Tetrapyrrolic

Tetrapyrrolic compounds are a class of macrocyclic organic molecules in which four pyrrole rings are linked by methine bridges to form a single, highly conjugated ring system. The canonical example is the porphyrin core, which presents a planar, aromatic framework that can coordinate a metal ion centrally through the four inner nitrogen atoms.

The best-known natural tetrapyrroles are porphyrins and their metal complexes. Heme is an iron-containing porphyrin essential for oxygen transport and catalysis in hemoproteins; chlorophylls are magnesium-containing porphyrins that harvest light for photosynthesis. Other biologically important tetrapyrroles include siroheme, an electron carrier in sulfur metabolism, and bacteriochlorophylls, which are variants adapted for bacterial photosynthesis.

Biosynthesis proceeds via the tetrapyrrole biosynthetic pathway. Delta-aminolevulinic acid (ALA) is condensed to form porphobilinogen and then assembled into the macrocycle protoporphyrin IX. Insertion of Mg2+ forms chlorophylls; insertion of Fe2+ forms heme. The side chains and degree of oxidation influence color and function.

Because of their strong light-absorbing properties and ability to coordinate metals, tetrapyrrolic compounds have applications in medicine (photodynamic therapy, imaging), catalysis, and materials science. Synthetic porphyrins and porphyrinoids are studied as models for natural systems and as functional dyes and catalysts.