Structure of Chlorophyll

Chlorophylls are the Mg containing green pigments of plants which serve the purpose of oxygen transport for photosynthesis. The empirical formula of chlorophyll a is C₅₅H₇₂O₅N₄Mg. Chlorophyll-a is a blue-green microcrystalline solid, consisting of head and a tail. The head consists of a porphyprin ring or tetrapyrrole nucleus from which extends a tail made up of a 20 carbon grouping called the phytol. The porphyrins are complex carbon nitrogen molecules that usually surround a metal. In chlorophyll the porphyrin surrounds a magnesium while in haemoglobin surrounds an iron. The cytochromes of the electron transport system also have porphyrin rings. The basic unit of the porphyrin ring is the porophobilinogen molecule. Four such pyrrole units make up the tetrapyrrole structure. Phytol is a long straight chain alcohol containing a single double bond. It may be regarded as a hydrogenated carotene (vitamin A). Its formula is C₂₀H₃₉.

Chlorophyll-b has the empirical formula C₅₅H₇₀O₅N₄Mg. It is a green-black microcrystaline solid, it differs from chlorophyll-a in having aldehyde (CHO) group attached to carbon atom 3, instead of a methyl (CH₃) group.

The presence of Mg makes the entire molecule rigid so that the energy is not readily lost thermally as well as it increases the rate at which the short lived singlet state passes into the long lived triplet state and hence can transfer its excitation energy to the redox system.

As we know that the amount of light (energy) absorbed at different wave lengths constitutes the absorption spectrum. It has also been seen that the clorophyll of higher plants absorbs mainly in the blue and red regions of the spectrum. There is some difference in the absorption spectra of chlorophyll-a and chlorophyll-b. Chlorophyll-a has a major absorption peak at 675 mμ and a minor peak at 450 mμ. The major absorption spectrum for chlorophyll-b is at 450 mμ. Recent work has shown that extraction procedures change the absorption properties of chlorophyll. Thus, the absorption spectra for extracted chlorophyll and chlorophyll in situ are not same. In situ chlorophyll-a absorbs mainly far red light (700 mμ) while chlorophyll-b primarily absorbs lower wavelengths of light.

Bacteriochlorophyll absorbs in the infra-red and blue-violet regions.