Jp. Collman et al., AZA-CROWN-CAPPED PORPHYRIN MODELS OF MYOGLOBIN - STUDIES OF THE STERIC INTERACTIONS OF GAS BINDING, Journal of the American Chemical Society, 119(15), 1997, pp. 3481-3489
A series of myoglobin active site analogues (1-6) has been synthesized
and characterized. These synthetic models differ in their cavity dime
nsions, and have been designed to demonstrate the effects of steric fa
ctors on O-2 and CO binding affinities. Quantitative gas titrations we
re employed to measure these affinities, yielding M values that are st
rikingly lower than those reported for hemoglobin and myoglobin. The 1
,4,7-triazacyclononane-capped porphyrin 1 has about 1200 times the CO
affinity but only about 10 times the O-2 affinity of the cyclam-capped
porphyrin 2, suggesting a more open gas binding cavity for 1. The cav
ity dimensions and conformation of 2 were determined by single-crystal
X-ray structural analysis of the Zn analogue 7. This paper unequivoca
lly demonstrates that steric effects can control the ratio of O-2/CO b
inding constants.