Structure of chlorophylls

Willstatter s most important work for which he won the 1915 Nobel Prize in chem istry was directed toward determining the structure of chlorophyll... 

Richard Willstatter 11872-19421 was born in Karlsruhe, Germany, and obtained his Ph.D. from the Technische Hochschule, Munich (18951. He was professor ol chemistry at the universities of Zurich, Berlin, and then Munich (1916-1924). In 1915, he won the Nobel Prize in chemistry for his work on elucidating the structure of chlorophyll. Nevertheless, as a Jew, he was subjected to anti-Semitic pressure that caused him to resign his position at Munich in 1924. He continued to work privately. 

In accordance with the structure of chlorophyll c, it is hypothesized that its biosynthesis comes from protochlorophyllide a by dehydrogenation of the side chain at C-17. Chlorophyll d should arise from chlorophyll a by oxidation of the C-3-vinyl residue, but at which stage of chlorophyll biosynthesis this occurs is unknown. The biosynthesis of bacteriochlorophylls seems to follow the same general pathway of higher plants, according to studies performed with chlorophylhde and bacterio-chlorophyU enzymes. ... 

FIGURE 13.3 The structures of chlorophylls a and b. Other chlorophylls and their decomposition products can be derived from these structures as described in the text. 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

In chlorophyll iron as complex-forming metal is replaced by magnesium (Willstatter). The structure of chlorophyll differs from that of haemin as follows. In chlorophyll one propionic acid chain (a) in oxidised form has condensed with a methine carbon atom to form a cyclopentane ring which takes the position at (c) of the vinyl ethyl. Further the two carbonyl groups are esterified and one of the four pyrrole rings is partially hydrogenated... 

Chlorophylls are macrocyclic tetrapyrrole derivatives containing in their natural form a chelated magnesium ion. The basic structure of chlorophyll pigments is shown in Fig. 2.122. 

Fig. 19.18 Structure of chlorophyll. The long alkyl chain at the bottom is the phytyl group.

Based on our knowledge of the structure of chlorophyll as well as the results of studies on the photo behavior of chlorophyll in vitro, it is possible to summarize some of the features of the chlorophyll system which enhance its usefulness as a pigment in photosynthesis.68 First, there is extensive conjugation of the poiphyrin ring. This lowers the energy of the electronic transitions and shifts the absorption maximum into... 

An additional impetus for pyrrole and indole research derives from the recognition of the physiological significance of these ring systems. In the case of pyrroles the early work centered around hemin from hemoglobin and the bile pigments. The structure of chlorophyll was also shown to be pyrrole-derived. The biosynthetic connection between simple pyrroles and the macrocyclic hemin and chlorin have remained of interest up to the present (75ACR201). 

While the structure of chlorophyll a shown in Fig. 23-20 is the predominant one, other forms exist, e.g., with... 

Figure 20-6 The structure of chlorophyll a and chlorophyll b, showing cis-trans relationships of the substituents...

Chlorophyll is a magnesium tetrapyrrole of the chlorin class that has a unique alicyclic ring. Figure 19 shows the structure of chlorophyll, together with parts of the structure of chlorophyll b and bacteriochlorophyll a. The fifth and sixth coordination positions may be filled by solvent water, and also offer the possibility of dimer or higher aggregate formation. 

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.

Tetrapyrrole chemical substance consisting of four pyrrole rings, joined together into a single large ring known as a macrocycle, serves at the basic structure of chlorophyll. 

Figure 6-14 Structure of Chlorophyll a. (Chlorophyll b differs in having a formyl group at carbon 3). Source Reprinted with permission from J.R. Whitaker, Principles ofEnzymology for the Food Sciences, 1972, by courtesy of Marcel Dekker, Inc.

Chlorophylls chlorophylls have a variety of functions in photosynthetic systems, including collection of photons, transfer of excitation energy, operation of the primary photoinduced charge separation, and transfer of the resulting photoelectrons. Resonance Raman spectroscopy offers the possibility of selectively observing chlorophylls in their native structures (Lutz, 1984 Koyama et al., 1986 Tasumi and Fujiwara, 1987 Lutz and Robert, 1988 Nozawa et al., 1990 Lutz and Mantele, 1991). Transient Raman spectroscopy is a unique method of revealing the excited state structures of chlorophylls. The T1 and SI states were revealed by nanosecond Raman spectroscopy (Nishizawa et al., 1989 Nishizawa et al., 1991 Nishizawa and Koyama, 1991). 

Robert B. Woodward et al. Proof of structure of chlorophyll by total synthesis... 

Figure 3 Structures of chlorophylls and other photo synthetic pigments...

---