Chlorophyll monomer

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Fig. 1. Structures of model systems and the photosynthetic unit, (a) Rhodamine B adsorbed on the ab-plane of the anthracene single crystal (1), (b) steroid skeleton with biphenyl as donor and acceptor A (8), (c) donor and acceptor molecules bonded by methylene chain (28), (d) molecule with completely rigid skeleton (13), (e) methylviologen-capped porphyrin (9), (f) reaction center of Rps. viridis (29). D Chlorophyll dimer, M Chlorophyll monomer, PrPheophytin, QrQuinone.

The fluorescence quantum yield of monomeric chlorophyll-a in solution hardly varies with solvent [10], and a value of 0.32 was used for the fluorescence quantum yield of the uncoupled chlorophyll. Chlorophyll-a in ether exhibits a lifetime of 6.7 ns, which is similar to the lifetime of the uncoupled chlorophyll, which is assumed to be in a protein environment. This suggests that chlorophyll monomers in these different environments will have similar fluorescence quantum yields. 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

In the biosynthesis of the pigments oflife, uroporphyrinogen III (0-12) is formed by cyclotetramerization of the monomer porphobilinogen (0-11) (Scheme 0.4). Uroporphyrinogen III (0-12) acts as precursor of inter alia heme, chlorophyll, as well as vitamin B12 [13]. 

Fig. 1. The structure of the LHCII monomer as derived from electron crystallography [51], A proposed topography of the polypeptide in the photosynthetic membrane. Letters A, B and C indicate the three hydrophobic ix-helices spanning the membrane. Chlorophyll molecules are arranged into two rings roughly parallel to the membrane plane. B Approximate position of the chlorophyll in the upper level (left) and lower level (right) on the membrane plane. Dashed lines outline a-helices A, B and C. Chlorophyll molecules are oriented perpendicular to the membrane plane and are thus represented as black bars. Chlorophylls numbered as 6,7 and 8 are closer to those belonging to the lower layer than the other pigment molecules...

FIGURE 19-42 A light-harvesting complex, LHCII. The functional unit is an LHC trimer, with 36 chlorophyll and 6 lutein molecules. Shown here is a monomer, viewed in the plane of the membrane, with its three transmembrane a-helical segments, seven chlorophyll a molecules (green), five chlorophyll b molecules (red), and two molecules of the accessory pigment lutein (yellow), which form an internal cross-brace. 

Figure 23-34 Structure of PSII with assignment of protein subunits and cofactors. (A) Arrangement of transmembrane a-helices and cofactors in PSII. One monomer of the dimer is shown completely, with part of the second monomer related by the local-C2 axis (filled ellipse on the dotted interface). Chlorophyll a head groups and hemes are indicated by black wire drawings. The view direction is from the luminal side, perpendicular to the membrane plane. The a-helices of Dl, D2, and Cyt b-559 are labeled. D1/D2 are highlighted by an ellipse and antennae, and CP43 and CP47 by circles. Seven unassigned a-helices are shown in gray.

For example, extract the proanthocyanidin mixture with chloroform to remove chlorophyll, carotenoids, and waxy material. Use ethyl acetate if substantial amounts of flavan-3-ol monomers are present. Tissues that would benefit from a chloroform extraction include leafy tissues that contain chlorophyll (i.e., tea leaves) and seeds that contain oils (i.e., grape seeds). Ethyl acetate would be useful in plants such as apples, berries, grapes, and teas (i.e., tissues known to contain significant amounts offlavan-3-ol monomers). 

It is important to purify proanthocyanidins, particularly for determining their conversion yield. It is also advantageous to do so to eliminate extraneous material that might otherwise react with the proanthocyanidins. A combination of liquid-liquid extraction and adsorption chromatography is effective in removing impurities. The use of chloroform in liquid-liquid extraction is very effective in removing fat-soluble compounds such as carotenoids, chlorophyll, oils, and waxes. These compounds would be expected in leafy plant tissues (carotenoids and chlorophyll) as well as seeds and fruits (oils and waxes). Ethyl acetate is effective in the selective removal of flavan-3-ol monomers, which are also typically present with proanthocyanidins. 

Because of the varied nature of the plant tissues from which the proanthocyanidin extracts are derived, it is difficult to anticipate the expected outcome. As an example of how these procedures can be adapted to specific tissues and analyses, using grape tissues, fruit is harvested and the tissues of interest (e.g., skins and seeds) are removed from the remainder of the berry. They are rinsed well and then extracted as whole tissues using the conditions described in these protocols. For grape skins, aliquid-liq-uid extraction with chloroform has been successful in the removal of chlorophyll and waxes, yet no extraction with ethyl acetate has been performed because of the small proportion of flavan-3-ol monomers (Kennedy et al., 2001). For grape seeds, these protocols have... 

Thermosynechococcus (T.) vulcanus crystallizes as a dimer (Figure 3.4.2) that contains about 2,800 solvent water molecules [7], Each monomer consists of about 20 protein subunits that harbor 77 cofactors 35 chlorophylls (Chi a) 11 p-carotenes 2 plastoquinones (PQ) 2 pheophytines (Pheo a) 1 Mn4OsCa complex 2 heme Fe 1 nonheme Fe and 1 hydrogen carbonate, HCO3/CO3. The overall reaction of PSII is that of a light-driven water plastoquinone oxidoreductase [1] ... 

Hanson L.K. Molecular orbital theory of monomer pigments. In Chlorophylls. Scheer H, ed. 1991. CRC Press, Boca Raton, FL. pp. 993-1014. 

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