1768 CHEMISTRY OF CHLOROPLAST PIGMENTS CHAP. 37B 



above-described observations by Smith and Koski suggest the reverse 

 hypothesis — that only "Chi 678" is photosynthetically active. (The 

 capacity for photoxidation may well be antiparallel, rather than parallel, 

 to photosynthetic activity.) In Chapter 37C (section 6b) we will quote 

 another argument, pointing to the same conclusion: Resonance energy 

 transfer always is directed toward the pigment with the lower excited state 

 (while Krasnovsky's hypothesis calls for energy transfer in the opposite 

 direction). The 670 mn absorption band may belong to as yet "unor- 

 ganized" (but already protein-bound) chlorophyll, while the 678 m^i band 

 may be that of chlorophyll arranged in monomolecular layers; the 

 shift may then be due to the electrostatic interaction of the pigment mol- 

 ecules in a regular array, to be discussed in section 3 of Chapter 37C. 



Yocum (1946) observed the inhibition of chlorophyll formation in excised, etiolated 

 bean leaves by poisons. Carbon monoxide reduced both respiration and chlorophyll for- 

 mation in red light, in approximately the same ratio; strong blue light reversed the 

 inhibition in both cases. Cyanide (10~^ .1/) reduced the formation of protochlorophyll 

 in the dark, but did not affect the conversion of protochlorophyll to chlorophyll. 



The hiodecomposition of chlorophyll was discussed by Noack (1943), 

 who attributed the destruction of the green pigment in autumn to the action 

 of hydrogen peroxide, not decomposed by the, now inactivated, catalase, on 

 water-soluble chlorophyllides formed by the action of chloroph3'llase. 



2. Biogenesis of Chlorophyll ; Earlier Precursors 



If one wants to speculate on the mechanism of chlorophyll synthesis 

 in the living cell beyond the experimentally established photochemical con- 

 version of protochlorophyll to chlorophyll, one must take recourse to in- 

 direct evidence. 



Granick (1948,^-^ 1950,^'2 1951, 1954) obtained such evidence from ex- 

 periments in which Chlorella cells were exposed to x-rays, producing a vari- 

 ety of mutants. Several of these — ^'iable in glucose solution but incapable 

 of photosynthesis — were found to contain no chlorophyll: instead, some of 

 them carried pigments of the porphin type not normally encountered in 

 algae. One mutant, in particular, dark brown in color, contained globules 

 of pivtoporphyrin 9. In another mutant, which, when grown on a solid 

 nutrient medium, developed an orange-brown color, small amounts of the 

 magnesium derivative of the same protoporphyrin could be identified (in 

 addition to the protoporphyrin itself). A third, yellow mutant yielded 

 magnesium-vinyl pheoporphyrin 05, i. e., protochlorophyll without the 

 phytol chain. 



Granick postulated that the appearance of these pigments signifies 



