title: Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins.
abstract: The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.
authors.affiliation: Biomolecular Materials and Interfaces Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
authors.forename: Yujiang
authors.initials: Y
authors.lastname: Song
authors.forename: Raid E
authors.initials: RE
authors.lastname: Haddad
authors.forename: Song-Ling
authors.initials: SL
authors.lastname: Jia
authors.forename: Saphon
authors.initials: S
authors.lastname: Hok
authors.forename: Marilyn M
authors.initials: MM
authors.lastname: Olmstead
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Nurco
authors.forename: Neal E
authors.initials: NE
authors.lastname: Schore
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.forename: Jian-Guo
authors.initials: JG
authors.lastname: Ma
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Smith
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Gazeau
authors.forename: Jacques
authors.initials: J
authors.lastname: Pécaut
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Marchon
authors.forename: Craig J
authors.initials: CJ
authors.lastname: Medforth
authors.forename: John A
authors.initials: JA
authors.lastname: Shelnutt
mesh_terms: D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry; D008024:Ligands; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*; D010088:Oxidoreductases / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D013816:Thermodynamics
chemical_list: D008024:Ligands; D008665:Metalloporphyrins; D006418:Heme; D009532:Nickel; D010088:Oxidoreductases; C021572:methyl coenzyme M reductase