BENTLEY GLASS 865 



tivity of the ESR spectrometer it becomes possible to tolerate samples 

 containing licjuicl water and to attain scanning speeds ol '^0 to 90 

 seconds. Atlcquate resolntion ol the photosynthetic ESR signals 

 irom chlorojilasts oi spinach and tobacco revealed the signal's com- 

 plex structine. In the light, the once single ESR signal was resolved 

 into two, and in the dark a hyperfine structure of five peaks was dis- 

 closed. One of the peaks seen in the light was a new ESR signal (I) 

 lacking hyperfine structure. This signal had a relatively rapid 

 onset and decay. Signal II, the dark signal, was much slower in onset 

 and decay, and occurs in both light and dark. 



Observations on living Chlorella and on green and colorless forms 

 of Eiigleua further tied the two signals to the photosynthetic process. 

 The effects of 5% CO2, of different light intensities, and of various 

 temperatures were studied. The interpretation reached was that 

 there are two sequential steps (I and II) involving the formation of 

 intermediates with unpaired electrons (i.e., of free radicals). Compon- 

 ent I is considered the more proximal to the initial photochemical 

 event, but it can scarcely be an actual light-induced excited state 

 because it decays far too slowly. Since Component II is always 

 present in the dark, it must also be producible in some oxidation- 

 reduction occurring in the dark. There is no evidence that Com- 

 ponent I represents a free radical form of chlorophyll itself. By 

 growing Chlorella in D^O, a great compression of both signals is to 

 be observed, but especially in the case of Component II. This altera- 

 ation of the ESR signal indicates that the unpaired electron is asso- 

 ciated with hydrogen atoms, and that the hyperfine structure is due 

 to protons. Tlie association excludes the possibility that the unpaired 

 electrons are conduction electrons in a semiconductor and establishes 

 the existence of free radicals (at least Component II) in photosyn- 

 thesis. Since Calvin also postulates a formation of free radicals on 

 the basis of his phthalocyanine-quinone model, and since he does 

 not insist on a flow of electrons but rather on a charge migration, 

 there would seem to be no fundamental difference between Com- 

 moner and Calvin in the interpretation of the ESR signals. The 

 basic difference of interpretation lies in the identification of the 

 molecules that give rise to the free radicals. Calvin sees these as 

 possibly the chlorophyll and plastoquinone; Commoner avoids iden 

 tifying them for the present, but excludes chlorophyll as the source 

 of his Component I and does not regard the signals as immediately 

 related to the primary photochemical event. 



