338 PHOTOMECHANICAL CONSIDERATIONS 



CHROMATOPHORES FROM Rhodospirillum rubra m 



(-0.4 V) EXCESS No2S20^ Em (Fe^*/Fe^* Cyanide) -0.44 volts ' 

 pH - 73 



1.0 



0.8 \r 

 0.6 

 0.4 

 0.21- 



J I L 



(-0.4) -0.2 +0.2 +04 



REDOX POTENTIAL (volts) 



Fig. 3. Redox titration of the chemically induced and photoinduced EPR signal 

 in chromatophores from RhodospiriUuni ruhrnm (-O2; pH = 7.3). Potential 

 calculated: Ejn ( 0.02 mole of Fe3+/Fe2+ cyanide) = +0.44 volts. 



signal is made, a direct test of any model by EPR experiment is not 

 possible. 



Three additional parameters, which depend on physical variables 

 in the system, may be observed. These are the dependence of the EPR 

 signal on the wavelength of irradiating light, the dependence on tem- 

 perature, and the kinetics of signal production by light. 



SPECTRAL RESPONSE 



The magnitude of the light-induced EPR signal in/?, nib ru 772 h3.s 

 been observed to be dependent on the wavelength of light used to ir- 

 radiate the sample (action spectra), BChl in R. mbna)! has an optical 

 absorption maximum at x = 8800 A, The maximum EPR signal also 

 occurs at this wavelength (11), indicating that BChl is an important 

 participant in the chain of events leading to the production of a light- 

 induced EPR signal. 



The photoproduction of spins by distinct absorption bands should 

 show up as structural features in the action spectra. For example, it 



