title: A quantum chemical survey of metalloporphyrin-nitrosyl linkage isomers: insights into the observation of multiple FeNO conformations in a recent crystallographic determination of nitrophorin 4.
abstract: Using density functional theory-based geometry optimizations, we have searched for eta(1)-NO, eta(1)-ON (isonitrosyl), and eta(2)-NO (side-on bound NO) linkage isomers of a number of metalloporphyrin-NO complexes, M(Por)(NO)(L), where Por = porphinato dianion, M = Mn(II), Fe(II), Fe(III), Ru(II), Ru(III), Co(II), and Rh(II), and L = no ligand, SMe, Ph, and imidazole. The eta(1)-NO isomer had the lowest energy in all cases, and the isonitrosyl isomer was also located as a higher energy potential energy minimum in a number of cases. The eta(2)-NO isomer was only located as a minimum for Mn(II) (L = no ligand), Fe(III) (L = no ligand), and Ru(III) (L = Ph, imidazole, pyrdine), suggesting that an [MNO](6) electron count is important for stabilization of the eta(2) mode of ligation. However, in the presence of axial ligands L, the side-on isomers of [FeNO](6) complexes were not stable and opened up to an unusual geometry where the FeN(O) and NO vectors were tilted in opposite directions relative to the heme normal. Exactly such a geometry, as well as a "normal" upright geometry, has been observed in a recent crystallographic determination of nitrophorin 4 (Nature Struct. Biol. 2000, 7, 551), a salivary protein from the blood-sucking insect Rhodnius prolixus. Together, the calculated and experimental result illustrate the extreme softness of the FeNO potential energy surface toward various forms of tilting and bending deformations.
authors.affiliation: Department of Chemistry, Faculty of Science, University of Tromsø, N-9037 Tromsø, Norway.
authors.forename: T
authors.initials: T
authors.lastname: Wondimagegn
authors.forename: A
authors.initials: A
authors.lastname: Ghosh
mesh_terms: D018360:Crystallography, X-Ray; D006420:Hemeproteins / Q000737:chemistry; D006454:Hemoglobins; D007536:Isomerism; D008024:Ligands; D008665:Metalloporphyrins / Q000737:chemistry*; D019216:Metals, Heavy / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009603:Nitroso Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012471:Salivary Proteins and Peptides / Q000737:chemistry
chemical_list: D006420:Hemeproteins; D006454:Hemoglobins; D008024:Ligands; D008665:Metalloporphyrins; D019216:Metals, Heavy; D009603:Nitroso Compounds; D012471:Salivary Proteins and Peptides; C090560:nitrophorin; C008657:nitrosyl hemoglobin