Porphyrin ring system isomerism

For further clarity, a word about the possibilities of isomerism in the porphyrin system The positions on the pyrrole rings which can bear substituents are numbered consecutively from 1 to 8. If each pyrrole ring is to have one short and one long side chain (one acetic add —CHiCOOH, symbolized by —O, and one propionic acid —CHj—CHz—COOH, symbolized by O), then there are four possibilities, designated as types I-IV (cf. diagram of formulas). 

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