BIOINORGANIC CHEMISTRY A Short Course Second Edition

MYOGLOBIN AND HEMOGLOBIN 355

(not shown in Figure 7.9B ) occupying the other, proximal side of the porphyrin
ring. Design of such systems have been many and varied and have led, for
example, to models for the T and R states as shown in Figure 7.10.^12 The T - state
model (deoxy form in Figure 7.4 ) uses steric hindrance to prevent the iron ion
from entering the plane of the porphyrin ring, while the R - state model (oxy
form in Figure 7.4 ) permits the iron ion into the porphyrin plane. Geometric
data for hemoglobin, myoglobin, and a picket - fence model compound, shown
in Figure 7.9B , is given in Tables 7.1 and 7.2.
In summary, researchers found a number of methods for avoiding μ - oxo
dimer formation and preserving a fi ve - coordinate Fe(II) in iron - containing

7.2.6 Iron-Containing Model Compounds,

1. Modifying the imidazole to preserve the T - state (shown in Figure
   7.10A ).^22


2. Modifying the porphyrin to sterically prevent addition of a large sixth
   ligand. The most well - known version of these is the “ picket - fence ”

Figure 7.9 (A) Prevention of μ - oxo dimer formation by distal - side porphyrin modifi -
cations. (Adapted with permission from the reference 19. Copyright 1985, Division of
Chemical Education, Inc.) (B) Picket - fence porphyrin Fe(T piv )PP, meso - tetrakis( α , α ,
α , α - o - pivalamidephenyl)porphyrin described in references 6, 17 , 18 , and 21.

Fe Fe

O

O

Fe O 2

Proximal

Base

Proximal

Base

Proximal

Base

Fe

O

HN

CO

NH

C

O

N

N

N

NH

O C

HN

C

N

A

B