Chlorophyll disappearance

The whole process of chlorophyll disappearance in vascular plants is a complex multistep pathway, much as chlorophyll biosynthesis is, but for didactic reasons it can be abbreviated into two main stages. The first group of reactions produces greenish derivatives while the more advanced steps produce colorless compounds by an oxidative ring opening, analog to the oxygenolytic rupmre of the porphynoid macrocycle of haem. It is a very rapid process and despite considerable efforts, the detection of intermediates is difficult. ... 

THE BEAUTIFUL COLORS that develop in trees in the fall appear w/hen the tree ceases to produce chlorophyll, w/hich imparts the green color to the leaves during the summer. Some of the color we see has been in the leaf all summer, and some develops from the action of sunlight on the leaf as the chlorophyll disappears. 

The carotenoids represent one of the most important and widespread groups of pigments in nature. The structures of some 500 of them are known, and they are responsible for many of the yellow and red colors of flowers, fruits, birds, insects, and other animals. " Carotenoids are synthesized de novo in all organisms except for animals, where the pigments are of dietary origin." Their universal presence in photosynthetic tissues is only noticeable at the onset of leaf senescence, when the chlorophylls disappear. The dramatic changes in color of ripening fruits also reflect the disappearance of chlorophyll and the concomitant, massive increase in carotenoids. 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

FIGURE 20.21 The initial chemical step in photosynthesis, as carried out by the purple bacterium R. sphae-roides, is the transfer of an electron from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 17 A away. (Section 20.6 describes the structure of the reaction center in greater detail.) This electron transfer occurs very rapidly and with essentially 100% efficiency. Ultrafast spectroscopy monitors the electron transfer by detecting the disappearance of the excited state of the pair of chlorophyll molecules through the decay of its fluorescence. The data shown here reveal that the electron moves to the pheophytin with a half-life of 2.8 picoseconds (2.8 X 10 s). 

Figure 21 presents UV-visible spectra of two types of sediments, surface sediments (immersed or not) and deeper sediments (10-15 cm of depth). The colour of the former ones is brown, and it is black for the latter ones. This difference can be noticed in the visible range of the spectra a clear peak at 664 nm, specific of chlorophyll, can be noticed for surface sediments, which disappears for deeper sediments. This decrease of absorbance in the visible range goes with an increase of absorbance and a specific pattern of the spectrum in the UV region (200-300 nm), where degradation products may absorb. It may be supposed that, in this case, the process of decomposition of the organic matter is more advanced. This phenomenon is especially clear for deep sediments 1 and 2. 

The chlorophyll pigments are not stable to heat. The green color disappeared after the canning process largely due to the breakdown of chlorophylls into pheophytins. This was also evidenced by the large change in a value from -8.1 to +0.3 (see Table VIII). 

Fig. 2A shows the similarly dynamic changes in DF intensity and Pn in the presence of AAR at pH 3 for 2 h. After 15 min, DF intensity and Pn declined to about 65.6 and 66.7% of the original levels, respectively. They then continued to decrease gradually until they disappeared almost completely at 2 h (Fig. 2A). However, within the first 30 min of treatment, we have observed no marked changes in chlorophyll content. Up to 1 h, there occurred a significant decrease in chlorophyll content at P < 0.01 as compared to the control (Fig. 2B).

Fraits with a total disappearance of chlorophylls, usually having a characteristic yellow colour due to unmasked carotenes and xanthophylls following the degreening process (i.e., banana, plantain). 

Acceleration index of disappearance trom the body (shown in parentheses) equals (percent body burden ot PCDD and PCDF congeners ot rats in the chlorophyll diet) / (percent body burden ot PCDD and PCDF congeners of rats fed the control diet) x 100. 

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