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Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)].

ABSTRACT: The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)iron(II), [Fe(TPP)(NO)], has been explored over the temperature range of 33-293 K. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the single nitrosyl oxygen atom. The phase transition to the triclinic crystal system leads to a partial ordering with the molecule now having inversion symmetry and disorder of the axial nitrosyl ligand over only two positions. The increase in ordering allows subtle molecular geometry features to be observed; in particular, an off-axis tilt of the Fe-N(NO) bond from the heme normal is apparent. The transition of the reversible phase change begins at about 250 K. This transition has been confirmed by both X-ray diffraction studies and a differential scanning calorimetry study.

SUBMITTER: Silvernail NJ

PROVIDER: S-EPMC2680005 | biostudies-literature | 2009 Feb

REPOSITORIES: biostudies-literature

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Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)].

Silvernail Nathan J NJ Olmstead Marilyn M MM Noll Bruce C BC Scheidt W Robert WR

Inorganic chemistry 20090201 3

The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)iron(II), [Fe(TPP)(NO)], has been explored over the temperature range of 33-293 K. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the si ...[more]

PMID: 19128024

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Project description:This paper presents oriented single-crystal Nuclear Resonance Vibrational Spectroscopy (NRVS) data for the six-coordinate (6C) ferrous heme-nitrosyl model complex [(57)Fe(TPP)(MI)(NO)] (1; TPP(2-) = tetraphenylporphyrin dianion; MI = 1-methylimidazole). The availability of these data enables for the first time the detailed simulation of the complete NRVS data, including the porphyrin-based vibrations, of a 6C ferrous heme-nitrosyl, using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Importantly, the Fe-NO stretch is split by interaction with a porphyrin-based vibration into two features, observed at 437 and 472 cm(-1). The 437 cm(-1) feature is strongly out-of-plane (oop) polarized and shows a (15)N(18)O isotope shift of 8 cm(-1) and is therefore assigned to nu(Fe-NO). The admixture of Fe-N-O bending character is small. Main contributions to the Fe-N-O bend are observed in the 520-580 cm(-1) region, distributed over a number of in-plane (ip) polarized porphyrin-based vibrations. The main component, assigned to delta(ip)(Fe-N-O), is identified with the feature at 563 cm(-1). The Fe-N-O bend also shows strong mixing with the Fe-NO stretching internal coordinate, as evidenced by the oop NRVS intensity in the 520-580 cm(-1) region. Very accurate normal mode descriptions of nu(Fe-NO) and delta(ip)(Fe-N-O) have been obtained in this study. These results contradict previous interpretations of the vibrational spectra of 6C ferrous heme-nitrosyls where the higher energy feature at approximately 550 cm(-1) had usually been associated with nu(Fe-NO). Furthermore, these results provide key insight into NO binding to ferrous heme active sites in globins and other heme proteins, in particular with respect to (a) the effect of hydrogen bonding to the coordinated NO and (b) changes in heme dynamics upon NO coordination. [Fe(TPP)(MI)(NO)] constitutes an excellent model system for ferrous NO adducts of myoglobin (Mb) mutants where the distal histidine (His64) has been removed. Comparison to the reported vibrational data for wild-type (wt) Mb-NO then shows that the effect of H bonding to the coordinated NO is weak and mostly leads to a polarization of the pi/pi* orbitals of bound NO. In addition, the observation that delta(ip)(Fe-N-O) does not correlate well with nu(N-O) can be traced back to the very mixed nature of this mode. The Fe-N(imidazole) stretching frequency is observed at 149 cm(-1) in [Fe(TPP)(MI)(NO)], and spectral changes upon NO binding to five-coordinate ferrous heme active sites are discussed. The obtained high-quality force constants for the Fe-NO and N-O bonds of 2.57 and 11.55 mdyn/A can further be compared to those of corresponding 5C species, which allows for a quantitative analysis of the sigma trans interaction between the proximal imidazole (His) ligand and NO. This is key for the activation of the NO sensor soluble guanylate cyclase. Finally, DFT methods are calibrated against the experimentally determined vibrational properties of the Fe-N-O subunit in 1. DFT is in fact incapable of reproducing the vibrational energies and normal mode descriptions of the Fe-N-O unit well, and thus, DFT-based predictions of changes in vibrational properties upon heme modification or other perturbations of these 6C complexes have to be treated with caution.

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Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)].

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Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)].

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