862 LIGHT AND LIFE 



the positive holes, and reducing power, in the form of the quinone 

 double negative ions. 



That this a good model of the chloroplast system is far from sub- 

 stantiated, but a number of observations support such a view. The 

 chloroplast has an electron acceptor. Coenzyme Q2.-5 or plastoquinone, 

 which might function like o-chloranil in the model system. Unshared 

 electrons are indeed generated and disappear in the chloroplast sys- 

 tem. At room temperature as well as at — 150°C, whole spinach chloro- 

 plasts give a clear ESR signal when illuminated with red light, and 

 its generation just as rapidly at the low temperature as at a normal 

 temperature indicates that this signal is not a product of an ordinary 

 chemical reaction requiring activation energy. ESR signals can be 

 obtained from chloroplast extracts, too, but not comparable with 

 those from whole chloroplasts, either in gauss or in rapidity of decay. 

 By using a mutant Rhodopseudomoyias, which does not contain caro- 

 tenoids, and comparing it with the wild type, which does, Calvin 

 could show that the presence of the electron spin signal does not 

 dejDend upon the presence of the carotenoid pigment. The action 

 spectra for the ESR signals in the green plant chloroplasts and in 

 Rhodopseudomonas have a maximum at 7200 A, at the red-infrared 

 boundary, whereas, the absorption peak falls at 6800 A. This phe- 

 nomenon corresponds to the recent demonstration by Brody of an 

 emission j^eak in Chlorella at 7180 A. Both cases indicate the presence 

 of a previouly unknown active state — not the triplet emission, which 

 lies still farther in the infrared. It might be the lowest 11,17* state. 

 Excitation to this state, whatever it is, would give rise to the unpaired 

 electron. 



Calvin is led to postulate a relation in the chloroplast between 

 chlorophyll and plastoquinone such that, although the latter cannot 

 accept an electron from chlorophyll in the ground state it can do so 

 after the chlorophyll is raised to an excited state, by illumination at 

 7200 A. Transfer of a second electron to the quinone negative ion 

 radical would be possible if the chlorophyll were excited to a still 

 higher state (say by radiation of 6800 A); and the quinone double 

 negative ion thus produced would be sufTiciently potent to reduce 

 pyridine nucleotide. The positive chlorophyll ion would ultimately 

 obtain electrons from water, via some donor such as iron, possibly in 

 a cytochrome. Spatially, the chlorophyll layer might have the quinone 

 on one side, in a lipid layer, and the cytochrome on the other side, 

 in the aqueous phase. To quote: 



