Aza-Crown-Capped Porphyrin Models of Myoglobin: Studies of the Steric Interactions of Gas Binding

A series of myoglobin active site analogues ( 1 − 6 ) has been synthesized and characterized. These synthetic models differ in their cavity dimensions, and have been designed to demonstrate the effects of steric factors on O 2 and CO binding affinities. Quantitative gas titrations were employed to measure these affinities, yielding M values that are strikingly 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 cavity dimensions and conformation of 2 were determined by single-crystal X-ray structural analysis of the Zn analogue 7 . This paper unequivocally demonstrates that steric effects can control the ratio of O 2 /CO binding constants.