PHOTOMECHANICAL CONSIDERATIONS 



AA 



8 minutes 



Fig. 10. Restoration curve of photobleaching of bacteriochlorophyll in meth- 

 anol, with ascorbic acid) and in chromatophores of Rps. spheroidcs. In this 

 sample about 20% of the absorption around 880 m// was bleached reversibly, 

 Usually, however, the percentage was much smaller but the time curve simi- 

 lar. 



Phenomenon 2) is not shown with any of the in vitro experiments of 

 bacteriochlorophyll. This, of course, does not mean that a reversible 

 oxidation resulting in an absorption shift of about 5 m.^ is not possible 

 in vitro. In such a case the reversible shift at 800 m.^ might represent 

 a second pigment system in photosynthetic bacteria; one might think of, 

 e.g., a system taking the place of the water- splitting system in green 

 plants (cf. eg., Goedheer, 23). However, up till now there are no indi- 

 cations whatsoever that such a second pigment system exists (enhance- 

 ment effect, luminescence "quenching," action spectra). Using a cir- 

 culating flow system, we found that the shift at 800 m^w occurs in bac- 

 teria also if irradiated at 880 m^, where the bacteriochlorophyll 

 molecules responsible for the 800 m/^ band are assumed to have no 

 absorption at all. The time curves of 800 m^ shift and 880 m/^ revers- 

 ible bleaching were found to be, in first approximation, equal and in- 

 dependent of wavelength. 



As, therefore, a close correlation between both phenomena is ap- 

 parent in vivo, whereas in vitro no reversible shift is found, we will 

 consider a possible explanation of the reversible 800 m/i shift in a 

 way different from a direct participation in some photochemical act. 

 The difference in position of absorption bands in vivo and in diluted 

 organic solvents is due to electronic interaction with neighbouring 

 molecules, e.g.. proteins, other pigment molecules or cytochromes. 

 If there is a change in electronic properties of these molecules as a 

 result of illumination, such as oxidation of a bacteriochlorophyll 



