Split Soret

Optical Spectra. The UV-vis spectrum of pz (219) shows a Soret band at 366 nm and a split Q band at 586 and 656 nm, which is characteristic for metal free porphyrazines. The Ni(II)(pz) (220) shows a Soret band at 343 nm and a single Q band at 613 nm (31). Porphyrazines 219 and 220 undergo a rapid color change from blue to purple upon dinitration in CH2C12. The dinitroporphyrazincs 221 and 222 are somewhat unusual in showing similar absorption spectra with split Soret bands (353 and 373 nm for 221, 348 and 370 for 222) as well as split Q bands (523 and 657 nm for 221, 582 and 659 for 222). 

Eight Al(Por)R complexes (For = OEP,TPP R = Me, "Bu, CeHs, C6F4H) were prepared from the reaction of Al(Por)Cl with an alkyl or aryllithium reagent. Mass spectra of the compounds showed low intensities for the molecular ion peaks, consistent with facile cleavage of the Al—C bond. The chloride complexes Al(Por)Cl show normal porphyrin UV-visible spectra, whereas the organometallic complexes Al(Por)R show a split Soret band typical of hyperporphyrin spectra. 

Figure 1 shows the Raman spectrum of Hb obtained with 406.7-and 413.1-nm excitation and the spectrum of monomeric, four-coordinate Ni protoporphyrin in aqueous micellar solution (9). Excitation at 413.1 nm is at resonance with the red component of the split Soret band of Ni-reconstituted hemoglobin at 406.7 nm the blue component of the Soret band is selectively probed. Comparison of the spectra shows that two sets of marker line frequencies exist. One set (labeled 4 in Figure 1) is enhanced by resonance with the blue Soret component the other set (labeled 5) is enhanced by excitation of the red Soret component. Thus, the shifts in the core-size lines in going from set 4 - 5 are -39 cm (i/-q at 1657 cm ), -20 cm cm ), and -34 cm 1 19 cm ). 

Recently, syntheses of the model compounds 156 and 157 were reported [107,108], which are closely related to earlier approaches [103,109] (Fig. 25). In agreement with theoretical calculations the CO complexes of the Fe(II)por-phyrins 156 and 157 display a split Soret band at 370/446 nm and 383/456 nm, respectively, but no experiments with molecular oxygen were reported. But it was demonstrated that 157 catalyzed the formation of stable aryloxy radicals from the corresponding phenols in the presence of e.g. feri-butylhydroperoxide (TBHP). These results indicate a thiolate mediated 0-0 bond cleavage of TBHP accelerated 240 fold in comparison to iron(III)tetraphenylporphyrin [108],... 

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Some anomalous features appear occasionally, such as the weak 14 kK band of cytochrome-c (5, 52, 53), the split Soret band of cyto-chrome-cc (54), and the high energy of the Soret band in haem dimers and in some proteins (55). 

Figure 1 (a) Superposition of heme pairs displaying the parallel heme stacking motif. The hemes shown are from the D. desulfuricans ATCC 2111A Split-Soret cytochrome (PDB code 1DCC, hemes 1 and... 

Split-Soret Cytochrome. The split-Soret cytochrome (Ssc) and NapB proteins have no relationship except the fact that both contain a single stacked parallel diheme motif Ssc is a dimer of two identical 26.3 kDa subunits, which has so far been isolated only from the D. desulfuricans ATCC 27774, a species that can use sulfate or nitrate as terminal electron acceptors. Each monomer contains two c-type hemes with bis-histidinyl coordination and redox potentials of —168 and —330 mV. The name of this cytochrome derives from the fact that the reduced form displays a spht-Soret band with maxima at 420 mn and a shoulder at415mn. 

Further evidence for this conclusion came from spectroscopic studies. For instance, the UV-vis spectrum of 4.117 revealed a strong, split-Soret absorbance at 440/464 nm (s = 207900/104400cm ) with Q-bands in the region of 672-790... 

The Manganese111 Porphyrin Type Spectrum. The main features of this type of spectrum are the split Soret band, broad visible... 

The most stable manganese oxidation state in porphyrins is +3, and Mn111 complexes of ETIO-type porphyrins are the most prominent example of metalloporphyrins with a split Soret band. Instead of the usual intense band at 400 nm, these chelates possess two bands at 350 and 460 nm with an intensity ratio of 2 1. This change was related to a strong porphyrin-metal n-d(XZjyZ) interaction in addition to the usual a interaction between the four pyrrole nitrogens and the metal [Boucher (11)]. Theoretical calculations do indeed predict dxz,yg-n interactions because the metal d orbitals in the first transition series roughly match... 

The atypical electronic spectrum is reminiscent of a cation radical spectrum, and shows very broad visible absorption bands and a split Soret band of low intensity [608 (2,200) 511 (15,000) 427 (66,000) 362 (54,000)] [Lexa (123)]. Co1 ETIO reacts with acyl or aryl halides to form Com acyl or aryl adducts, which reminds one of the supernucleophilic catalytic activities of vitamin Bi2 ... 

The spectra of metal complexes are more informative in terms of nonplanarity [147,148]. All metal complexes usually show a split Soret and number of low energy Q-bands. Due to lower symmetry of the thiaporphyrin complexes, the low energy portion of the spectrum is more complex than is found for the corresponding porphyrin complexes. 

A diheme cytochrome has been isolated from one species of sulfate-reducing bacteria, Desulfovibrio desulfuricans ATCC 27774 [92]. It has been named as the Split-Soret cytochrome as it displays a shoulder at 415 nm for the Soret band of the reduced form. It is a dimer of a 26 kDa subunit, each containing two hemes c. The function of this cytochrome has not been elucidated and it was observed that it is present in cells grown both on nitrate or sulfate. A diheme cytochrome of 11 kDa was isolated from Wolinella succinogenes, but the low yield of purified protein prevented detailed structural studies [38]. 

UV/visible spectroscopy in DMSO solution or aqueous suspension showed a split Soret band (384/460 or 363/446 nm) which was similar to the hyper spectrum of carbonylated cytochrome P450. 

Other H-type aggregates of various chromophores [40]. The arrangement of two porphyrins with a finite dihedral angle causes, in principle, a splitting in the energy bands due to the existence of both face-to-face and head-to-tail orientations of the two in-plane transition moments of the porphyrin. This is also the case for slipped cofacial arrangements, where the existence of two similar transition moments produces split Soret bands. 

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