Porphyrin basicity

For systemic administration, the photosensitizer usually has to be delivered into the bloodstream by intravenous injection. Since the photosensitizer is a solid, this means that a solution or a stable suspension has to be provided. Metal complexes of the basic porphyrin and phthalocyanine nuclei are insoluble in water, so that some effort has to be made to render the system water soluble, or at least amphiphilic, by placing various substituents (e.g., S03H, C02H, OH, NR3+, polyether, aminoacid, sugar) on the periphery of the molecule. The aromatic character of the ligand offers a suitable opportunity for such substitutions to be made. Examples will appear frequently in the following sections. 

Porphyrin Basicity. The reduction rate is decreased significantly as the porphyrin is made more basic (Figure 6). The bis-cyanide complex of ferric octaethyl porphyrin, the more basic porphyrin, is reduced the slowest while the bis-cyanide complex of ferric TPP, the least basic (16), is reduced the fastest. Increasing the porphyrin basicity places more electron density on the iron, making it more difficult to accept another electron. 

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. 

The stabilization imparted by aromaticity has been exploited by nature in the use of the porphyrin ring system. Shown to the right is the basic porphyrin skeleton. 

There are four points at which the main stracmral modifications of the chlorophyll molecule start (1) the chelate, (2) the ester bond of the phytol alcohol (C-17 ), (3) the isocyclic ring (C-13 ), and (4) the basic porphyrin structure. Rgure 7.4 shows the possible transformations of the chlorophyll molecule. 

The electron absorption spectrum of the chlorophylls and their derivatives is very characteristic and is attributed to the system of conjugated double bonds making up the basic porphyrin structure. Electron transitions in the chlorophyll molecules, detected by photosensitive optical equipment, produce absorption bands [8,64,65], The intensity of a particular absorption band is normally expressed by its molar specific coefficient of extinction s), which, according to the Beer-Lambert law, depends on the optic density (A), the molar concentration (C), and the optical pathway of the light beam through the solution, expressed in centimeters (L). 

The absorption spectra of all the porphyrin pigments are characterized by a number of relatively pronounced bands in the yellow, red, and near-infirared regions, and a band of strong absorption in the violet or near-violet region denominated the Soret band. The presence of the latter indicates that there has been no cleavage of the basic porphyrin stmcmre [66]. 

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