Hemins metal complexes

For the construction of artificial metalloproteins, protein scaffolds should be stable, both over a wide range of pH and organic solvents, and at high temperature. In addition, crystal structures of protein scaffolds are crucial for their rational design. The proteins reported so far for the conjugation of metal complexes are listed in Fig. 1. Lysozyme (Ly) is a small enzyme that catalyzes hydrolysis of polysaccharides and is well known as a protein easily crystallized (Fig. la). Thus, lysozyme has been used as a model protein for studying interactions between metal compounds and proteins [13,14,42,43]. For example, [Ru(p-cymene)] L [Mn(CO)3l, and cisplatin are regiospecificaUy coordinated to the N = atom of His 15 in hen egg white lysozyme [14, 42, 43]. Serum albumin (SA) is one of the most abundant blood proteins, and exhibits an ability to accommodate a variety of hydrophobic compounds such as fatty acids, bilirubin, and hemin (Fig. lb). Thus, SA has been used to bind several metal complexes such as Rh(acac)(CO)2, Fe- and Mn-corroles, and Cu-phthalocyanine and the composites applied to asymmetric catalytic reactions [20, 28-30]. 

Early peroxidase models were based on simple metal complexes or modified hemin derivatives. Anionic metalloporphyrins adsorbed on ion-exchange resins have an efficient peroxidase activity. 

In 1929 Fischer and his co-workers succeeded in the total synthesis of hemin (the chloride of ferriheme). For the exciting developments during this period and the chemistry of porphyrins and their metal complexes in general, the reader is referred to the authoritative book by Fischer and Orth (1937). 

Classical metal ions with a positive catalytic effect on the luminol reaction are Fe-(III)-(CN)6-ions or the hemin-Fe-complex. 

Fe(III)-phthalocyanine (Pc) compounds have structures similar to heme proteins. The Fe(III)-Pc compounds catalyze the decomposition of hydrogen peroxide. Sigel et al. have studied the mechanism of the HjO decomposition reaction Fe(III)- and Co(II)-tetrasulfo-Pc [77,78]. Shirai et al. reported the decomposition of H.O. by using Fe(lll)-3,4,3, 4, 3",4",3", 4" -octacarboxyphthalocyanine [Fe(III)-oaPc] (56) as a catalyst [79]. Fe(IIl)-oaPe is a remarkably effective catalyst for the reaction and is comparable with hemin and other metal complexes. From the kinetic studies, the mechanism for the reaction was proposed as follows, at pH 7.0 ... 

In the most common method for chemiluminescent immunoassay (GLIA), after the immunological reaction and any necessary separation steps, the labeled compounds or complexes react with an oxidizer, eg, hydrogen peroxide, and an enzyme, eg, peroxidase, or a chelating agent such as hemin or metal... 

The CL reaction of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) (1) is one of the more commonly used nonenzymatic CL reactions and has been extensively studied [49, 57-59], It is well known that the luminol CL reaction is catalyzed by many kinds of substances, e.g., ozone, halogen-Fe complex, hemin, hemoglobin, persulfate, and oxidized transition metals. The most acceptable scheme is shown in Figure 10. Luminol forms a six-membered ring of peroxide (3) from a diazaquinone intermediate (2) and then, by the decomposition of 3, N2 gas and the Si-excited state of the phthalate dianion are produced, yielding... 

The oxidation is catalyzed by various heavy metal ions such as Cu , Fe (hemin complex), Ni and Co and their complexes,and more importantly, the addition of these ions leads to the selective formation of disulfides without any overoxidized products. The cluster (Bu"4N)2[Fe4S4(SR)4], the analog of the active site of nonheme iron-sulfur proteins, catalyzed extremely smooth oxidation of thiols by oxygen to disulfides in acetonitrile at 0 C (equation 4), while in the case of FeCb or FeCb catalysts oxygen uptake was very slow." The catalysis by AI2O3 for aerobic oxidation is also common. Thus, by stirring thiols in benzene with exposure to air at room temperature for 4-6 h disulfides were obtained almost quantitatively except in the hindered case of Bu SH. 

The arrangement of four nitrogen atoms in the center of the porphyrin ring enables porphyrins to chelate various metal ions. Protoporphyrin that contains iron is known as heme ferroheme refers specifically to the complex and ferri-heme to Fe. Ferriheme associated with a chloride counter ion is known as hemin, or hematin when the counter ion is hydroxide. 

On the other hand, for the first time, we have synthesized a nonnatural ribozyme using an indirect method (96. 97). A pool of RNAs consisting of random sequences was obtained in the presence of 2 -aminocytidine triphosphate instead of CTP. The pool was incubated with an A-methylmesoporphyrin-immobilized gel, and the bound nonnatural RNAs were then collected and amplified by RT-PCR. The selected nonnatural RNA catalyzed the metalation of porphyrin using the same mechanism. In addition, the selected RNA exhibited a peroxidase activity by complex formation with hemin. 

Fig. 7 (a) Porphyrin metallation catalyzed by DNAzymes. (b) Proposed mechanism for the peroxidation reaction catalyzed by the DNAzyme/hemin complex [97]... 

SERRS spectra of adsorbed Fe(III) protoporphyrin IX chloride (FePPCl) in aqueous base media (pH 10.5)and its dimethylester form in acetonitrilewere recently published. Using a rather low excitation wavelength at 406.7 nm, near the strong electronic absorption B (or Soret) of metalloporphyrin, Sanchez tmd Spiro were able to monitor the oxidation state of the Fe ions as a function of the silver electrode potential, see Fig. 29. From metal sensitive Raman bands, attributed to Fe(III) and Fe(II) complexes, the authors found good agreement between the voltam-metric and the RR results. The mid-point of the transition for aqueous hemin, —0.65 V... 

The intensive development of chelate-type MCMs can be taken into account. Fragmentary data (most often without either quantitative data or study of the reaction mechanism) on radical copolymerizations of MCMs - Schiflf bases with vinyl groups, macrocyclic complexes (hemins, phthalocyanines etc.) - point us in this direction. Probably, polymerizable crown ethers with exocyclic groups and metal ions in the ring (ciyptands, podands etc.) will be synthesized. 

Two different catalysts for hydrogen peroxide decomposition, the enzyme peroxidase (isolated from the horseradish root, HRP), and polymer-supported catalyst (acid form of poly-4-vinylpyri-dine functionalized by ferric sulfate, apFe) [99,100], are examined with an aim to compare their activity. The active center in the peroxidases is the ferric ion in protoporphyrin IX. Besides the complex made of ferric ion and protoporphyrin IX, that is ferricprotoporphyrin IX, also known as ferric heme or hemin, peroxidase possesses a long chain of proteins [101,102]. On the other hand, the macroporous acid form of polyvinyl pyridine functionalized by ferricsulfate is obtained from cross-linked polyvinyl pyridine in macroporous bead form [103]. Pyridine enables it to form coordination complexes or quaternary salts with different metal ions such as iron (111) [104]. An active center on the polymeric matrix functionalized by iron, as metallic catalyst immobilized on polymer by pyridine, has similar microenvironment conditions as active center in an enzyme [105]. 

---