Chlorophyll

Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός and φύλλον. Chlorophyll allows plants to absorb energy from light. Those pigments are involved in oxygenic photosynthesis, as opposed to bacteriochlorophylls, related molecules found only in bacteria and involved in anoxygenic photosynthesis.
Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, is less absorbed. Two types of chlorophyll exist in the photosystems of green plants: chlorophyll a and b.

History

Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.
The presence of magnesium in chlorophyll was discovered in 1906, and was the first detection of that element in living tissue.
After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940. By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming, and in 1990 Woodward and co-authors published an updated synthesis. Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010; a molecular formula of C₅₅H₇₀O₆N₄Mg and a structure of -chlorophyll a were deduced based on NMR, optical and mass spectra.

Photosynthesis

Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light.
Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves three functions:

The function of the vast majority of chlorophyll is to absorb light.
Having done so, these same centers execute their second function: The transfer of that energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems.
This specific pair performs the final function of chlorophylls: Charge separation, which produces the unbound protons and electrons that separately propel biosynthesis.

The two currently accepted photosystem units are and which have their own distinct reaction centres, named P700 and P680, respectively. These centres are named after the wavelength 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.
The function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation. The removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an electron transport chain. The charged reaction center of chlorophyll is then reduced back to its ground state by accepting an electron stripped from water. The electron that reduces P680⁺ ultimately comes from the oxidation of water into O₂ and H⁺ through several intermediates. This reaction is how photosynthetic organisms such as 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 as it accepts the electron, via many intermediates in the thylakoid membrane, by electrons coming, 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 pump H⁺ ions across the thylakoid membrane, setting up a proton-motive force a chemiosmotic potential used mainly in the production of ATP or to reduce NADP⁺ to NADPH. NADPH is a universal agent used to reduce CO₂ into sugars as well as other biosynthetic reactions.
Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of a single chlorophyll molecule doing so under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins 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. These pigments complement chlorophyll by absorbing photons at wavelengths outside of chlorophyll's narrow absorption spectrum and deliver additional electrons to the photosystem.

Chemical structure

Several chlorophylls are known. All are defined as derivatives of the parent chlorin by the presence of a fifth, ketone-containing ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, which are reduced relatives of porphyrins. They share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which contain iron bound to the N4 center, most chlorophylls bind magnesium. The axial ligands attached to the Mg²⁺ center are often omitted for clarity. Appended to the chlorin ring are various side chains, usually including a long phytyl chain. The most widely distributed form in terrestrial plants is chlorophyll a. Chlorophyll a has methyl group in place of a formyl group in chlorophyll b. This difference affects the absorption spectrum, allowing plants to absorb a greater portion of visible light.
The structures of chlorophylls are summarized below:

	Chlorophyll a	Chlorophyll b	Chlorophyll c₁	Chlorophyll c₂	Chlorophyll d	Chlorophyll f
Molecular formula	C₅₅H₇₂O₅N₄Mg	C₅₅H₇₀O₆N₄Mg	C₃₅H₃₀O₅N₄Mg	C₃₅H₂₈O₅N₄Mg	C₅₄H₇₀O₆N₄Mg	C₅₅H₇₀O₆N₄Mg
C2 group	−CH₃	−CH₃	−CH₃	−CH₃	−CH₃	−CHO
C3 group	−CH=CH₂	−CH=CH₂	−CH=CH₂	−CH=CH₂	−CHO	−CH=CH₂
C7 group	−CH₃	−CHO	−CH₃	−CH₃	−CH₃	−CH₃
C8 group	−CH₂CH₃	−CH₂CH₃	−CH₂CH₃	−CH=CH₂	−CH₂CH₃	−CH₂CH₃
C17 group	−CH₂CH₂COO−Phytyl	−CH₂CH₂COO−Phytyl	−CH=CHCOOH	−CH=CHCOOH	−CH₂CH₂COO−Phytyl	−CH₂CH₂COO−Phytyl
C17−C18 bond	Single	Single	Double	Double	Single	Single
Occurrence	Universal	Mostly plants	Various algae	Various algae	Cyanobacteria	Cyanobacteria

Chlorophyll e is reserved for a pigment that has been extracted from algae in 1966 but not chemically described. Besides the lettered chlorophylls, a wide variety of sidechain modifications to the chlorophyll structures are known in the wild. For example, Prochlorococcus, a cyanobacterium, uses 8-vinyl Chl a and b.

Measurement of chlorophyll content

Chlorophylls can be extracted from the protein into organic solvents. In this way, the concentration of chlorophyll within a leaf can be estimated. Methods also exist to separate chlorophyll a and chlorophyll b.
In diethyl ether, chlorophyll a has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll b has approximate maxima of 453 nm and 642 nm. The absorption peaks of chlorophyll a are at 465 nm and 665 nm. Chlorophyll a fluoresces at 673 nm and 726 nm. The peak molar absorption coefficient of chlorophyll a exceeds 10⁵ M⁻¹ cm⁻¹, which is among the highest for small-molecule organic compounds. In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; peaks for chlorophyll c₁ are 442 nm and 630 nm; peaks for chlorophyll c₂ are 444 nm and 630 nm; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm.
Ratio fluorescence emission can be used to measure chlorophyll content. By exciting chlorophyll a fluorescence at a lower wavelength, the ratio of chlorophyll fluorescence emission at and can provide a linear relationship of chlorophyll content when compared with chemical testing. The ratio F₇₃₅/F₇₀₀ provided a correlation value of r² 0.96 compared with chemical testing in the range from 41 mg m⁻² up to 675 mg m⁻². Gitelson also developed a formula for direct readout of chlorophyll content in mg m⁻². The formula provided a reliable method of measuring chlorophyll content from 41 mg m⁻² up to 675 mg m⁻² with a correlation r² value of 0.95.
Also, the chlorophyll concentration can be estimated by measuring the light transmittance through the plant leaves. The assessment of leaf chlorophyll content using optical sensors such as Dualex and SPAD allows researchers to perform real-time and non-destructive measurements. Research shows that these methods have a positive correlation with laboratory measurements of chlorophyll.