Reduced Ring Tetrapyrroles

There has been substantial recent interest in the structures of reduced porphyrin derivatives. Reduced derivatives mean the reduction of the porphyrin system by the addition of one or more moles of hydrogen. This chemical change can have important effects on physical properties. However, in this review, the effect of such modified macrocycles on the coordination parameters of the metalloderivatives is the major question. Characterized species include three levels of reduction, di-, tetra- and hexahydroporphyrins. Examples of the dihydroporphyrins are chlorins (G) and porphodimethenes (H). Tet-rahydroporph)nins include bacteriochlorins (I) and isobacteriochlorins (J). Characterized hexahydroporphyrins are the pyrrocorphins (K) and hexahydroporphyrins (L). 

We first simply note that there are i) a number of free base compounds of known structure and ii) a number of magnesium derivatives of known structure that are related to the naturally occuring chlorophyls. 

One effect of reducing one or more pyrrole rings is to increase the metal-nitrogen bond distance to the reduced ring(s) if everything else remains equal. This is cleanly seen in the structure of [Zn(TPC)(Py)] In this complex, the Zn-Np bond distances average 

These data for the isobacteriochlorins also suggest one other possible trend in the coordination parameters, namely that as the number of reduced rings is increased the differences in the M-N bond distances to the two types of rings will decrease. Data presented by Kratky for a number of tetra- and hexahydrooctaethylporphyrin derivatives are in accord with this idea. Unfortunately, most of the metallo species are four- 

Finally, it should be noted that the expected increase in the Cb-Cb and Cj-Cb bond distances of the five-membered ring are observed upon reduction. It is expected and generally observed that the pyrroline ring is no longer planar. 

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Structure of products From its absorption properties, the photoproduct still appears to be a cyclic-conjugated tetrapyrrole. The nmr spectrum indicates a reduced ring-current. Since only two methine signals appear in both the educt and the photoproduct, the C-20 position lacks a hydrogen substituent in the product, too. The spectra are compatible with a 20-oxonia-chlorophyll (17). 

Porphyrins consist of a tetrapyrrole ring with variable side chains. The physiological compounds are the reduced forms, the porphyrinogens that do not fluoresce. They react spontaneously with oxygen in the air to the strongly fluorescent porphyrins. 

Figure 19.5. Chlorophyll. Like heme, chlorophyll a is a cyclic tetrapyrrole. One of the pyrrole rings (shown in red) is reduced. A phytol chain (shown in green) is connected by an ester linkage. Magnesium ion binds at the center of the structure.

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