Chlorophyll molecules sensitization

In a broad sense photosynthesis in plants is a photoinduced electron transport reaction. Chlorophyll molecules in the green plants are the main light harvesting molecules. They are assisted by carotenoids and phycocyanins in this act. These molecules have absorption in the visible region covering the whole spectrum from blue to red. The energy absorbed by all these molecules is transferred to chlorophyll a (Chi a), which is the main light sensitive molecule, by mechanisms discussed in Section 6.6.4. 

Photosynthesis in green plants is mediated by two kinds of membrane-bound, light-sensitive complexes—photosystem I (PS I) and photosystem II (PS II). Photosystem I typically includes 13 polypeptide chains, more than 60 chlorophyll molecules, a quinone (vitamin Kj), and three 4Fe-4S clusters. The total molecular mass is more than 800 kd. 

Chlorophyll a fluorescence induaion is a widespread method to evaluate the photosynthetic activity. This method is noninvasive, highly sensitive, fast, and easily measured. When chlorophyll molecules in photosystem II absorb light, that light may be assimilated into the hght reactions of photosynthesis or may be released as fluorescence or heat energy. In vivo fluorescence increases when photosynthesis declines or is inhibited. Numerous environmental f ors can affect the rate of electron transport between photosystem II and photosystem I due to interference with electron carriers between the two photosystems. For example, when the diuton is added in the measured sample, electron transport from photosystem II to photosystem I is blocked resulting in maximum fluorescence. This method was often employed to detect the photosynthetic activity of immobilized photosynthetic material. ... 

Livingstone and Owens have shown that the triplet state is an essential intermediate in chlorophyll-sensitized photochemical reactions occurring in homogeneous solutions (17). It has not yet been satisfactorily proven whether or not the triple state participates in in viw chlorophyll reactions, but nevertheless there is considerable evidence that the sequence of biochemical reactions in photosynthesis is initiated by a chlorophyll molecule in its triplet state (5). 

The chemical answer to this is to identify the sites on the porph)rrin molecule where reactivity occurs and to bl(x k them. These sites are the mcso-carbons and the metal centre. Nature, of course, solved this problem eons ago by embedding the sensitive chlorophyll molecule in the protein-lipid matrix of the chloroplast membrane. Synthetically, this problem is solved by substituting water-solubilising groups in the wicso-positions so that a molecular chain covers that position and the metal centre like an umbrella. 

The second volume of Laser Spectroscopy covers the different experimental techniques, necessary for the sensitive detection of small concentrations of atoms or molecules, for Doppler-free spectroscopy, laser-Raman-spectroscopy, doubleresonance techniques, multi-photon spectroscopy, coherent spectroscopy and time-resolved spectroscopy. In these fields the progress of the development of new techniques and improved experimental equipment is remarkable. Many new ideas have enabled spectroscopists to tackle problems which could not be solved before. Examples are the direct measurements of absolute frequencies and phases of optical waves with frequency combs, or time resolution within the attosecond range based on higher harmonics of visible femtosecond lasers. The development of femtosecond non-collinear optical parametric amplifiers (NOPA) has considerably improved time-resolved measurements of fast dynamical processes in excited molecules and has been essential for detailed investigations of important processes, such as the visual process in the retina of the eye or the photosynthesis in chlorophyl molecules. 

In Nature (on membranes of chloroplasts) water is oxidized by the four-electron mechanism and the life-time of chlorophyll can be as long as one day. In a model octane/water system, oxygen evolution occurs also during several hours. This is indirect evidence in favor of a many-electron reaction. Therefore we shall consider a four-electron mechanism of water oxidation sensitized by chlorophyll adsorbed at the oil/water interface proposed by Volkov [81]. It was shown above that the interface is the most likely site for the water photooxidation reaction. Thus it is assumed that water can be oxidized by a reaction complex adsorbed at the interface that consists of a hydrated oligometer of chlorophyll, a hydrophilic electron acceptor and hydrophobic proton acceptor [81,82,86]. The water in the reaction complex is linked coordinatively with the magnesium of one of the chlorophyll molecules, by hydrogen bonds with the carbonyl group of another chlorophyll molecule, and with the phenol anion also... 

The photosensitized transformation of carotenoids has been studied using several sensitizer molecules, such as chlorophylls, iodine, rose bengal (RB), and methylene blue (MB) and in general terms isomerization is the major pathway of reaction. 

Irradiation of olefins and dienes in the presence of oxygen and various sensitizers produces hydroperoxides and cyclic peroxides. Effective sensitizers include not only high-energy carbonyl triplets, but also low-energy organic dyes such as methylene blue, rose bengal, chlorophyl, and riboflavin. The same species that quench the phosphorescence of these complex molecules also quench their photosensitizing ability. 

In widespread use since 1982 (Barber et al., 1982), FAB and LSIMS are matrix-mediated techniques. The most effective matrix for static FAB/LSIMS analysis of chlorophylls and their derivatives is 3-nitrobenzyl alcohol (van Bree-men et al., 1991a), whereas glycerol provides adequate sensitivity and a more robust system during continuous-flow FAB/LSIMS (van Breemen et al., 1991b). Ionization and desorption of the chlorophyll analyte occur together during the bombardment of the matrix by fast atoms (or ions) to produce molecular ions, M+-, and protonated molecules, [M+H]+. 

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