The Chlorophylls are a group of tetrapyrrolic pigments with common structural elements
and functions. In chemical terms, they are cyclic tetrapyrroles of the porphyrin, chlorin, or
bacteriochlorin oxidation State (Scheme 1), which are characterized by a fifth, isocyclic ring
that is biosynthetically derived from the C-13 propionic acid side chain of Protoporphyrin.*
In chemical terms, Chlorophylls are conventionally also characterized by a central magnesium
atom. In a biological context, the definition is somewhat shifted. Only those among the
pigments defined above should be regarded as true Chlorophylls which function in photosynthesis.
This biological definition excludes precursors or degradation products which
conform to the chemical definition. It includes, on the other hand, the pheophytins, e.g.,
the derivatives which lack the central magnesium, because they are active in photosynthetic
electron transport.
The additional str