382 PHOTOMECHANICAL CONSIDERATIONS 



Also, the intensities of the light-induced 435, 720, and 1250 m/i bands 

 are in the same proportion as the heights of the BChl bands at 375, 

 590, and 870-890 m/i (27), No explanation is offered for the blue- shifts 

 at 375, 590, and 800 mfi. 



The carotenoid reaction cannot be essential for photosynthesis, be- 

 cause carotenoidless forms of purple bacteria exhibit normal photo- 

 synthetic metabolism and growth. The light-induced bleaching and red- 

 shift of carotenoids might result from interactions with BChl in an 

 excited triplet state, or from the diffusion of electrons or holes (elec- 

 tron vacancies) to the neighborhood of carotenoid molecules (29). The 

 carotenoid reaction interferes with the observation of other light re- 

 actions; it can be eliminated selectively through the use of carotenoid- 

 less phenotypes or, in chromatophores, by treatment with deoxycho- 

 late (29). 



The UQ reaction is identified as such on the basis of light-induced 

 absorbancy changes in the region 240-340 m/i (30). These changes have 

 the same form as the difference spectrum (reduced minus oxidized) of 

 UQ. In chromatophores these absorbancy changes can be produced 

 chemically by reductants, and not by oxidants. 



A number of cytochromes are oxidized reversibly by light in cells 

 of photosynthetic bacteria; perhaps four inChromatium (32). In washed 

 chromatophores the reaction of a single cytochrome appears to pre- 

 dominate in each species, and it becomes permissible to speak of 

 "the Cyt reaction." The possibility of observing a Cyt reaction in 

 chromatophores depends on the speed with which the Cyt is reduced 

 in darkness, following its oxidation by light (29). If the reduction is 

 extremely slow (as in dried chromatophores), the light-induced oxida- 

 tion will be observed only in thoroughly dark- adapted preparations. 

 The rate of reduction is influenced by the manner in which the chroma- 

 tophores have been prepared, by the oxidation potential of their en- 

 vironment, and by prior illumination (light inactivates endogenous re- 

 ducing systems reversibly). These factors are useful because they 

 permit study of the P870-890 and UQ reactions with and without a 

 concomitant Cyt reaction. 



The maximum light reactions of Cyt, UQ, and P870 or P890, in 

 washed chromatophores, correspond to the presence of one light- 

 reacting molecule of each kind for every 30 to 50 BChl molecules 

 (27-30,34). These figures are in satisfying agreement with the size of 

 the bacterial photosynthetic unit as determined by flashing light ex- 

 periments (see earlier). 



The quantum requirement for the UQ reaction has not been meas- 

 ured. For the reactions of P870-890 and Cyt the requirement is less 

 than three quanta (absorbed by BChl) per electron-transfer (28,33,35, 

 36). 



