Chlorophylls structures

Thus far we have used the word chlorophyll as if this term related to a unique chemical species. In actuality there are a number of structurally related molecules present in photosynthetic organisms which are collectively referred to as chlorophyll. The general chlorophyll structure is ... 

Another important metal-containing enzyme, chlorophyll (structure 16.24), is structurally related to myoglobin and is responsible for photosynthesis, the conversion of water and carbon dioxide, with the aid of solar energy, to carbohydrates. 

Inhomogeneity is quite naturally predicted to occur in this class of complexes. This is not only due to the different possible breakdown pathways in soils, sediments, etc., but is also due to the multiplicities of naturally occurring precursors in living organisms. For example, a number of known chlorophyll structures are shown in (23). The structural complexity is even more elaborate for the haems. Mixtures in geological media thus are to be expected. However, it should be pointed out that the amount of porphyrins from plant sources is overwhelming compared to that... 

Figure 1 Chlorophyll structures The three basic types are shown on the top, with the lUPAC numbering of the atoms and the ligands to the central metal given for the central phytochlorin system, (a) Only sometimes esterified and (b) acrylic side chain at C-17 (17 -1 7 double bond). See the internet version for color coding.

Figure 15. Copepod and chlorophyll profiles sampled at 1900 h (a) and the corresponding estimated production profiles (b) from the Peruvian shelf at 9° S. The vertical chlorophyll structure shows both a surface and subsurface...

The family of tetrapyrroles can be arranged in two classes depending on whether they are closed like chlorophyll (typically green) or open like phycocyanobilins (red or blue) (Table II). Since the pioneer works by Willstater and Stoll (1913) on the chlorophyll structure, more than 50 different chlorophyll-related molecules have been reported (Scheer, 1996). 

From a proteic point of view, the location of the light-induced reactions in PS1 appears to be very different from that in PS2 or in bacterial RCs. Indeed, in the bacterial RCs the membrane-embedded part of the photosystems that carry the first electron donors and acceptors consists of two proteic subunits of 250-350 amino acids, whereas psa A and psa B are each constituted of 750 amino acids. If one compares the function of the different photosystems, it clearly appears that many of the electron tranfer steps are similar between PS2 and bacterial RCs. In these RC, after excitation of the primary electron donor, the electron rapidly jumps from a chlorophyllic structure (a dimer of BChl in bacterial RCs) to a (bacterio)pheophytin. From the (bacterio)pheophytin the electron is transferred to a quinone then to a second quinone. In PS1, after the excitation of the primary electron donor, the electron jumps rapidly from the primary electron donor P700 (most likely a dimer of Chls) to a chlorophyll (Aq) from Aq it is transferred to the A acceptor, and thence to a series of iron-sulfur clusters (Fx. Fg and Fb) (4). If some structural analogy may be found between all the photosystems. It obviously will concern the proteic features related to the first electronic steps, e.g. those which are located in the local environment of the primary donor and/or primary acceptors of electrons. 

Chlorophylls, structure and occurrence. Note that the macrocyclic ring displays different degrees of reduction, i.e. a chlorophyll may be a porphyrin, a dihydroporphyrin or a tetrahydroporphyrin. [Data from S.l. Beale J.D. Weinstein in Biosynthesis of Heme and Chlorophylls (H. A. Dailey, ed.) pp.287-391, McGraw-Hill Publishing Co., New York, 19901... 

When ferrous iron is inserted into protoporphyrin, the ubiquitous iron porphyrin or heme is formed. Because of the resonating structure, an electron donor or acceptor molecule need not come in contact with the iron atom directly to oxidize or reduce the iron atom it is probably sufficient that contact be made with any portion of the resonating molecule for the iron atom to be oxidized or reduced. The oxidative properties of the iron atom in heme are modified by the iron being held in this ring and are further modified by the heme being attached to specific proteins. In nature the other metal that complexes with protoporphyrin is magnesium Mg protoporphyrin is an intermediate compound in the biosynthetic chain of chlorophyll synthesis. The movements of the ir electrons in the porphyrin are undoubtedly intimately connected with the functioning of the heme and chlorophyll structures, but of this we know very little. 

System 1 Sucrose (icing sugar) layer (0.4 mm, Tate Lyle), light petroleum (40-60°C)-2-propanol (99 1). System 2 Cellulose layer (0.1 mm, Merck), light petroleum (40-60°C)-acetone (80 20). System 3 Silica gel 60 (0.25 mm, Merck), diethyl eiher-acetone-isooctane (20 20 60). Petroselinum crispum was used as pigment source. The following abbreviations are used Phy = phytyl Chi = chlorophyll. Structure indication refers to Fig. 14. 

Thus, as a consequence of the mobile x-electrons a rather large stable colored molecule, the porphyrin, is obtained. The molecule is flat and contains within itself a cyclic circuit along which the x-electrons may be considered to move. The energy levels of the x-electrons in these circuits are undoubtedly intimately connected with the functioning of the heme and chlorophyll structures but of this we know very little. 

Baber, M. et al.. Fast atom bombardment mass spectrometry. Anal. Chem., 54,645,1982. Brereton, R.G. et al.. Positive and negative ion fast atom bombardment mass-spectro-metric studies on chlorophylls Structure of 4-vinyl-4-desethyl chlorophyll b tetrahedron, Letters, 24, 5775, 1983. 

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