Chlorophyll molecular structure

Chlorophyll, as extracted from plants, is actually made up of two closely related compounds, chlorophyll A and chlorophyll B. These differ slightly in molecular structure and can be separated because they have different tendencies to be adsorbed on a finely divided solid (such as powdered sugar). 

In contrast to the well-established methods for identifying and quantifying naturally occurring chlorophylls, very few reports concern quantitative analysis of chlorophyllin copper complexes in color additives and in foodstuffs. Analytical methods proposed are based on spectral properties, elemental analysis, chromatographic separation, and molecular structure elucidation or a combination of these procedures. 

Nonomura, Y. et al., Spectroscopic properties of chlorophylls and their derivatives. Influence of molecular structure on the electronic state, Chem. Phys., 220, 155, 1997. 

Woodward then moved across town in Cambridge to devote a year of postgraduate study at Harvard University. At the end of that year, he accepted an appointment to the Harvard chemistry faculty, a post he held for most of the rest of his life. One of his great interests at Harvard was the synthesis of large, complex molecules, the first of which was quinine in 1944. He followed that work with the elucidation of other molecular structures and the development of synthetic methods for each. Included among these molecules were penicillin (1945), patulin (1948), cholesterol and cortisone (1951), oxytetracycline (1952), strychnine (1954), lysergic acid (1954), reserpine (1956), chlorophyll (1960), colchicine (1963), cephalosporin C (1965), and vitamin (1971). 

Figure 2 Molecular structures and IUPAC numbering scheme of organic cofactors occurring in photosynthetic reaction centres (bRC, PS I, PS II). (Bac-teriolpheophytin is the free base of (bacterio)chlorophyll plastoquinone (PQ) is found in PS If phylloquinone or vitamin K, ( VK,) in PS I many bacteria contain ubiquinone (UQ). Shown is also the amino acid tyrosine (Tyr, Y) that is redox active in PS II.

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

Chow, H. C., Serlin, R., Strouse, C. E., Crystal and molecular structure and absolute configuration of ethyl chlorophyllide a-dihydrate. Model for the different spectral forms of chlorophyll a. J. Am. Chem. Soc. 1975, 97, 7230-7237. 

Schulten (110, 111) has used laser-assisted field desorption mass spectrometry to study some inorganic and organometallic systems. This method is intermediate between LAMMA and simple FD. Metal cations predominate from inorganic salts. The technique also showed clusters of the type reported from both FAB and SIMS studies. By carefully controlling the laser, a chlorophyll molecular ion could be obtained as well as fragments relating to its structure. 

It is interesting that no one of the spectroscopies mentioned above can give complete information about the electronic or molecular structure, but that they provide complementary information to each other. X-ray structures can stand by themselves, of course, but by themselves give no direct information about the electronic energy levels involved in the biochemical processes as in the absorption of light by chlorophyll for example. 

FIGURE 22.3 Molecular structures of chlorophyll a, chlorophyll 6, and bacteriochlorophyll a. 

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