PHOTOCHEMISTRY OF BACTERIOCHLOROPHYLL 411 



molecule, the band position of another closely adjacent pigment mole- 

 cule may be influenced. In this way a reversible shift of the small 

 800 m^ band in R. rubrum may be directly coupled with the oxidation 

 of the reactive part of the long wavelength band (P890). As both the 

 small 800 m/i band and the reactive part of the long wavelength band 

 cover a few per cent of the total number of bacteriochlorophyll mole- 

 cules, the hypothesis seems plausible that one 800 m^ bacteriochloro- 

 phyll molecule is intimately connected with one P890 molecule. As the 

 light- induced absorption difference spectra in the near infrared are 

 similar for various types of purple bacteria, notwithstanding their dif- 

 ferent absolute absorption spectra, such a combination might be es- 

 sential for bacterial photosynthesis. It seems tempting to see bac- 

 teriochlorophyll 800 m/i (in R. rubrum) as an "accessory pigment" to 

 P890, similar e.g. to chlorophyll b or phycocyanin. In our scheme the 

 function of such "accessory" pigments then might be seen as an aid in 

 effective separation of electrons leaving and electrons entering bacter- 

 iochlorophyll and chlorophyll a. To function as such, the accessory pig- 

 ment should be located close to the main pigment molecule, and there- 

 fore show an effective energy transfer, while the redox potential of the 

 accessory pigment should be more negative than that of the main 

 pigment. For the mentioned bacteriochlorophyll 800 m/i type and for 

 chlorophyll b these conditions seem to be fulfilled. 



It does not seem impossible that more reversible absorption 

 changes (e.g., the reversible shift in position of carotenoid absorption 

 bands, cf, Clayton, 24), might be due to change in electronic inter- 

 action. 



The foregoing may demonstrate that it is possible to devise a 

 mechanism based upon the photochemical properties of the individual 

 pigment molecules. Such a mechanism enables a continuous conver- 

 sion of radiation energy into electronic energy, without introducing 

 the assumption of solid state phenomena. 



REFERENCES 



1. Goedheer, J. C, Investigations on bacteriochlorophyll in organic solutions. 

 Biochim. Biophys. Acta, 27, 478 (1958). 



2. Rabinowitch, E., and Weiss, J., Reversible oxidation of chlorophyll. Proc. 

 Roy. Soc, Ser. A, 162, 2511 (1937). 



3. Goedheer, J. C., Horreus de Haas, G. H., and SchuUer, P., Oxidation- 

 reduction potentials of different chlorophylls in methanol. Biochim . Biophys . 

 Acta, 2S, 278 (1958). 



4. Komen, J, G., Observations on the infrared absorption spectrum of bac- 

 teriochlorophyll. Biochim. Biophys. Acta, 22, 9 (1956). 



5. Goedheer, J. C., Spectral and redox properties of bacteriochlorophyll in 

 its natural state. Biochim. Biophys. Acta, 38, 389 (1960). 



