PICOSECOND REACTION OF PICKET FENCE HEME WITH O-2 AND CO - GEMINATE RECOMBINATION IN THE SOLVENT CAGE

Hemes, types of iron prophyrins, have been developed over many years t o model aspects of ligand binding in heme proteins. Recently, the heme -ligand system has proved useful for exploring the detailed kinetics o f photodissociation-geminate recombination within a solvent cage. Oxyg en is a key ligand for both purposes, but most hemes are unstable in i ts presence. Picket fence heme is an exception, it is reasonably stabl e with O-2. Cage recombination efficiency in toluene solvent after pho tolysis with 314 nm laser light is about 43% when methylimidazole is u sed as the trans base, reduced to about 20% when dimethylimidazole is used to introduce trans strain. Previously, geminate rebinding of CO w as unmeasurable in any model heme system, but with picket fence heme a nd methylimidazole, recombination efficiency appears to approach 10%, again reduced significantly by dimethylimidazole. The lifetime of the caged geminate pair is longer in the picket fence system, about 30 ps in all cases. These observations fit well with expectations from prior studies but fill some crucial gaps in the emerging picture.