All plants have chlorophylls



The Chlorophyll (from Greek χλωρός, chlōrós - "light green, fresh" and φύλλον, phýllon - "sheet") or Leaf green refers to a class of natural dyes that are produced by organisms that carry out photosynthesis. Plants in particular get their green color from chlorophyll molecules.

Plants, algae and cyanobacteria have different types of chlorophyll, and different photosynthesis-driving bacteria have different types of Bacteriochlorophyll.

Structure and properties

Chemically speaking, the chlorophylls are organic complexes (based on chlorin) with an Mg2+-Ion as central ion. The organic part acts as a tetradentate ligand of the chelate complex. With chlorophyll a the chain-like part of the molecule is an esterified form of phytol.

The heme, which is part of the blood pigment (hemoglobin), myoglobin and cytochromes, has a very similar structure, with iron being the central atom in the heme instead of magnesium.

Chlorophyll is readily soluble in ethanol, acetone and other solvents with similar properties.

Surname structure C.3-Rest C.7-Rest C.8-Rest C.17-Rest C.17-18-Binding Molecular formula
Chlorophyll a -CH = CH2 -CH3 -CH2CH3 -CH2CH2COO-Phytyl Single bond C.55H72O5N4Mg
Chlorophyll b -CH = CH2 -CHO -CH2CH3 -CH2CH2COO-Phytyl Single bond C.55H70O6N4Mg
Chlorophyll c1 -CH = CH2 -CH3 -CH2CH3 -CH = CHCOOH Double bond C.35H30O5N4Mg
Chlorophyll c2 -CH = CH2 -CH3 -CH = CH2 -CH = CHCOOH Double bond C.35H28O5N4Mg
Chlorophyll d -CHO -CH3 -CH2CH3 -CH2CH2COO-Phytyl Single bond C.54H70O6N4Mg

Spectral properties

The absorption spectra of chlorophylls dissolved in solvents always have two distinct absorption maxima, one between 600 and 800 nm, known as QyBand, and one around 400 nm called the Soret band. The figure on the right shows these absorption maxima for chlorophyll a and b. In addition, there is the QxBand around 580 nm, which is perpendicular to Qyis polarized and is usually very weakly absorbed. For chlorophyll a it can still be seen in the figure for chlorophyll b it disappears underground.

From the spectra in the figure, it's easy to understand why leaves - these contain chlorophyll a and b - are green. Together they absorb chlorophyll a and b mainly in the blue spectral range (400–500 nm) and in the red spectral range (600–700 nm). In the green area, on the other hand, there is no absorption, so green light is scattered, which makes leaves appear green.

The absorption depends on the solvent and accordingly the position of the absorption maxima can vary by a few nanometers depending on the type of solvent. In the natural environment of chlorophylls, i.e. the protein environment, things are different. Here the position of the absorption maxima depends on two factors: (1) Depending on the partial charge of the surrounding amino acids and the bending of the side groups of the chlorophyll molecules, the absorption maxima can be at very different wavelengths. (2) In proteins, chlorophylls come very close to one another, so that they interact with one another (dipole-dipole interaction; at very short distances also exchange interaction). This interaction leads to a lowering of the energy level and thus to a redshift of the absorption maxima. This can be seen particularly impressively in the example of the antenna complex LH2 of purple bacteria. The LH2 complex consists of two groups of bacteriochlorophyll molecules arranged in a ring (see figure on the left). The upper ring (B850) contains 18 BChla-Molecules that are very close to one another, i.e. are strongly coupled. The lower ring (B800) consists of 9 BChla-Molecules that are significantly farther apart and are therefore much more weakly coupled.

Due to the strong coupling, the absorption of BChla shifted to red in the B850 ring. The absorption band is at 850 nm. The weakly coupled BChla of the B800 ring, on the other hand, absorb at 800 nm, i.e. roughly in the same range as BChl dissolved in solventaMolecules. In the absorption spectrum (figure on the right) of the LH2 complex, the absorption bands of the B800- and B850-BChl-a-Molecules clearly separated. In addition, bands that originate from carotenoid molecules are shown; these are not shown in the structure.

Types

There are several types of chlorophyll that differ in the side groups of the porphyrin. They have different absorption spectra and occur in different phototrophic organisms:

Chlorophyll type colour Absorption maxima
(in nm)[1]
Occurrence
Chlorophylla blue green430, 662Cyanobacteria and all phototrophic eukaryotes
Chlorophyllb yellow-green454, 643Green algae (Chlorophyta), Euglenozoa and all land plants
Chlorophyllc green444, 576, 626instead of chlorophyll b in brown algae (Phaeophyta), diatoms (Bacillariophyta),
Golden algae (Chrysophyta), yellow-green algae (Xanthophyta), Haptophyta, Dinophyta and Raphidophyceae
Chlorophylld 447, 688instead of chlorophyll b in red algae (Rhodophyta)
Bacteriochlorophyllagreen358, 577, 773Purple bacteria (Rhodospirillaceae, Chromatiaceae)
Bacteriochlorophyllb 368, 580, 794Purple sulfur bacteria (Chromatiaceae)
Bacteriochlorophyllcgreen432, 660Green sulfur bacteria (Chlorobiaceae)
Bacteriochlorophyllcs   Green non-sulfur bacteria (Chloroflexaceae)
Bacteriochlorophylld 458, 646Green sulfur bacteria (Chlorobiaceae)
Bacteriochlorophylle 424, 654 Green sulfur bacteria (Chlorobiaceae)
BacteriochlorophyllG 408, 418, 470, 575, 763Heliobacteria

Importance in photosynthesis

Chlorophylls have several roles in photosynthesis. By far the largest share is used for light absorption and the transmission of the absorbed energy. For this purpose, the chlorophyll molecules are organized in light-collecting complexes, which are arranged in such a way that, on the one hand, the largest possible absorbing surface is formed and, on the other hand, an energetic funnel is created that guides the absorbed energy to the so-called reaction center. In the reaction center, two chlorophylls serve as acceptors of this energy. They are so specially arranged that their excitation leads to a charge separation, which can be regarded as the first step in the actual photosynthesis. This pair of chlorophylls is called in English special pair designated.

There are many differences in the structure of the light-harvesting complexes in the very different organisms that drive photosynthesis, the reaction center, on the other hand, is almost always structured in almost the same way. The special pair is always caused by chlorophyll in plants, algae and cyanobacteria a, formed in bacteria by various bacteriochlorophylls.

history

The first studies on the chemical structure of chlorophyll come from Richard Willstätter. The chemist Hans Fischer resumed Willstätter's research in the 1930s, and in 1940 he was able to elucidate the structure of the molecule. Fischer's research was confirmed in 1960 by Robert B. Woodward's chlorophyll synthesis.

miscellaneous

An important property of chlorophyll is chlorophyll fluorescence. It is mainly used to determine the chlorophyll content and its activity as well as for other scientific analyzes.

As a food additive, chlorophyll has the code number E 140.

credentials

  1. Hugo Scheer (Editor): Chlorophylls. CRC Press, 1991. ISBN 0-8493-6842-1

literature

  • Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: biochemistry. 6th edition. Elsevier Spectrum Akademischer Verlag GmbH, Heidelberg 2007, ISBN 978-3-8274-1800-5

See also

Categories: Photosynthesis | Chemical compound | Vegetable dye