Pheophytins

Pheophytins are a class of porphyrin-based pigments derived from chlorophyll by removal of the central magnesium ion. In their common natural forms they retain the phytol side chain and are distinguished mainly by their origin from chlorophyll a or chlorophyll b, yielding pheophytin a and pheophytin b. The demetallated porphyrin ring is typically olive-brown to greenish-brown in color. Pheophytins arise in plants during chlorophyll breakdown or can be produced synthetically by demetallation of chlorophyll under acidic or basic conditions, and they can be isolated from plant tissues, photosynthetic bacteria, and laboratory preparations.

In photosynthesis, pheophytin plays a key role as the primary electron acceptor in reaction centers. When light excites the chlorophyll within the photosystem II reaction center, the excited electron is transferred to pheophytin a, which then passes it to plastoquinone QA, initiating the electron transport chain. Similar roles are found in bacterial reaction centers that use pheophytins as initial electron acceptors, illustrating a conserved function in photosynthetic electron transfer across diverse organisms.

Spectroscopic and practical notes: Pheophytins exhibit characteristic absorption bands in the blue and red regions of the spectrum, with spectral features shifted relative to chlorophyll due to the absence of the central magnesium ion. They are less fluorescent than chlorophyll and are used in biochemical studies of photosynthetic electron transfer and chlorophyll degradation pathways. In food and ornamental contexts, chlorophyll degradation to pheophytin can contribute to color changes from vivid green to olive-brown under acidic conditions.