pheophytin

The presence of pheophytin is a cause of the undesirable color.

In photosystem II, pheophytin plays a very similar role.

To promote brightness, it is helpful to cook the vegetable using methods that will minimize the creation of pheophytin.

The overall mechanisms, roles, and purposes of the pheophytin molecules in the two transport chains are analogous to each other.

Biochemically, pheophytin is a chlorophyll molecule lacking a central Mg ion.

This discovery was met with fierce opposition, since many believed pheophytin to be a byproduct of chlorophyll degradation.

The negatively charged pheophytin radical quickly passes its extra electron to two consecutive plastoquinone molecules.

The latter can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters.

Photo-reduction of pheophytin occurs at temperatures as low as 100K, and is observed after the reduction of plastoquinone.

The quantity of pheophytin is in direct proportion to the number of PSII reaction centers.

After P680 becomes excited to P680, it transfers an electron to pheophytin, which converts the molecule into a negatively charged radical.

Cooking in an uncovered vessel allows the escape of volatile acids that create pheophytin, and shorter cooking times also help preserve greenness.

This is followed by the step P680 pheophytin, and then on to plastoquinone, which occurs within the reaction center of PS II.

The electrons transfer from pheophytin to plastoquinone, then to plastocyanin, providing the energy for hydrogen ions (H) to be pumped into the thylakoid space.

Photo-reduction of pheophytin has been observed in various mixtures containing PSII reaction centers.

The reactions outlined above in the section concerning purple bacteria give a general illustration of the actual movement of the electrons through pheophytin and the photosystem.

This electron is subsequently captured by the primary electron acceptor, a pheophytin molecule located within photosystem II near P680.

Therefore, more experiments ensued to prove that pheophytin is indeed the primary electron acceptor of PSII, occurring between P680 and plastoquinone.

When the chlorophyll passes the electron to pheophytin, it obtains an electron from P. In turn, P can oxidize the Z (or Y) molecule.

Using several experiments, including electron paramagnetic resonance (EPR), they were able to show that pheophytin was reducible and, therefore, the primary electron acceptor between P680 and plastoquinone.

"The Identification of Potential Pheophytin Binding Sites in the Photosystem II Reaction Center of Chlamydomondas by Site-Directed Mutagenesis."

In the 1970s, scientists Karapetyan and Klimov performed a series of experiments to demonstrate that it is pheophytin and not plastoquinone that serves as the primary electron acceptor in photosystem II.

PS II is an extremely complex, highly organized transmembrane structure that contains a water-splitting complex, chlorophylls and carotenoid pigments, a reaction center (P680), pheophytin (a pigment similar to chlorophyll), and two quinones.

The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin.

When a chlorophyll molecule at the core of the photosystem II reaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred to the primary electron-acceptor molecule, pheophytin, through a process called photoinduced charge separation.