title: Trigonal-bipyramidal geometry induced by an external water ligand in a sterically hindered iron salen complex, related to the active site of protocatechuate 3,4-dioxygenase.
abstract: A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces the endogenous His2Tyr2 donor set with water as an external ligand. X-ray crystal structures of a series of iron model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies with the external ligand that is bound in the order Cl << EtO < H2O, which is consistent with the spectrochemical series. The distortion in the model system is not due to steric repulsions but electronic interactions between the external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.
authors.affiliation: Institute for Molecular Science & Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8787, Japan. hiro@ims.ac.jp
authors.forename: Takuya
authors.initials: T
authors.lastname: Kurahashi
authors.forename: Kenji
authors.initials: K
authors.lastname: Oda
authors.forename: Manabu
authors.initials: M
authors.lastname: Sugimoto
authors.forename: Takashi
authors.initials: T
authors.lastname: Ogura
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujii
mesh_terms: D001665:Binding Sites; D002396:Catechols / Q000737:chemistry / Q000378:metabolism; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D005029:Ethylenediamines / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D017434:Protein Structure, Tertiary; D011520:Protocatechuate-3,4-Dioxygenase / Q000737:chemistry* / Q000378:metabolism*; D013059:Spectrum Analysis, Raman; D013379:Substrate Specificity; D014867:Water / Q000737:chemistry*
chemical_list: D002396:Catechols; D005029:Ethylenediamines; D005290:Ferric Compounds; D008024:Ligands; D014867:Water; D011520:Protocatechuate-3,4-Dioxygenase; C034221:catechol