408 PHOTOMECHANICAL CONSIDERATIONS 



absorbed light is transferred to the absorption band of longest wave- 

 length (Duysens, 15), The absorption band of longest wavelength thus 

 is expected to be reversibly oxidised and reduced upon illumination. 

 In the chromatophores, the energy transfer can be reversibly dis- 

 turbed, without a marked altering of the absorption spectrum, by 

 affecting the carrier complex configuration with detergents (Bril, 16). 

 As mentioned before, reversible chemical bleaching could be measured 

 with bacteriochlorophyll in chromatophores as well as in organic 

 solvents. Reversibility, however, was only prominent for the long 

 wavelength band or shoulder (around 880 m/i) with chromatophores of 

 Rho do spirillum rubriim, Rhodops eudo monas spheroides and Chroma- 

 tium. 



Duysens (15) measured reversible changes in absorption of the long 

 wavelength bacteriochlorophyll bands in photosynthetic bacteria, which 

 he suggested to be due to photooxidation. The changes were only meas- 

 urable when the cells were aerobic and devoid of substrate. In the 

 presence of substrate the changes occurred only at high light inten- 

 sities {Duysens, 17). Later it was observed {Duysens, 18) that these 

 spectral changes occurred only when bacterial cytochrome c was in its 

 oxidised state, a situation which is schematically indicated in Fig. 8: 4. 



Difference spectra similar to those obtained upon illumination can 

 be obtained by addition of ferri-ferrocyanide mixtures of about 0.5 V 

 to bacterial chromatophores {Duysens, 19; Goedheer, 20), With stronger 

 oxidising mixtures the reversible bleaching of the long wavelength 

 band exceeds the in vivo one (cf. Fig. 4). 



The difference spectra obtained upon addition of ferri-ferrocyanide 

 mixtures or illumination appear to consist of two different parts: 1) a 

 reversible bleaching around 880 m/i, similar to the reversible bleach- 

 ing in organic solvents, and 2) a reversible shift of the small absorp- 

 tion band at 800 m/y. 



The reversible absorption difference spectra in the far red region 

 are analogous for Rhodo spirillum rubrum, Chromatium and Rhodo- 

 pseudomonas spheroides , although the absolute absorption spectra are 

 markedly different. 



Phenomenon 1) listed above is explained as a light- induced revers- 

 ible oxidation of bacteriochlorophyll absorbing around 880 miu. Re- 

 versible bleaching in intact bacteria around 880 m// is saturated when 

 only a few percent of the pigment molecules are in the oxidised state. 

 From fluorescence measurements Vredenberg and Duysens (21) con- 

 cluded that the major fraction of bacteriochlorophyll molecules at 

 880 m^ in R. rubrum transfer their absorbed light energy to a small 

 fraction called P890. From here the energy is assumed to enter the 

 photosynthetic system, Clayton (22) comes to a similar conclusion. He 

 states that this fraction (called BChl2) comprises 2-5 per cent of the 

 total BChl. 



