366 PHOTOMECHANICAL CONSIDERATIONS 



trated by Fig, 4, the quantum requirement of approximately two per 

 electron can be converted very nearly to one per electron. It is pos- 

 sibly premature to make this calculation at the present time since the 

 quantum requirement for the particular suspension of cells employed 

 in this experiment was not measured and further experiments along 

 these lines are needed. Nevertheless, a possible approach of the quan- 

 tum requirement to one per electron is of considerable practical and 

 theoretical interest. 



SUMMARY 



1. In 67 per cent glycerol, the rate of light-induced oxidation of 

 cytochrome of type c is accelerated while the rate of reduction on 

 interrupting illumination is decelerated. 



2. These results are interpreted in terms of increased intracellu- 

 lar viscosity and suggest a viscosity-independent cytochrome- 

 chlorophyll electron transfer in Chromatiuyn and R. nibrum and a 

 viscosity-dependent electron transfer to cytochrome of type c from 

 electron donors of the cyclic or noncyclic system. 



3. These results are consistent with the temperature independence 

 of the light-on reaction and the temperature dependence of the light- 

 off reaction in Chromatiiim. 



4. The effects of both temperature and viscosity support the con- 

 clusion that electron transfer between cytochrome of type c and 

 bacterial chlorophyll is an electron transfer reaction not requiring 

 mobility of the cytochrome— whereas electron donation to cytochrome 

 does require a collision mechanism. 



5. The increased velocity of cytochrome oxidation in the presence 

 of glycerol suggests a re- examination of quantum requirement and a 

 possible revision of currently accepted values from two quanta per 

 electron to approximately one quantum per electron. 



REFERENCES 



Chance, B., Spectra and reaction kineticsof respiratory pigments of homo- 

 genized and intact cells. Nature, 169, 215 (1952). 



Duysens, L. N. M., Reversible photooxidation of a cytochrome pigment in 

 phoiosynthesi7Ang Rhodospirillum rubrum. Nature, 173, 692 (1954). 

 Chance, B., and Smith, L., Respiratory pigments of Rhodospirillum rubrum. 

 Nature, 175, 803 (1955). 



Olson, J. M., A spectrophotometric study of Chromatium. Dissertation, 

 University of Pennsylvania, 1956. 



Chance, B., and Nishimura, M., On the mechanism of chlorophyll-cytochrome 

 interaction: The temperature insensitivity of light-induced cytochrome oxi- 

 dation in Chromatium. Proc. Natl. Acad. Sci. U. S., 46, 19 (1960). 



