444 CHLOROPHYLL CHAP. 16 



A comparison with the structure of bacteriochlorophyll (Formula 16.IV) 

 also speaks in favor of formulation B, since the semi-isolated double bond 

 (3-4 in structure B) offers itself as a natural location of the next hydro- 

 genation step. Another argument in favor of structure B is the absence 

 of a large dipole moment — which should be present if two polar mag- 

 nesium-nitrogen bonds would not point in opposite directions. (Of 

 course, this fact can also be explained by nonlocalized magnesium-nitrogen 

 bonds, i. e., by a mesomerism of structures A, B, and C). 



To sum up, it seems that, if one must choose between the structures A, 

 B, and C as isomers (and not mesomers), evidence speaks in favor of 

 structure B rather than A. The other two structures may correspond 

 to tautomers with a slightly higher energy. One is free to speculate on 

 a possible relationship between these structures and the chlorophyll 

 isomers a', h' , and d' , described by Strain and Manning {cf. page 403). 



In a complete analysis of the ground state of the chlorophyll molecule, 

 many more mesomeric structures should be considered, e. g., those with 

 the magnesium atom bearing a positive charge (according to Pauling, 

 a Mg-N bond is about "50% ionic"), as well as structures with positive 

 nitrogen and negative carbon. 



The ionic contribution to a carbon-nitrogen single bond is only about 6%; but in 

 the case of pyrrole, Pauhng estimated a 24% contribution of the four ionic structures: 



The keto group in the homocyclic ring V can be enolized by the 

 hydrogen atom in position 10. This tautomerism probably accounts for 

 some chemical reactions of chlorophyll, particularly in alkaline media. 



3. Bacteriochlorophyll and Protochlorophyll 



The hacteriochlorophijll of purple sulfur bacteria was investigated by 

 Schneider (1934) and in more detail by Fischer and coworkers (1940). 

 According to Fischer, it differs from chlorophyll a by oxidation of the 

 vinyl group, — CH=CH2, in position 2 to an acetyl group, — CO — CH3, 

 and the hydrogenation of a second double bond in one of the pyrrole 

 nuclei, probably the one in position 3-4 {cf. Formula 16.1 V). This 

 makes bacteriochlorophyll formally a hydrate of chlorophyll a, with the 

 composition, C55H7406N4Mg. Hydrogenation of nucleus II fixes the 

 conjugated double-bond system in bacteriochlorophyll in the "diagonal" 

 arrangement shown in formula 16. IV, and makes this compound appear 



