title: Spectroscopic studies of Rhodobacter capsulatus cytochrome c' in the isolated state and in intact cells.
abstract: Ferricytochrome c' from Rhodobacter capsulatus was investigated by 1H-NMR, EPR and optical spectroscopies. A haem-linked ionisation, occurring with a pKa of 8.4 at 25 degrees C, was observed and assigned to the ionisation of the axial histidine ligand by comparison with data for related proteins. At pH values below this pKa the spin-state of the haem Fe3+ is shown to be a quantum mechanically admixed S = 3/2, 5/2 state. Above the pKa the Fe3+ is high-spin. EPR studies of intact cells grown photoheterotrophically reveal that in situ cytochrome c' exists largely in the ferrous state. Upon the addition of [Fe(CN)6]3- the protein becomes oxidised and EPR spectra reveal that the Fe3+ spin-state is a quantum mechanically admixed S = 3/2, 5/2 state. These data indicate that the unusual spin-state of ferricytochrome c' is not a consequence of changes to the protein on its isolation, as had been suggested previously. They also indicate that in situ cytochrome c' is located in an environment with a pH less than 7.
authors.affiliation: Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, University of East Anglia, Norwich, UK.
authors.forename: F
authors.initials: F
authors.lastname: Monkara
authors.forename: S J
authors.initials: SJ
authors.lastname: Bingham
authors.forename: F H
authors.initials: FH
authors.lastname: Kadir
authors.forename: A G
authors.initials: AG
authors.lastname: McEwan
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
authors.forename: A G
authors.initials: AG
authors.lastname: Thurgood
authors.forename: G R
authors.initials: GR
authors.lastname: Moore
mesh_terms: D003574:Cytochrome c Group / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D003574:Cytochrome c Group; D007501:Iron