380 PHOTOMECHANICAL CONSIDERATIONS 



P700, Apparently the P700 is first oxidized and then receives electrons 

 from a cytochrome, probably Cyt/. 



The electron acceptor in the oxidation of P700 might be PQ. Klingen- 

 berg et al, (21) have described a light-induced bleaching in chloroplasts 

 that is centered at 255 mfi (corresponding to the loss of oxidized PQ) 

 and that has roughly the same kinetics as the oxidation of P700. How- 

 ever, the identification of PQ as a primary electron acceptor leads to 

 a difficulty concerning its oxidation potential. The potential of PQ in 

 vitro is about zero mv, but electrons from System I are able to reduce 

 such strong reductants as PPNR and NADP, One way to deal with this 

 problem is to assume that PQ operates in vivo at a potential of about 

 -400 mv (23). 



With reservations as to the role of PQ as primary electron acceptor, 

 the foregoing evidence suggests the model shown in Fig, 1, for System 

 I (the far red photochemical system) in chloroplasts. 



LIGHT- INDUCED ABSORPTION SPECTRUM CHANGES 

 IN PHOTOS YNTHE TIC BACTERIA 



Cells and chromatophores^ of photosynthetic bacteria exhibit a 

 variety of light- induced absorption spectrum changes (24-32). Many of 

 these changes suggest reactions of known molecular types, as indicated 

 in Table 1.^ For convenience the various light- induced absorbancy 

 changes will be referred to as "the P870 reaction," "the Cyt reaction," 

 etc., even though these designations may be presumptive (as with the 

 UQ reaction) or incomplete (e.g., where "Cyt reaction" refers to the 

 oxidation of more than one type of cytochrome). 



P870-890 is a pigment having an absorption band coincident with 

 the long wave absorption maximum of BChl in vivo (27,33). This maxi- 

 mum is at 870 m^ in Rhodopseudouiouas sphcroides and at about 890 

 ran in Rhodospirilliim riibrum andC/;ro;;?a^»n;?. As we shall see, P870- 

 890 is probably BChl in a specialized environment. Its reversible 

 photobleaching is a result of its oxidation (27); titration with ferri- 

 cyanide establishes this pigment to be a single-electron donor of po- 

 tential about 500 mv (25). The restoration of P870-890 after its light- 

 induced bleaching is accelerated by a reducing environment (R. K. 

 Clayton, unpublished). These properties indicate that P870-890 is the 

 bacterial counterpart of P700. 



3 The term "chromatophores " is used to denote a chromatophore fraction ob- 

 tained by sonic disruption of cells followed by centrifugation for 90 min at 

 100,000 X g. 



4 Difference spectra for these light reactions can be found in references 27, 29, 

 and 30. 



