Pyrrole rings, 2,2 -methine bridging

A macrocyclic ligand called the corrin ring, which bears various substituents. It resembles the porphine ring, but differs in various ways, notably in the absence of one methine, =CH, bridge between a pair of pyrrole rings. 

Metalloporphyrins, MP, represent derivatives of porphyrin, P, in which four pyrrole fragments are bound together by methine bridges (Fig. 13). The diversity of porphyrins is due to the possibility of variation for substituents R in the periphery of the porphyrin ring. A typical optical spectrum of a P solution is presented in Fig. 14. One can point out quite a number of characteristic bands in it. The most intensive short-wave peak in the P absorption spectrum (/max 400 nm) corresponds to the transition S0 -+ S2 and is referred to as Soret band. The extinction coefficient of this band is very large, as a rule, and amounts to 10 -106 M 1 cm-1. The less intensive long-wave bands of P absorption correspond to the S0 - Sx transition (bands I-IV in Fig. 14). Complexation with the metal results in a rise of the symmetry of the molecule, due to which MP molecules have only two bands in the long-wave part of the absorption spectra. Most of the metalloporphyrins are characterized by intense luminescence. The time of MP fluorescence decay (transition Si - S0) is short and amounts to 10"8 to 10 9 s. Besides the transition... 

Vitamin B12.24 The best known biological function of cobalt is its intimate involvement in the coenzymes related to vitamin Bl2, the basic structure of which is shown in Fig. 25-F-6. The macrocyclic ring is the corrin system it is reminiscent of the porphyrin system, the most notable difference being the absence of a methine (CH) bridge between one pair of pyrrole rings. 

The [40]decaphyrin (1.0.1.0.0.1.0.1.0.0.) turcasarin 10 is the largest expanded porphyrin to date derived by insertion of additional pyrrole subunits up to a total number of ten pyrrole rings in the macrotetracycle. [40]pentaphyrin(5.5.5.5.5) 11 combines expansion by an additional pyrrole subunit and elongation of the methine bridges of the basic porphyrin system. 

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