Nickel protoporphyrin

Figure 1. Raman spectra of nickel-reconstituted hemoglobin (pH 7.5) obtained with 413.1-nm excitation (a), with 406.7-nm excitation (b), and the spectrum of nickel protoporphyrin IX free acid in cetyltrimethylammonium bromide micelles in 0.1 M NaOH (c). Spectra in (b) and (c) were obtained simultaneously on a Raman difference spectrometer.

Figure 3. Transient Raman spectrum of nickel protoporphyrin IX in piperidine obtained with 10-ns laser pulses at 420 nm. (a) high power density (b) low power density.

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

Fig. 18.10. [Nickel-(protoporphyrin IX)dimethyester], which was electropolymerised on glassy carbon with nitrobenzene as template to prepare a nitrobenzene-selective sensor [135].

The nature of the bonds in this complex is somewhat uncertain, since, although the value of the magnetic moment is that which is associated with square coordination (as in nickel protoporphyrin ), there is little doubt that the configuration about the iron atom is octahedral. Four dsp covalent bonds directed to the comers of a square would utilize one d orbital in ferri-... 

We next turn to an extensive study of the Ni(II) complexes for unlike Mn(II) and Fe(II) protoporphyrins the simple complexes are low-spin. McLees and Caughey (57) have shown that the addition of bases to nickel porphyrins causes a Soret band shift of 30 nm (410 440)... 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

Recent reconstitution experiments open the possibility of selectively introducing other metallo bacteriochlorins (or chlorins) at the acceptor BChl or BPheo sites. Zinc substitutions are obvious choices since the optical and coordination properties of Zn complexes resemble those of Mg derivatives but the former are easier to reduce (and harder to oxidize) than the latter. Perhaps a less obvious candidate for metal replacement is nickel which offers several advantages Ni complexes are also easier to reduce and harder to oxidize than Mg compounds but, in addition, redox potentials of Ni porphyrins and chlorins are close to those of the metal-free derivatives the optical properties of Ni chlorins are similar to those of Mg ones they possess shortlived excited states as a result of which they have proved to be valuable for resonance Raman studies. A more practical advantage of Ni complexes is their enhanced stability towards demetallation, compared to Mg or Zn complexes, a property of merit if the chromophores are to be used in reconstitution reactions. (Ni protoporphyrins IX have been successfully exchanged for the histidine-bound hemes in hemoglobin.) ... 

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