390 PLANT METABOLISM 



The green pigment from higher plants can be separated into chlorophyll 

 a and chlorophyll h by repeated distribution between immiscible solvents 

 or by adsorption chromatography. The first method was used by AVill- 

 statter in his classic investigations of the chemistry of the chlorophylls. 

 Tswett's separation of plant pigments in 1906 introduced the useful 

 tool of adsorption chromatography ^ to chemists. Chlorophylls other than 

 a and h have been isolated from diatoms and algae, but their structures 

 have not been determined in detail. In addition, bacteriochlorophyll 

 has been isolated from purple sulfur bacteria and characterized. It differs 

 from chlorophyll a (Fig. 15-3) only in having an acetyl group (— COCH3) 

 at position 2 and extra hydrogens at 3 and 4. 



When seeds are germinated in the dark, the plants produced are termed 

 etiolated plants, being devoid of chlorophyll. However, some proto- 

 chlorophyll is present in etiolated plants, and when they are illuminated 

 the protochlorophyll is rapidly converted to chlorophyll a by a process 

 of reduction. It is suggested that chorophyll h then arises from chloro- 

 phyll a. Most plants have about 3 times as much chlorophyll a as h, 

 and the sum of these chlorophylls usually constitutes 0.7 to 1.3 per cent 

 of the dry weight of leaves. 



In addition to chlorophylls, the chloroplasts contain other pigments 

 that may function indirectly in photosynthesis. Some of the light that 

 they absorb can be utilized, but there has been no demonstration that 

 they can function photosynthetically in the complete absence of chloro- 

 phylls. 



The determination of the structure of chlorophyll by Willstatter, Stoll, 

 Conant, Hans Fischer, and their co-workers constitutes a brilliant con- 

 tribution to organic chemistry. The empirical formula for chlorophyll 

 a is C55H7205N4Mg, and for chlorophyll 5 is C55H7o06N4Mg. The cur- 

 rently accepted structural formula for chlorophyll a is shown in Fig. 15-3. 

 There remains some question whether the semi-isolated double bond - 

 occurs in pyrrole ring II or is in ring III; if it were in ring III, the Mg 

 would be bound between rings I and II rather than between I and III 

 as shown. 



There are a number of characteristics of the chlorophyll molecule 

 which should be noted. First, much of the stability of the molecule can 

 be attributed to the system of conjugated double bonds,^ designated 

 by the bold lines in the outline formula of Fig. 15-4. Second, the 



1 The procedure for adsorption chromatography is briefly as follows : a finely 

 powdered adsorbent such as magnesium oxide is padded in a glass tube to form a 

 column, and a solution of the mixture to be separated is poured on the top. When 

 a suitable solvent is passed through the column, the individual components of the 

 mixture move at different rates and separate into distinct bands. 



= This is the double bond that is not a part of the conjugated system, shown between 

 carbons 3 and 4 in Fig. 15-3. 



^ Alternate single and double bonds are said to be conjugated. 



