352 IRON-CONTAINING PROTEINS AND ENZYMES
Fe(T piv )PP - (1 - MeIm)O 2 , works better as a model for HbO 2 and MbO 2 than
does model 3, Fe(T piv )PP - (2 - MeIm)O 2 , although the φ and tilt values are not
good matches with MbO 2 in either model.
7.2.5 Some Notes on Model Compounds,
Kenneth Suslick^19 has written several pertinent statements about the study of
metalloproteins and their model compounds. This paradigm will guide the
study of the varied bioinorganic systems described in this chapter. Additions
to Suslick ’ s statement in point 2 stress the importance of modern structural
analysis (X - ray crystallography, EXAFS, EPR, and two - and three -
dimensional high - fi eld NMR) of the purifi ed metalloproteins and their model
compounds. As paraphrased from reference 19 , the following statements
outline the steps involved in the study of metalloproteins:
TABLE 7.2 Oxy - Heme Stereochemistry for Mb, Hb, and Model Compounds
MbO 27
HbO 2 ( α H 2 O) a
(β )^16Fe Model 2 b
(Oxy)^17Fe Model 3 c
(Oxy)^18Fe – N p , Å d 1.95 (6) 1.99 (5) 1.98 (1) 1.996 (4)
1.96 (6)
Fe – N Im , Å d 2.07 (6) 1.94 (9) 2.07 (2) 2.107 (4)
2.07 (9)
Fe – O, Å 1.83 (6) 1.66 (8) 1.75 (2) 1.898 (7)
1.87 (3)
O – O, Å 1.22 (6) 1.24 1.22 (2)
Fe – O – O, deg 115 (5) 153 (7) 131 129 (2)
159 (12)
Fe – Porph, Å 0.18 (3) 0.12 (8) − 0.03 0.09
− 0.11 (8)
φ , deg e 1 11 20 22
2 7
Tilt, deg e 4 3 0 7
5
a α and β refer to the particular protein chains in myoglobin or hemoglobin. One notes from the
table that the bond distances and angles are slightly different for the two moieties. In each table
cell, data are reported in order α and then β.
b Fe model 2 (oxy form) Fe(T piv )PP – (1 - MeIm)O 2. The porphyrin ligand, Fe(T piv )PP, is the picket
fence porphyrin, meso - tetrakis( α , α , α , α - o - pivalamidephenyl)porphyrin (see Figure 16 of refer-
ence 9 ). In this chapter see Figure 7.9B.
c Fe model 3 (oxy form) Fe(T piv )PP – (2 - MeIm)O 2. The ligand is (T piv )PP – (2 - MeIm).
d F e – N p is the bond distance to porphyrin nitrogen ligand atoms. Fe – N Im is the bond distance to
histidine (or imidazole in model compounds) nitrogen ligand atoms.
e φ is defi ned as the angle between the plane of the axial base (imidazole or histidine) and the
plane defi ned by the metal, the N ligands of the porphyrin ring, and the axial base. Tilt is defi ned
as the angle the axial base moves away from the normal to the metal – porphyrin plane and is
affected by methyl substituents such as those on Fe(T piv )PP – (2 - MeIm).