Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins.
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.
Song
Yujiang
Y
Biomolecular Materials and Interfaces Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
Haddad
Raid E
RE
Jia
Song-Ling
SL
Hok
Saphon
S
Olmstead
Marilyn M
MM
Nurco
Daniel J
DJ
Schore
Neal E
NE
Zhang
Jun
J
Ma
Jian-Guo
JG
Smith
Kevin M
KM
Gazeau
Stéphanie
S
Pécaut
Jacques
J
Marchon
Jean-Claude
JC
Medforth
Craig J
CJ
Shelnutt
John A
JA
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
D008024:Ligands; D008665:Metalloporphyrins; D006418:Heme; D009532:Nickel; D010088:Oxidoreductases; C021572:methyl coenzyme M reductase