Chlorophyll

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Image:Leavessnipedale.jpg

Chlorophyll gives leaves their green color

Space-filling model of the chlorophyll molecule

Chlorophyll is found in high concentrations in chloroplasts of plant cells.

Image:Chlorophyll ab spectra.png

Absorbance spectra of free chlorophyll a (green) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.

Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs light most strongly in the blue and red but poorly in the green portions of the electromagnetic spectrum, hence the green color of chlorophyll-containing tissues like plant leaves.

Chlorophyll and photosynthesis

Chlorophyll is vital for photosynthesis, which allows plants to obtain energy from light. Chlorophyll molecules are specifically arranged in and around pigment protein complexes called photosystems which are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundred per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. Because of chlorophyll’s selectivity regarding the wavelength of light it absorbs, areas of a leaf containing the molecule will appear green. There are currently two accepted photosystem units, Photosystem II and Photosystem I, which have their own distinct reaction center chlorophylls, named P680 and P700, respectively.^[1] These pigments are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent (such as acetone or methanol), these chlorophyll pigments can be separated in a simple paper chromatography experiment, and, based on the number of polar groups between chlorophyll a and chlorophyll b, will chemically separate out on the paper. The function of the reaction center chlorophyll is to use the energy absorbed by and transferred to it from the other chlorophyll pigments in the photosystems to undergo a charge separation, a specific redox reaction in which the chlorophyll donates an electron into a series of molecular intermediates called an electron transport chain. The charged reaction center chlorophyll (P680⁺) is then reduced back to its ground state by accepting an electron. In Photosystem II, the electron which reduces P680⁺ ultimately comes from the oxidation of water into O₂ and H⁺ through several intermediates. This reaction is how photosynthetic organisms like plants produce O₂ gas, and is the source for practically all the O₂ in Earth's atmosphere. Photosystem I typically works in series with Photosystem II, thus the P700⁺ of Photosystem I is usually reduced, via many intermediates in the thylakoid membrane, by electrons ultimately from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700⁺ can vary. The electron flow produced by the reaction center chlorophyll pigments is used to shuttle H⁺ ions across the thylakoid membrane, setting up a chemiosmotic potential mainly used to produce ATP chemical energy, and those electrons ultimately reduce NADP⁺ to NADPH a universal reductant used to reduce CO₂ into sugars as well as for other biosynthetic reductions.

Reaction center chlorophyll-protein complexes are capable of directly absorbing light and performing charge separation events without other chlorophyll pigments, but the absorption cross section (the likelihood of absorbing a photon under a given light intensity) is small. Thus, the remaining chlorophylls in the photosystem and antenna pigment protein complexes associated with the photosystems all cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment-protein antenna complexes.

Chemical structure

Chlorophyll is a chlorin pigment, which is structurally similar to and produced through the same metabolic pathway as other porphyrin pigments such as heme. At the center of the chlorin ring is a magnesium ion. The chlorin ring can have several different side chains, usually including a long phytol chain. There are a few different forms that occur naturally, but the most widely distributed form in terrestrial plants is chlorophyll a. The general structure of chlorophyll a was elucidated by Hans Fischer in 1940, and by 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule as then known.^[1]. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming^[1], and in 1990 Woodward and co-authors published an updated synthesis.^[1]

The different structures of chlorophyll are summarized below:

	Chlorophyll a	Chlorophyll b	Chlorophyll c1	Chlorophyll c2	Chlorophyll d
Molecular formula	C₅₅H₇₂O₅N₄Mg	C₅₅H₇₀O₆N₄Mg	C₃₅H₃₀O₅N₄Mg	C₃₅H₂₈O₅N₄Mg	C₅₄H₇₀O₆N₄Mg
C3 group	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CH=CH₂	-CHO
C7 group	-CH₃	-CHO	-CH₃	-CH₃	-CH₃
C8 group	-CH₂CH₃	-CH₂CH₃	-CH₂CH₃	-CH=CH₂	-CH₂CH₃
C17 group	-CH₂CH₂COO-Phytyl	-CH₂CH₂COO-Phytyl	-CH=CHCOOH	-CH=CHCOOH	-CH₂CH₂COO-Phytyl
C17-C18 bond	Single	Single	Double	Double	Single
Occurrence	Universal	Mostly plants	Various algae	Various algae	cyanobacteria

Image:Chlorophyll a.svg Structure of chlorophyll a	Image:Chlorophyll b.svg Structure of chlorophyll b	Image:Chlorophyll d.svg Structure of chlorophyll d
Image:Chlorophyll c1.svg Structure of chlorophyll c1	Image:Chlorophyll c2.svg Structure of chlorophyll c2

Spectrophotometry

Measurement of the absorption of light is complicated by the solvent used to extract it from plant material, which affects the values obtained,

In diethylether, chlorophyll a has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll b has approximate maxima of 453 nm and 642 nm.^[1]
The absorption peaks of Chlorophyll a are at 665 nm and 465 nm. Chlorophyll a fluoresces at 673 nm. The peak molar absorption coefficient of chlorophyll a exceeds 10⁵ M⁻¹ cm⁻¹, which is among the highest for organic compounds.