized and dithionite-reduced Azotobacter vinelandii ferredoxin I (FdI), a [( 4Fe-4S]2+/1+, [3Fe-4S]1+/0) protein, are reported. Unlike the CD of oxidized FdI, the CD of dithionite-reduced FdI exhibits significant pH dependence, consistent with protonation-deprotonation at or near the cluster reduced: the [3Fe-4S] cluster. The MCD of reduced FdI, which originates in the paramagnetic reduced [3Fe-4S]0 cluster, is also pH-dependent. Detailed studies of the field dependence and temperature dependence of the MCD of oxidized and reduced FdI, in the latter case at pH 6.0 and 8.3, are reported. The low-field temperature dependence of the MCD of oxidized FdI, which originates in the paramagnetic oxidized [3Fe-4S]1+ cluster, establishes the absence of a significant population of excited electronic states of this cluster up to 60 K. The low-field temperature dependence of the MCD of reduced FdI establishes that the ground-state manifold of the reduced [3Fe-4S]0 cluster possesses S greater than or equal to 2 at both pH 6.0 and 8.3. Analysis, assuming S = 2 and an axial zero-field splitting Hamiltonian, leads to D = -2.0 and -3.5 cm-1 at pH 6.0 and 8.3, respectively. The site of the (de)protonation affecting the spectroscopic properties of the [3Fe-4S] cluster remains unknown.
authors.affiliation: Department of Chemistry, University of Southern California, Los Angeles 90089-0482.
authors.forename: P J
authors.initials: PJ
authors.lastname: Stephens
authors.forename: G M
authors.initials: GM
authors.lastname: Jensen
authors.forename: F J
authors.initials: FJ
authors.lastname: Devlin
authors.forename: T V
authors.initials: TV
authors.lastname: Morgan
authors.forename: C D
authors.initials: CD
authors.lastname: Stout
authors.forename: A E
authors.initials: AE
authors.lastname: Martin
authors.forename: B K
authors.initials: BK
authors.lastname: Burgess
mesh_terms: D001395:Azotobacter / Q000378:metabolism*; D002942:Circular Dichroism; D005288:Ferredoxins / Q000737:chemistry* / Q000302:isolation & purification; D008433:Mathematics; D011487:Protein Conformation; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D005288:Ferredoxins