Nitrosyl porphyrins

The current example illustrates PVDOS formulation as an effective basis for comparison of experimental and theoretical NIS data for ferrous nitrosyl tetraphe-nylporph3Tin Fe(TPP)(NO), which was done [101] along with other ferrous nitrosyl porphyrins. Such compounds are designed to model heme protein active sites. In particular, the elucidation of the vibrational dynamics of the Fe atom provides a unique opportunity to specifically probe the contribution of Fe to the reaction dynamics. The geometrical structure of Fe(TPP)(NO) is shown in Fig. 5.16. 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

Hoshino M, Kogure M. Photochemistry of nitrosyl porphyrins in the temperature range 180-300 K and the effects of pyridine on photodenitrosylation of nitrosyliron tetraphe-nylporphyrin. J Phys Chem B 1989 93 5478. 

Mossbauer Spectroscopy. Mossbauer spectroscopy has been used as a powerM technique to probe the electronic stracture of the five- and six-coordinate ferrous nitrosyl porphyrins. The isomer shifts of both types of complexes (5 0.35 mms ) are similar and show temperature... 

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

The work comprising new complexes is actively expanding. IR results suggest that both MS states are formed on irradiation at 20 K of several MNO 6 metalloporphyrins [(OEP)Ru(NO)L] (L = 0-i-C5Hn, SCH2CF3, Cl, py).83,84 For the first time, MSi was IR-characterized for two MNO 7 nitrosyl porphyrins, [Fe(TTP)NO] and [Fe(OEP)NO].83,84 Low-7 photo-induced, reversible linkage isomerization of stable r]1- N2 to the 72-N2 form has been observed in [Os(NH3)5N2]2+, showing that these processes are not limited to NO chemistry (see below for N20).83,84... 

The IR spectrum of Feni(TPP)(0N02)N0 at low-temperatures has vFe-NO at 548 cm-1 284 The resonance Raman spectrum of NO-bound ferric derivatives of wild-type and mutated (BIO Tyr - Phe) FIbN (a haemoglobin from Mycobacterium tuberculosis) showed vFe-NO and 8Fe-N-0 at 591, 579 cm-1 respectively.285 Nuclear resonance vibrational spectroscopy has been used to identify a number of modes involving motion of iron in the plane of the porphyrin in nitrosyl porphyrins, e.g. Fe NO torsion modes at 27 and 54 cm 1 in Fe(TPP)NO 286... 

Bonding and Dissociation Reactions in Nitrosyl Porphyrins How is NO Released from the Ferri-Hemes ... 

Well-characterized complexes of the MNO 8 class are scarce (Table 7.2).62 Five-coordinate species with bound NO and square pyramidal geometries are dominant,10 18 probably related to the great trans-effect of NO-. Most common are the cobalt nitrosyl porphyrins.18 By reacting with monodentate ligands (NCS-, Cl-, Br-, etc.), six-coordinate complexes may be obtained. Reaction 7.28 describes the preparation of the first compound of this type studied by X-ray methods.13s... 

Keywords HNO NO Nitroxyl Azanone Nitroxyl anion Nitrosyl Porphyrin Heme Iron Manganese Ruthenium Cobalt Reductive nitrosylation Kinetics Oxidation Protein Myoglobin NOS. 

The reactions can be followed spectroscopically based on the corresponding reported spectral changes characteristic of the starting ferric and nitrosyl porphyrins, respectively. The observed changes in the position of the Soret and Q bands are, however, moderate. 

Kinetic analysis show that the association rate constant for the reaction of the reconstituted globin with azanone is practically the same than that for the free porphyrinate, suggesting that the protein environment is not involved in the reaction mechanism. However, oxidation of the nitrosyl porphyrin inside the protein is ca. 1000 times slower than for the porphyrin in solution, a feature that is ascribed to the role played by the distal residues which protect the nitrosyl product inside the protein matrix (see Scheme 7). 

Electrochemical measurements of Co(Por) boimd to electrodes show the presence of three redox couples as well as the electrode bound nitrosyl porphjrrin complex. The most significant feature of the electrochemical data is that for Co VCo couple (Eq. (31)) the obtained Ey value is shifted ca. 400mV to lower potentials compared to the Ey2 in solution, strongly suggesting that Co(Por) adsorption on the gold electrode facilitates Co oxidation. On the other hand, for the nitrosyl porphyrin, the shift due to the gold surface effect is much smaller, and of only 60 mV... 

Once again, the trans cr-bonding alkyl or aryl ligand exerts a powerful influence on the M-N-O bond angle. Studies on transient intermediates during photolysis of Ru-NO nitrosyls have revealed different modes of binding (and dissociation) of coordinated NO at the ruthenium centers. Metastable NO linkage isomers have been observed for MNO (M = Fe, Ru, Os) and for MNO ° complexes of Ni, as well as for FeNO iron nitrosyl porphyrins [208-212]. 

Scheidt WR, Barabanschikov A et al (2010) Electronic structure and dynamics of nitrosyl porphyrins. Inorg Chem 49 6240-6252... 

Cheng L, Novozhilova I et al (2000) First observation of photoinduced nitrosyl linkage isomers of iron nitrosyl porphyrins. J Am Chem Soc 122 7142-7143... 

Fomitchev V, Coppens P et al (1999) Photo-induced metastable linkage isomers of ruthenium nitrosyl porphyrins. Chem Commun 2013-2014... 

Figure 34 Formation of the r/ -ON and -NO isomers in some six-coordinate ruthenium nitrosyl porphyrins. (A) (0EP)Ru(N0)(0-/-C5Fln) and (B) (OEP)Ru(NO)(SCFl2CF3).

NO to ferrous nitrosyl-porphyrins to yield fra s-[Fe(por)(NO)2] in low-temperature solutions (i39). These are also EPR-silent complexes, with similar IR properties as described for Ij. From theoretical calculations, a trans-syn (C2v) conformation has been proposed for [Fe(TPP)(NO)2]- We found a similar picture for [Fe(CN)4(NO)2], also reproducing the trans-syn geometry, with fairly consistent no values. 

Since the rate of reductive nitrosylation was shown to be controlled by the electrophihcity of the coordinated NO, as well as the chemical nature of the nucleophilic reactant (N02, HONO or HO ), systematic temperature and pressure studies on the nitrite-catalyzed reduction of ferric nitrosyl porphyrin complexes (see Equation 11) as a function of pH, nitrite concentration, and nature of the porphyrin substituents were performed (118,119). 

Fujita, E. and J. Fajer (1983). Models for nitrite reductases. Redox chemistry of iron-nitrosyl porphyrins, chlorins, and isobacteriochlorins and p cation radicals of cobalt-nitrosyl isobacteriochlorins. J. Am. Chem. Soc. 105, 6743-6745. 

A significant number of nitrosyl metalloporphyrins has been synthesized over the past two decades [78, 80]. (see Periodic Table of nitrosyl porphyrins in Fig. 6) and the most well-characterized of these complexes have been those with Fe(II) [5, 66, 67, 71, 76, 77, 81-89] or Co(II) [27, 67, 70, 90-93] central metal ions. 

Manganese porphyrins with a nitrosyl axial ligand have also been examined for their electrochemical properties. The oxidation of (TPP)Mn(NO) to its Mn(III) form is accompanied by a loss of the NO ligand (Fig. 7). However, the two reductions of the compound are reversible and stepwise generate a Mn(II) nitrosyl porphyrin 7T-anion radical and a nitrosyl porphyrin dianion at more negative potentials [100]. High valent manganese porphyrins have... 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

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