Hydrogen atom transfer reactions of iron-porphyrin-imidazole complexes as models for histidine-ligated heme reactivity.
Hydrogen atom transfer (HAT) reactions of the bis(histidine) cytochrome active site models (TPP)FeII(ImH)2 (FeIIImH) and (TPP)Fe(Im)(ImH) (FeIIIIm) have been examined in acetonitrile solvent (TPP = tetraphenylporphyrin, ImH = 4-methylimidazole). The ascorbate derivative 5,6-isopropylidine ascorbate, hydroquinone, and the hydroxylamine TEMPOH all rapidly add H* to FeIIIIm to give FeIIImH. Similarly, the phenoxyl radical 2,4,6-tBu3C6H2O* and excess TEMPO* each oxidize FeIIImH to give FeIIIIm. On the basis of redox potential, pKa, and equilibrium measurements, the N-H bond in FeIIImH was found to have a bond dissociation free energy (BDFE) of 70 +/- 2 kcal mol(-1). A hydrogen atom transfer mechanism (concerted transfer of e- and H+) is indicated based on data for the ascorbate and TEMPO* reactions.
Warren
Jeffrey J
JJ
University of Washington, Department of Chemistry, Box 351700, Seattle, Washington 98195, USA.
Mayer
James M
JM
D006418:Heme / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*
D007093:Imidazoles; D008024:Ligands; D011166:Porphyrins; D006418:Heme; D006639:Histidine; C029899:imidazole; D006859:Hydrogen; D007501:Iron