Methylated hydroporphyrins

Mg11 insertion into labile hydroporphyrins is effected by the use of IMg(2,6-di-f-butyl-4-methyl-phenoxide)-Li(2,2,6,6-tetramethylpiperidide).169... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

Table 3. Redox potentials for Ni(II) complexes derived from anhydromesorhodochlorin XV methyl ester and other hydroporphyrin complexes in CH2CI2 [96]...

C42H5 ClN4NiO,7, Mr 1006.04, uv, (pentamethyl ester, F 430 M in HjO) 431,418,296,274 nm. F. belongs to the structural class of the hydroporphyrins. Occurrence F. (coenzyme F 430), is the prosthetic group of methyl-coenzyme M reductase, which catalyzes the last step of methane formation in methano-genic bacteria. The mechanism of formation of methane from methyl-coenzyme M (H3C-S-CH2-CH2-SOJ) is still unknown. However, the central nickel atom in F. most certainly participates in the reduction. Biosynthesis F. is formed from 5-amino-4-oxova-leric acid via the normal biosynthetic route for porphi-noid compounds. The 2- and 7-methyl groups originate from 5-adenosylmethionine. 

---