11 



L.N.M. Duysens 



The primary photooxidanta of system _1 : P 89O and P 700 



The first observed reversible change in absorbancy upon il- 

 lumination of photo synthesizing organisms was a decrease in ab- 

 sorption close to the baoteriochlorophyll peak at around 89O m^ 

 in purple bacteria^-^* ■^', In various species studied, this de- 

 crease was accompanied by an decrease at about 810 m^i and an 

 increase of absorption at about 790 m^i. These absorption chan- 

 ges were attributed to the oxidation of a small(about 27^) spe- 

 cial fraction of the bacteriochlorophyll, which fraction we 

 now call PH 89O. Since the maximum of the differegge spectrum 

 of PH 890 varies iHpdifferent purple bacteria ^-^' and under 

 various conditions^ ^in a similar way as the maximum of the 

 absorption spectrum of the bacteriochlorophyll-type B 89O (see 

 the next paragraph), PH 89O is probably closely related to bac- 

 teriochlorophyll, but not necessarily chemically identical. The 

 shape of the difference spectrum indicates that PH 89O consists 

 of at least two bacteriochlorophyll-like molecules, one with 

 a maximum at about 8IO, the other with a maximum between 87O 

 and 890 mn, depending upon the species. Upon photoconversion 

 of PH 890 to P 890 the maxima shift from about 810 and 89O m^ 

 to about-7^0 and I25O m|i. The last maximum was observed by 

 Clayton wO). Similar changes in the infrared region can be 

 brought about by adding mixtures of potassium ^errj-r and fer^i- 

 cyanide. The E'n value of P was found to be 0.^1^ ', ^'^^^K^N 

 and about O.44 (Duysens, unpublished observation). Clayton^ 

 provided direct evidence indicating that the absorption peeik 

 at 890 mil was. almost completely bleached upon oxidation. Arnold 

 and Clayton made the important observation that the lig^* 

 induced changes in infra-red absorption occurred even at 1 K 

 in dried extracts of bacteria. 



In purple bacteria, the bulk of the pigments consists of ca- 

 rotenoids and three bacteriochlorophyll types, called B 800, 

 B 850 and B 890 after the approximate location of the absorp- 

 tion maxima, which vary somewhat from species to species. Only 

 B 890 shows fluorescence. Light energy absorbed by the carote- 

 noids and the other bacteriochlorophyll types besomea only ac- 

 tive in photosynthesis through transfer to B 890^^*-^. The 

 fluorescence yield increases upon increasing the intensity of 

 the actinic light. The increase^occurs at lower intensities in 

 the absence of hydrogen donors^ -^. A quantitative correlation 

 was found under various conditions between the increase i^'i^) 

 fluorescence and the decrease in absorption aroxaid 89O mix • 

 These observations can be quantitatively explained by the hy- 

 pothesis that excitation energy is transferred from B 89O to 



