382 IRON-CONTAINING PROTEINS AND ENZYMES
the [Fe IV (O)(TMCS)] + (Figure 7.23 , complex 1) did not react with triphenyl-
phosphine, PPh 3 , to form triphenylphosphine oxide, OPPh 3 (an oxo - atom two -
electron oxidation), while the complex did react with dihydroanthracene at
− 40 ° C in CH 3 OH, undergoing a one - electron reduction to a red species. (See
Figure 7.23 .) This product exhibits an electrospray ionization mass spectros-
copy (ESI - MS) ion at m / z = 388.1944, corresponding to [Fe(TMCS)(OCH 3 )] +
(Figure 7.23 , complex 2). The authors postulate that [Fe IV (O)(TMCS)] + decays
by abstracting a hydrogen atom from a dihydroanthracene molecule to form
[Fe III (TMCS)(OH)] + that converts to [Fe III (TMCS)(OCH 3 )] + in the CH 3 OH
solvent. Thus, in this model compound, the Fe IV = O unit is not a two - electron
oxo - atom transfer agent but a one - electron oxidant hydrogen abstraction
reagent. The conclusion reached is that thiolate ligands in the axial position
of [Fe IV (O)(TMCS)] + P450 model compounds cause more drastic changes
in reactivity than found for other axial ligands in similar model compounds.
For instance the compound [Fe IV (O)(TMC)(NCCH 3 )]+ (Figure 7.23 , complex
3) where TMC =1,4,8,11 - tetramethyl - 1,4,8,11 - tetraazacyclotetradecane and
NCCH 3 is acetonitrile, does produce the oxo - atom two - electron product, OPPh 3
when reacted with PPh 3 (see Figure 7.23 ).
7.4.6 Cytochrome P450 Conclusions,
Cytochrome P450 is a monooxygenase that carries an Fe(II)/Fe(III) heme b
cofactor identical to that of hemoglobin and myoglobin. The most - studied
cytochrome P450, P450 CAM , hydroxylates the camphor molecule. Spectroscopic
methods and X - ray crystallography have been used to identify many inter-
mediates in the P450 catalytic cycle, although questions remain as to the
exact mechanism of P450 - catalyzed hydroxylations, epoxidations, and other
monooxygenase - type reactions. The study of P450 model compounds has
assisted in identifying possible intermediates in reaction pathways. Some con-
clusions from study of model compound reactions include the following: (1)
Nucleophilic ferric - peroxo porphyrins may epoxidize electron defi cient alkenes
and deformylate aldehydes; (2) isolation and characterization of oxoiron(IV)
porphyrin cation radicals have been very successful in P450 model compounds
in contrast to controversy that still surrounds the participation of this inter-
mediate in the P450 enzyme ’ s catalytic cycle; (3) participation of oxidant –
iron(III) porphyrin adducts in P450 model compounds may still be questionable;
and (4) the mechanism for the activity of oxoiron(IV) porphyrins in oxygenat-
ing organic substrates remains largely ill - defi ned.
7.5 Cytochrome b(6)f: A Green Plant Cytochrome,
7.5.1 Introduction,
Cytochrome b(6)f protein complex, found in green plants, is part of the
photosynthetic system — the principal producer of both oxygen and organic