J. A. BASSHAM AND M. CALVIN 



unit of about 10^ A^ volume, or 100 A diameter, it is capable of 

 producing oxygen in the Hill reaction even if partially frag- 

 mented. 



Earlier, Milner et al. (42), using the photochemical reduction 

 of 3,6-dichlorobenzene indophenol as a measure of activity, 

 studied the Hill reaction with particles of subgranar size and 

 obtained activity with a mixture of particles, the majority of 

 which were about 20 A in diameter. The same workers (43), 

 after measuring Hill reaction activity with these particles of about 

 one-fourth that of intact chloroplasts, found that the particles 

 could be aggregated by precipitation with a variety of salts in 

 the presence of 15% to 20% methanol in such a way as to 

 produce particles with increased activity. Loss of lipid material 

 resulted in loss of activity. 



These experiments indicate that there are physical units, 

 about 100 A in diameter and containing about 200 chlorophyll 

 molecules, which are capable of carrying out the Hill reaction 

 nearly as efficiently (^-^60%) as intact chloroplasts. If these units 

 are broken down further, Hill reactivity falls off rapidly, but is 

 present to some extent with considerably smaller particles, 

 especially with high light intensities. Moreover, much of the 

 original activity can be restored by reaggregation. The require- 

 ments for photodecomposition of water with chlorophyll seem 

 to be some aggregation of chlorophyll, lipids, and protein. 

 Rodrigo (51) has studied associations of a few molecules of 

 chlorophyll, finding no shift of the red absorption peak to 6800 

 A. However, when chlorophyll was mixed with some ground-up 

 leaves which contained no chlorophyll initially, some shift of the 

 red absorption peak toward 6800 A and some oxygen evolution 

 in light with quinone were observed. The implication of this 

 result seems to be that a degree of aggregation of the chlorophyll 

 molecules sufficient to shift the red peak as far as 6800 A is 

 required before the Hill reaction can function. It is interesting 

 to note in this connection that the absorption spectra of chloro- 

 phyll in crystals of varying size formed from ethyl chlorophyllide 

 in acetone has been studied by Jacobs and Holt (37) and a shift 



44 



