BIBLIOGRAPHY TO CHAPTER 16 467 



4. Elimination of Magnesium and Phytol 



The best investigated chemical reaction of magnesium in chlorophyll 

 and similar compounds is its elimination by acids (substitution of two 

 hydrogen atoms leading to the formation of two imino groups) which 

 produces pheophytins (if phytol is intact) or pheophorbides (if phytol 

 has been eliminated). 



The kinetics of the conversion of chlorophyll into pheophytin can be 

 followed spectrophotometrically by observing the gradual weakening of 

 the red absorption band, as has been done by Joslyn and Mackinney 

 (1938, 1940) and Mackinney and Joslyn (1941). They found the 

 reaction to be of the first order with respect to [H+], and probably also 

 with respect to chlorophyll. Chlorophyll a reacts eight to nine times 

 more rapidly than chlorophyll h. They assume that the entrance of the 

 first hydrogen atom is the slow rate-determining process. 



That "pheophytinization" is a two-step reaction (as envisaged in 

 equations 16.3) was confirmed by observations by Rabinowitch and 

 Weiss on the effect of light on this reaction (see page 493). While the 

 removal of magnesium from chlorophyll occurs very easily (it can be 

 replaced not only by hydrogen, but also by other divalent metals, e. g., 

 copper, zinc, iron), the reintroduction of magnesium into pheophorbide 

 is much more difficult, although it can be achieved by means of Grignard 

 reagents. 



Ruben, Frenkel, and Kamen (1942) found that pure chlorophyll does not exchange 

 its magnesium for radioactive magnesium in the presence of magnesium salts, but that 

 this exchange can be observed in crude leaf extracts. 



Another part of the chlorophyll molecule which can be reversibly 

 replaced, is phytol. This can be achieved by means of the enzyme 

 chlorophyllase, whose presence in plants was discovered by Willstatter 

 and Stoll (1913) (c/. Chapter 14, page 380). It is plausible that the 

 purpose of chlorophyllase is to assist in the synthesis of chlorophyll 

 rather than to participate in the photosynthetic process. 



Bibliography to Chapter 16 

 Molecular Structure and Chemical Properties of Chlorophyll 



A. Molecular Structure 

 Books and Reviews 



1909 Marchlewski, L., Die Chemie des Chlorophylls. F. Vieweg and Son, 



Braunschweig, 1909. 



1910 Tswett, M., Chromophylls in the Plant and Animal World (in Russian), 



Warsaw, 1910. 

 1913 Willstatter, R., and Stoll, A., Untersuchungen iiber Chlorophyll. 

 Springer, Berlin, 1913; Investigations on Chlorophyll. Science Press, 

 Lancaster, Pa., 1928. 



