Macrocyclic system, chlorophylls

There is also evidence that individual chlorophyll derivatives exhibit cytostatic and cytotoxic activities against tumor cells. Studies have been started on electronic structures, in particular the electronic state of the phorphyrin macrocycle, and progress in this area is expected regarding photodynamic therapy for tumors, since the strong absorption of hght in the visible region is effective for laser excitation. Nevertheless, httle is known to date about the influences of peripheral groups on the electronic state of the macrocycle n system in chlorophyll derivatives. ... 

The phthalocyanine [1-4] system is structurally derived from the aza-[18]-annulene series, a macrocyclic hetero system comprising 18 conjugated n-electrons. Two well known derivatives of this parent structure, which is commonly referred to as porphine, are the iron(III)complex of hemoglobin and the magnesium complex of chlorophyll. Both satisfy the Htickel and Sondheimer (4n + 2)- electron rule and thus form planar aromatic systems. 

An additional impetus for pyrrole and indole research derives from the recognition of the physiological significance of these ring systems. In the case of pyrroles the early work centered around hemin from hemoglobin and the bile pigments. The structure of chlorophyll was also shown to be pyrrole-derived. The biosynthetic connection between simple pyrroles and the macrocyclic hemin and chlorin have remained of interest up to the present (75ACR201). 

The beautiful colors associated with porphyrin and chlorophyll systems are manifest in their characteristic electronic absorption spectra. The most intense band in the spectra, around 400 nm, is known as the Soret band it is intrinsic to the large macrocyclic conjugated pathway and has molar extinction coefficients usually between 150 000-400 000. This extinction value is lower in chlorins than in porphyrins, and the band is absent in porphyrinogens (6) and ring-opened bile pigments. 

Another porphobilinogen-derived system is vitamin B12, the structure of which is significantly different, though related to chlorophyll and haem. The parent, unsubstituted macrocycle is called corrin. 

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