Crystallographic and EXAFS studies of conformationally designed nonplanar nickel(II) porphyrins

Barkigia, Kathleen M.; Renner, Mark W.; Furenlid, Lars R.; Medforth, Craig J.; Smith, Kevin M.; Fajer, J.

doi:10.1021/ja00062a029

Cited by 181 publications

(155 citation statements)

References 5 publications

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“…A decrease of the (LUMO − HOMO) of a porphyrin together with a destabilization of its -cation, was correlated with structural data on the deformation of the porphyrin ring from a planar to a saddle configuration [22]. It is then probable that the alkyl porphyrins P1, P2 and P3 studied in this work are structurally deformed.…”

Section: Redox Processes: General Datasupporting

confidence: 69%

Electrochemical and spectroelectrochemical characterization of meso-tetra-alkyl porphyrins

Paliteiro

Sobral

2005

Electrochimica Acta

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A series of three meso-tetra-alkyl porphyrins (H 2 TAPs) were synthesised and their electrochemical properties were studied in dichloromethane containing either tetra-butyl ammonium perclorate (TBAP) or tetra-butyl ammonium hexafluorophosphate TBA(PF 6 ) as supporting electrolyte (SE) by cyclic voltammetry and in situ UV/vis spectroelectrochemistry. Values of E 1/2 for the recorded four redox processes of these porphyrins and of the meso-tetra-phenyl porphyrin (H 2 TPP) on glassy carbon and platinum were measured and compared. The redox data collected on the first oxidation and the first reduction processes predict that the value of (LUMO − HOMO) for the H 2 TAPs is lower than for meso-tetra-phenyl porphyrin and, therefore, that the first Q band of the electronic spectra of the H 2 TAPs is red shifted relatively to the same band in the spectrum of H 2 TPP; this shift was indeed found. The second oxidation process of the H 2 TAPs is influenced by the supporting electrolyte used and the dication then formed is consumed by an irreversible chemical reaction. The species produced in this reaction is irreversibly reduced at a considerably lower potential. The electrochemical and spectroelectrochemical data collected are consistent with the occurrence of structural distortion in the macrocycle rings of the H 2 TAPs.

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Section: Redox Processes: General Datasupporting

confidence: 69%

Electrochemical and spectroelectrochemical characterization of meso-tetra-alkyl porphyrins

Paliteiro

Sobral

2005

Electrochimica Acta

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“…The multiple signals indicate that there are multiple species in frozen solution which, since there was no motion upon reduction, arise from multiple conformation of the Ni II parent complex. As discussed by many researchers, 51,55,56,69,114,115 Ni II tetrapyrroles are highly flexible (towards ruffling, saddling etc) and the heterogeneity observed here by EPR is not surprising. The sample was then briefly (ca.…”

Section: (Tmc)]supporting

confidence: 67%

Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

Telser

2010

J. Braz. Chem. Soc.

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Complexos de cobre(II) (3d 9 , S = 1/2) são estáveis e amplamente investigados por espectroscopia de ressonância paramagnética eletrônica (EPR). Já o isoeletrônico níquel(I) é muito menos comum e muito menos estudado. No entanto, níquel(I) tem interesse biológico, uma vez que o sítio ativo da metil coenzima M redutase (MCR) , S = 1/2) complexes are stable and widely investigated by electron paramagnetic resonance (EPR) spectroscopy. In contrast, isoelectronic nickel(I) is much less common and much less investigated. Nickel(I), however, is of biological interest as the active site of methyl coenzyme M reductase (MCR) contains a tetraaza macrocyclic ligand, F 430 , which coordinates Ni I in its active form, MCR red1 . As result, the redox behavior and spectroscopy of tetraaza macrocyclic complexes of nickel is of importance in biomimetic chemistry. Such efforts are complicated by the difficulty in generating Ni I from their stable, Ni II , precursors. Reduction of Ni II macrocyclic complexes can afford Ni I in certain cases, but in many other cases can lead instead to reduction of the macrocycle to generate an organic radical anion. Previous studies on the formation of tetraaza macrocyclic complexes of Ni I are discussed in terms of the competition between metal-centered and ligandcentered reduction. EPR results are particularly important in making the distinction between these two reduction processes, as formation of Ni I gives characteristic EPR spectra similar to those for Cu II , while ligand-centered reduction gives narrow EPR spectra at g = 2.00, typical of organic radicals. Even if metal-centered reduction occurs, the geometry of the resulting Ni I macrocyclic complex is highly variable and, as a result, the EPR spectral appearance is highly variable. In this case, the comparison is between the extremes of spectra typical for tetragonally distorted complexes (dx2-y2 1 ground state, which includes tetragonally distorted octahedral, square pyramidal and square planar geometries) and those for trigonal bipyramidal complexes (dz2 1 ground state). Previous work on Cu II was related to the situation for Ni I . The different types of EPR spectra for such systems are specifically discussed using previously unpublished examples of several tetraaza macrocyclic complexes of nickel, including F 430 and MCR itself.

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“…The two pairs of pyrrole rings are twisted in the opposite direction, resulting in a saddle shape for the porphyrin. This type of deviation has been observed in the crystal structures of a number of substituted metalloporphyrins (23)(24)(25)(26). The distortions are most likely to be a consequence of the short Ni-N(pyrrole) distances ( here to assume a planar conformation.…”

Section: Resultsmentioning

confidence: 75%

A DNA–porphyrin minor-groove complex at atomic resolution: The structural consequences of porphyrin ruffling

Bennett

Krah

Wien

et al. 2000

Proc. Natl. Acad. Sci. U.S.A.

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The crystal structure of a B-type DNA hexanucleotide duplex complexed with the porphyrin molecule nickel-[tetra-N-methylpyridyl] porphyrin has been solved by multiwavelength anomalous diffraction phasing and refined to an R factor of 11.5% at a resolution of 0.9 Å. The structure has been solved and refined as two crystallographically independent duplexes, stacked end to end. Contrary to expectation, the porphyrin molecule is not intercalated into the duplex but is stacked onto the ends of the two-duplex stack. The porphyrin molecule is highly buckled as a consequence of the nickel coordination, which produces large changes in local DNA structure. A second mode of porphyrin binding is apparent as a consequence of crystal packing, which places the ligand in the minor groove of an adjacent duplex. This structure thus provides, to our knowledge, the first atomic visualization of minor-groove binding for a porphyrin molecule. The geometry of groove binding provides a ready explanation for porphyrin-induced DNA strand cleavage at deoxyribose residues.DNA structure ͉ minor groove binding ͉ end capping T he interactions of cationic porphyrins with nucleic acids have received considerable attention (1, 2). Several have clinical potential as anticancer agents in photodynamic therapy (3, 4), probably as a consequence of their ability to selectively accumulate on the surface of tumor cells, become internalized, bind to genomic DNA, and then induce DNA strand cleavage. One such compound is tetra-(N-methylpyridyl) porphyrin (TMPy: Fig. 1a), which can be coordinated with a range of transition metals (1, 2, 5, 6). TMPy has more direct antitumor activity, possibly as a consequence of its ability to inhibit the telomerase enzyme (7). Binding to quadruplex DNA as well as to duplexes has been reported for TMPy and various complexes (8-10). Stabilization of G-quadruplex structures has been shown to be the mechanism whereby TMPy inhibits telomerase from catalyzing the synthesis of further linear telomere repeats. NMR and photocleavage data have been interpreted in favor of a model with a TMPy molecule externally stacking onto guaninequadruplex structures (8-10).Noncovalent interactions of TMPy and its metal complexes with duplex DNA are sequence dependent (2, 11), with intercalation occurring at 5Ј-CpG sites, as shown by NMR and other biophysical studies (12, 13) as well as molecular modeling (13,14). A crystallographic analysis (15) of the Cu 2ϩ complex of TMPy bound to the sequence 5Ј-d(CGTACG) 2 also found the ligand intercalated at this site, but with the 5Ј-end cytosine swung out in an extrahelical arrangement. By contrast, biophysical and modeling (14, 16) studies have suggested that TMPy itself (and many of its metal complexes) bind in the minor groove at A͞T sequences and perturb the double helix only to a minimal extent at such sites (16). No structural data has been reported to date on groove-binding modes. TMPy itself is a planar molecule (14, 15), and it has been assumed that the groove binding would therefore be analogou...

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Crystallographic and EXAFS studies of conformationally designed nonplanar nickel(II) porphyrins

Cited by 181 publications

References 5 publications

Electrochemical and spectroelectrochemical characterization of meso-tetra-alkyl porphyrins

Electrochemical and spectroelectrochemical characterization of meso-tetra-alkyl porphyrins

Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

A DNA–porphyrin minor-groove complex at atomic resolution: The structural consequences of porphyrin ruffling

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