Phytol chain

The early stages of catabolism correspond to the replacement of Mg by two H atoms under acidic conditions and/or by the action of Mg-dechelatase and the cleavage of the phytol chain by the enzyme chlorophyllase. The still greenish intermediates are pheophytins, chlorophyUides, and pheophorbides with intact tet-rapyrrole rings. - ... 

In addition to the porphyrin nucleus, the phytol tail that esterifies the propionic acid side chain at C-17 may be hydrolyzed enzymatically during storage or processing. Cleavage of the phytol chain during digestion is unlikely. Free phytol is quickly... 

A hydrophobic phytol chain, which keeps the chlorophyll molecule embedded in the photosynthetic membrane. 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

We are primarily concerned with several typical modes of the Chi interfacial layer relevant to photoelectric and photoelectro-chemical systems. Since the Chi molecule, a surfactant pigment consisting of a partially hydrophilic porphyrin ring and a totally hydrophobic phytol chain, is insoluble in water, it is generally employed for photoelectric and photoelectrochemical measurements in the form of a solid (or quasi-solid) film deposited on an electrode surface. Such Chi films are divided into three typical modes according to molecular configuration. 

Figure 9.37 Chemical structures of chlorophylls-a and b which contain a propionic acid esterified to a C20 phytol chlorophylls-cj and C2 have an acrylic acid that replaces the propionic acid. Also included are the pheopigments, the four dominant tetrapyrrole derivatives of chloropigments (pheopigments) found in marine and fresh-water/estuarine systems (chlorophyllide, pheophorbide, pheophytin, pyropheophorbide.) More specifically, chlorophyllase-mediated de-esterification reactions (loss of the phytol) of chlorophyll yield chlorophyllides. Pheophytins can be formed when the Mg is lost from the chlorophyll center. Pheophorbides are formed from removal of the Mg from chlorophyllide or removal of the phytol chain from pheophytin, and pyrolyzed pheopigments, such as pyropheophorbide and pyropheophytin, are formed by removal of the methylcarboxylate group (-COOCH3) on the isocylic ring from the C-13 propionic acid group.

Figure 19.5. Chlorophyll. Like heme, chlorophyll a is a cyclic tetrapyrrole. One of the pyrrole rings (shown in red) is reduced. A phytol chain (shown in green) is connected by an ester linkage. Magnesium ion binds at the center of the structure.

Fig. 26.1. Structure of the LH2 complex from Rps. acidophila as determined by X-ray diffraction. Part (a) shows the whole pigment-protein complex in a top view part (b) displays the spatial arrangement of the BChl a in a tilted side-view. The B800 BChl a pigments are shown in hght gray and the B850 BChl a molecules are shown in dark gray. The numbers indicate the centre to centre distances of the pigments in A. The arrows indicate the direction of the Qy transition moments. The phytol chains of the pigments are removed for clarity. Adapted from [3]...

In 27 the carotenoid moiety is linked through an amide function which is derived from the carboxylic acid which bore the phytol chain of the original chlorophyll. There are a total of three saturated carbon atoms separating the n-electron systems. Although this structure could allow a folded con-... 

Chlorophylls a and b are found in almost all photosynthesizing organisms. They possess a complex cyclic structure (called a porphyrin) with a magnesium atom at its center. Chlorophyll a possesses a methyl group attached to ring II of the porphyrin, whereas chlorophyll b has an aldehyde group attached to the same site. Pheophytin a is similar in its structure to chlorophyll a. The magnesium atom is replaced by 2 protons. Chlorophylls a and b and pheophytin a all possess a phytol chain esterified to the porphyrin. The phytol chain extends into and anchors the molecule to the membrane. Lutein and /5-carotene are the most abundant carotenoids in thylakoid membranes. 

Steric hindrance, attributed to the phytol chain of pheophytin, may decrease the rate of complex formation. In the study by Berezin and Koifman (20), pheophorbide a reacted 4 times faster with copper(II) ion than did pheophytin a in ethanol at 25°C. Moreover, electrostatic interaction of the C-7 propionic... 

One feature which makes these /0-dimers special is that both the ligand and the 17-propionic acid residue, which carries the long phytol chain, are on the a-side of the macrocycle. This enables the /0-sides to interact quite closely, allowing in principle efficient excitonic coupUng to create a red-shifted absorption in comparison to the monomeric Chls (which absorb around 670 nm). 

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