Pheophytin or phaeophytin (abbreviated Pheo) is a chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 that serves as the first electron carrier intermediate in the electron transfer pathway

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 of photosystem II

Photosystem

Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons...

 (PS II) in plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, and the photosynthetic reaction center (RC P870) found in purple bacteria

Purple bacteria

Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...

. In both PS II and RC P870, light drives electrons from the reaction center through pheophytin, which then passes the electrons to a quinone

Quinone

A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

 (Q_A) in RC P870 and RC P680. The overall mechanisms, roles, and purposes of the pheophytin molecules in the two transport chains are analogous to each other.

Structure

Biochemically, pheophytin is a chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 molecule lacking a central Mg²⁺ ion. It can be produced from chlorophyll by treatment with a weak acid

Weak acid

A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

, producing a dark bluish waxy pigment. The probable etymology

Etymology

Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...

 comes from this description, with pheo meaning dusky and phyt meaning vegetation.

History and discovery

In the 1970s, scientists Karapetyan and Klimov performed a series of experiments to prove that it is pheophytin and not plastoquinone

Plastoquinone

Plastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...

 that serves as the primary electron acceptor in photosystem II. Using several experiments, including electron paramagnetic resonance

Electron paramagnetic resonance

Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...

 (EPR), they were led to believe that pheophytin was reducible and, therefore, the primary electron acceptor between P680

P680

P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...

 and plastoquinone. This discovery was met with fierce objection, since many believed pheophytin to be a byproduct of chlorophyll degradation. Therefore, more testing ensued to prove that pheophytin is indeed the primary electron acceptor of PSII, occurring between P680 and plastoquinone. The data that was obtained is as follows:

1. Photo-reduction of pheophytin has been observed in various mixtures containing PSII reaction centers.
2. The quantity of pheophytin is in direct proportion to the number of PSII reaction centers.
3. Photo-reduction of pheophytin occurs at temperatures as low as 100K, and is observed after the reduction of plastoquinone.

These observations are all characteristic of photo-conversions of reaction center components.

Reaction in purple bacteria

Pheophytin is the first electron carrier intermediate in the photoreaction center (RC P870) of purple bacteria. Its involvement in this system can be broken down into 5 basic steps. The first step is excitation of the bacteriochlorophylls (Chl)₂ or the special pair of chlorophylls. This can be seen in the following reaction.

• (Chl)₂ + 1 exciton → (Chl)₂^* (excitation)

The second step involves the (Chl)₂ passing an electron to pheophytin producing a negatively charged radical (the pheophytin) and a positively charged radical (the special pair of chlorophylls), which results in a charge separation.

• (Chl)₂^* + Pheo → ·(Chl)₂⁺ + ·Pheo^- (charge separation)

The third step is the rapid electron movement to the tightly bound menaquinone, Q_A, which immediately donates the electrons to a second, loosely bound quinine (Q_B). Two electron transfers convert Q_B to its reduced form (Q_BH₂).

• 2·Pheo^- + 2H⁺ + Q_B → 2Pheo + Q_BH₂ (quinone reduction)

The fifth and final step involves the filling of the “hole” in the special pair by an electron from a heme in cytochrome c. This regenerates the substrates and completes the cycle, allowing for subsequent reactions to take place.

Involvement in photosystem II

In photosystem II, pheophytin plays a very similar role. It again acts as the first electron carrier intermediate in the photosystem. After P680 becomes excited to P680^*, it transfers an electron to pheophytin, which converts the molecule into a negatively charged radical. The negatively charged pheophytin radical quickly passes its extra electron to two consecutive plastoquinone molecules. Eventually, the electrons pass through the cytochrome

Cytochrome

Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.They are found either as monomeric proteins or as subunits of bigger enzymatic complexes that catalyze redox reactions....

b₆f molecule and leaves photosystem II. See the reactions above in the purple bacteria to get a better idea of the actual movement of the electrons through pheophytin and the photosystem in general. The overall scheme is:

1. Excitation
2. Charge separation
3. Plastoquinone reduction
4. Regeneration of substrates

Relationship to food preparation

In Western cultures, it is generally held that bright-green cooked vegetables are more appealing than darker, olive-colored vegetables. The presence of pheophytin is a cause of the undesirable color. Pheophytin can be produced by an acidic cooking environment or by prolonged cooking. To promote brightness, it is helpful to cook the vegetable using methods that will minimize the creation of pheophytin. Cooking in an uncovered vessel allows the escape of volatile acids that create pheophytin, and shorter cooking times also help preserve greenness.