title: Modeling the 2-His-1-carboxylate facial triad: iron-catecholato complexes as structural and functional models of the extradiol cleaving dioxygenases.
abstract: Mononuclear iron(II)- and iron(III)-catecholato complexes with three members of a new 3,3-bis(1-alkylimidazol-2-yl)propionate ligand family have been synthesized as models of the active sites of the extradiol cleaving catechol dioxygenases. These enzymes are part of the superfamily of dioxygen-activating mononuclear non-heme iron enzymes that feature the so-called 2-His-1-carboxylate facial triad. The tridentate, tripodal, and monoanionic ligands used in this study include the biologically relevant carboxylate and imidazole donor groups. The structure of the mononuclear iron(III)-tetrachlorocatecholato complex [Fe(L3)(tcc)(H2O)] was determined by single-crystal X-ray diffraction, which shows a facial N,N,O capping mode of the ligand. For the first time, a mononuclear iron complex has been synthesized, which is facially capped by a ligand offering a tridentate Nim,Nim,Ocarb donor set, identical to the endogenous ligands of the 2-His-1-carboxylate facial triad. The iron complexes are five-coordinate in noncoordinating media, and the vacant coordination site is accessible for Lewis bases, e.g., pyridine, or small molecules such as dioxygen. The iron(II)-catecholato complexes react with dioxygen in two steps. In the first reaction the iron(II)-catecholato complexes rapidly convert to the corresponding iron(III) complexes, which then, in a second slow reaction, exhibit both oxidative cleavage and auto-oxidation of the substrate. Extradiol and intradiol cleavage are observed in noncoordinating solvents. The addition of a proton donor results in an increase in extradiol cleavage. The complexes add a new example to the small group of synthetic iron complexes capable of eliciting extradiol-type cleavage and provide more insight into the factors determining the regioselectivity of the enzymes.
authors.affiliation: Organic Chemistry and Catalysis Group, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors.forename: Pieter C A
authors.initials: PC
authors.lastname: Bruijnincx
authors.forename: Martin
authors.initials: M
authors.lastname: Lutz
authors.forename: Anthony L
authors.initials: AL
authors.lastname: Spek
authors.forename: Wilfred R
authors.initials: WR
authors.lastname: Hagen
authors.forename: Bert M
authors.initials: BM
authors.lastname: Weckhuysen
authors.forename: Gerard
authors.initials: G
authors.lastname: van Koten
authors.forename: Robertus J M Klein
authors.initials: RJ
authors.lastname: Gebbink
mesh_terms: D001665:Binding Sites; D002264:Carboxylic Acids / Q000737:chemistry*; D002384:Catalysis; D002396:Catechols / Q000737:chemistry*; D018360:Crystallography, X-Ray; D049308:Dioxygenases / Q000737:chemistry*; D005290:Ferric Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D006860:Hydrogen Bonding; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry
chemical_list: D002264:Carboxylic Acids; D002396:Catechols; D005290:Ferric Compounds; D008024:Ligands; D049308:Dioxygenases; D010100:Oxygen