Five- to Six-Coordination in (Nitrosyl)iron(II) Porphyrinates:  Effects of Binding the Sixth Ligand

Abstract

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole, 4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously unobserved structural features. The tight range of bonding parameters for the [FeNO] moiety for these three complexes suggests a canonical representation for six-coordinate systems (Fe-N(p) = 2.007 A, Fe-N(NO) = 1.753 A, angle FeNO = 138.5 degrees ). Comparison of these data with those obtained previously for five-coordinate systems allows the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe-N(NO) bond and a decrease in the Fe-N-O angle. Several other aspects of the geometry of these systems are also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate [FeNO](7) heme system. We also report the first examples of Mössbauer studies for these complexes. Measurements have been made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required, consistent with differing g-tensor orientations in the five- vs six-coordinate species.

Role of Heme Iron Coordination and Protein Structure in the Dynamics and Geminate Rebinding of Nitric Oxide to the H93G Myoglobin Mutant

“…Previous density functional calculations gave comparable results for 6-coordinate porphyrins (23), whereas Hartee-Fock calculations do not predict a bond between the iron and bound NO, but instead give values that are more consistent with a van der Waal's complex. The x-ray data of the 5C-NO-porphyrin at 295 K (47) revealed that the iron is displaced by 0.21 Å from the heme center toward NO, a value very close to those obtained by DFT calculations (Table 2). On the contrary the heme has a more planar configuration for 6C-NO porphyrin.…”

“…The Fe-N-O bonds are bent for both 5-and 6-coordinate calculations and x-ray structures with an angle in the range 139 -144° (Table 2), which is within 5°of the experimental angle in all cases (47). As expected, the angle Fe-N-O is higher for the calculated model than for Mb-NO (close to 111°; see Ref.…”

“…Three calculations were carried out for comparative purposes, varying both the functional density and the basis set. The quality of the calculation is assessed by comparison of the calculated structures with known data from x-ray crystal structures (47,48). The calculated and experimental structural parameters are listed in Table 2.…”

S-Nitrosylation-induced Conformational Change in Blackfin Tuna Myoglobin

“…4), which produces more confidence in their refined positions. The geometry of the NO-ligated heme in the 0.95 Å ferrous nitrosyl blackfin tuna myoglobin structure is very similar to that observed in both small molecule crystal structures of model compounds (45) and in high level theoretical calculations (46), with a Fe-NO bond length of 1.74 Å and a Fe-N-O angle of 134°( Table 2). The only other atomic resolution protein crystal structures that feature a nitric oxide-and histidine-ligated heme are of the nitrophorin 4 (NP4) protein from Rhodnius prolixus, a non-globin heme protein.…”

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…This has been further demonstrated in a series of temperature-dependent crystal structures and IR investigation [87]. There are other effects on the structure around iron on the addition of the sixth ligand that have been summarized in references [87][88][89]. We have also characterized additional iron(III) species, espcially six-coordinate species and the original references include the following [90][91][92][93][94].…”