Heme nitrosyl porphyrinate complexes

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

Reactions of NO were also studied with the synthetic heme protein discussed earlier, namely the recombinant human serum albumin (rHSA) with eight incorporated TPPFe derivatives bearing a covalently linked axial base, were also investigated. The UV-vis absorption spectrum of the phosphate buffer solution at physiological pH showed absorption band maxima at 425 and 546 nm upon the addition of NO to form the nitrosyl species, which was also formed when the six-coordinate CO-adducts were reacted with NO gas. EPR spectroscopy revealed that the albumin-incorporated iron(II) porphyrin formed six-coordinate nitrosyl complexes. It was observed that the proximal imidazole moiety does not dissociate from the central iron when NO binds to the trans position. The NO-binding affinity P1 /2no was 1.7 X 10 torr at pH 7.3 and 298 K, significantly lower than that of the porphyrin complex itself, and was interpreted as arising from the decreased association rate constant (kon(NO), 8.9 x 10 M s" -1.5 x 10 M s ). Since NO-association is diffusion controlled, incorporation of the synthetic heme into the albumin matrix appears to restrict NO access to the central iron(II). ... 

Due to the many biological functions of ferrous heme nitrosyls, many corresponding model complexes have been synthesized and structurally and spectroscopically characterized, viz. tetraphenylporphyrin (TPPH2), octaethylporphyrin (OEPH2) and protoporphyrin IX diester (PPDEH2), ms o-tetrakis (4-carboxyphenyl) porphyrin (TCPP), meso-tetrakis [4-(N,N,N-trimethyl) aminophenyl] porphyrin (TTMAPP),... 

The NIS investigation of heme complexes includes various forms of porphyrins (deuteroporphyrin IX, mesoporphyrin IX, protoporphyrin IX, tetraphenylpor-phyrin, octaethylporphyrin, and picket fence porphyrin) and their nitrosyl (NO) and carbonyl (CO) derivatives, and they have been the subject of a review provided by Scheidt et al. [109]. 

The Fe(II)-NO complexes of porphyrins 66-68) and heme proteins 24, 49, 53, 69-76) have been studied in detail by EPR spectroscopy, which allows facile differentiation between five-coordinate heme—NO and six-coordinate heme—NO(L) centers. However, only a few reports of the Mossbauer spectra of such complexes have been published 68, 77-82), and the only Fe(III)-NO species that have been studied by Mossbauer spectroscopy include the isoelectronic nitroprusside ion, [FeCCNlsCNO)] (7S), the five-coordinate complexes [TPPFe(NO)]+ 68) and [OEPFe(NO)]+ 82), and two reports of the nitro, nitrosyl complexes of iron(III) tetraphenylporphjrrins, where the ligand L is NO2 82, 83). 

Fig. 7 also shows that base binding in the trans position weakens the metal to nitrosyl backbonding in that the Fe-NO bond length increases by 0.3 A and the Fe-N-O bond angle decreases 9°. In concert with this, the displacement of the Fe from the porphyrin plane decreases from 0.21 A in the square pyramidal structure to 0.07 A in the octahedral complex. The equilibria of NO binding to hemes is summarized in Fig. 8 [64], which indicates that NO binds to Fe" better... 

The first HNO complex, Os(PPh3)2(CO)(HNO)Cl2, was reported in 1970 upon exposure of HC1 to Os(PPh3)2(CO)(NO)Cl (173), and the X-ray crystal structure was published in 1979 (174). Recent interest has resulted in isolation of additional examples of HNO complexes, and the structures of three similar complexes have been reported [(Ru(HNO)(2,6-bis(2-mercapto-3,5-di-fert-butyl-phenylthio)dimethylpyridine) (175), ReCl(CO)2(PR3)2(HNO) (176), and IrHCl2 (PPh3)2(FINO) (177)]. Preparative routes generally involve protonation, hydride addition, or reduction of a coordinated nitrosyl (175, 176, 178-186). Farmer and co-workers also described the first synthesis of an HNO complex directly as a result of exposure to a donor compound (187) while Lee and Richter-Addo recently observed the HNO adduct of a heme model complex [ruthenium porphyrin (188)]. 

I 7.6. Section 17.5 discusses the bending In metal nitrosyl complexes. In this problem, we are going to go one step further with M-O2 complexes. The heme adducts of O2 involve a low spin Fe-porphyrin, a model of which is shown below (in actual fact this is a nitrosyl complex). 

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