BIOINORGANIC CHEMISTRY A Short Course Second Edition

354 IRON-CONTAINING PROTEINS AND ENZYMES

Fe II()+⇔O 22 Fe III O( )−− (7.3)

Fe III O()−+ 22 −−Fe II()⇔Fe III O()− −^2 Fe III() (7.4)

Fe III O()−− 22 − Fe III()→Fe IV()=O (7.5)

Fe IV O Fe II Fe III O Fe III

-oxo dimer

()=+()→()−−()

μ

(7.6)

Successful iron - containing Mb and Hb models have simulated the protein
surroundings of the heme porphyrin through introduction of steric hindrance
on the distal side of the porphyrin (Figures 7.9 and 7.10 ). The porphyrin ’ s
decoration is necessary to prevent irreversible oxidation of the Fe(II) by
dioxygen and the formation of the μ - oxo dimer shown in Figure 7.8. Other
strategies used to make Hb and Mb model compound irreversible oxidation
slower are to lower the temperature, to attach the Fe(II) complex to a solid
support, or to make the observation time faster by using stopped fl ow
kinetics.^19
In order to prepare and isolate solid - state, crystalline, oxygenated iron –
heme model complexes, chemists learned to synthesize (by self - assembly
methods) and oxygenate many types of hindered porphyrins. For instance,
“ capped ” porphyrins were synthesized by direct condensation of a suitable
tetraaldehyde with four pyrrole molecules.^22 “ Picket - fence ” porphyrins such
as [Fe(TPP)( N - MeIm)] (where TPP = meso - tetraphenylporphyrin and N -
MeIm = N - methylimidazole) were synthesized and purifi ed by separation of
isomeric mixtures of tetra( O - substituted phenyl)porphin.^23 Another picket -
fence porphyrin, Fe(T piv )PP, meso - tetrakis ( α , α , α , α - o - pivalamidephenyl)
porphyrin, synthesized and studied by Collman ’ s group in the 1970s, is shown
in Figure 7.9B.^21 Structural and geometric data on deoxy and oxy forms of this
model porphyrin are listed in Tables 7.1 and 7.2. Many other representative
examples of hemoglobin model compounds, some of which became known
as the “ basket ” and “ pocket ” porphyrins, are shown in Figures 8 and 9 of
reference 9.
All successful myoglobin and hemoglobin model compounds provide steric
bulk on the distal side of the porphyrin ring with a hydrophobic pocket for
complexation of dioxygen as well as a bulky alkyl imidazole proximal ligand

Figure 7.8 Formation of μ - oxo dimer. (Adapted with permission from the reference
19. Copyright 1985, Division of Chemical Education, Inc.)

O

O

O O

μ-oxo dimer

Fe O 2
Proximal
Base Fe
+O 2 FeProximal Fe Fe Fe Fe
Base