Steric effects on geminate recombinations

Steric effects on the binding of isonitrile ligands to iron(II) porphyrins were investigated by picosecond flash photolysis. Two different types of steric effects were distinguished and characterized: (1) steric restrictions to porphyrin planarity and (2) blocking of the pathway for ligand approach....

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Bibliographic Details
Published in:Journal of the American Chemical Society 1990-09, Vol.112 (19), p.6875-6880
Main Authors: Traylor, Teddy G, Taube, Douglas J, Jongeward, Karen A, Magde, Douglas
Format: Article
Language:English
Subjects:
400201 - Chemical & Physicochemical Properties
550200 - Biochemistry
BASIC BIOLOGICAL SCIENCES
Bioinorganic chemistry
CARBONIC ACID DERIVATIVES
CARBOXYLIC ACIDS
CHEMICAL BONDS
CHEMICAL REACTIONS
Chemistry
DECOMPOSITION
Exact sciences and technology
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
Inorganic chemistry and origins of life
INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY
ISONITRILES
KINETICS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
PORPHYRINS
PROTEINS
RECOMBINATION
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Summary:Steric effects on the binding of isonitrile ligands to iron(II) porphyrins were investigated by picosecond flash photolysis. Two different types of steric effects were distinguished and characterized: (1) steric restrictions to porphyrin planarity and (2) blocking of the pathway for ligand approach. Heme planarity was restricted by coordinating 1,2-dimethylimidazole trans to the ligand binding site being investigated. Blocking of the binding site was explored by using adamantane heme 6,6-cyclophane, in which the adamantane moiety forms a cap over the binding site. Results of picosecond kinetic measurements demonstrate that the first effect, heme nonplanarity or trans strain, influences the bond-making step, whereas the second effect, ligand blocking, involves a conformational preequilibrium prior to bond making. Relevance of these findings for contact pair recombination, in general, and for heme protein ligation, in particular, are discussed.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja00175a022