47 



Bessel Kok 



be related to it. 



II. Correlation between P700, C7OO and the low temperature 

 emission bands. 



With leaves at yy^K Butler (.k) observed a band at 705 mu 

 in the absorption spectrum as well as in the excitation spec- 

 trum of fluorescence. He assumed (a) that this pigment "C705" 

 was the emitter of the strong 730 mu fluorescence band found 

 earlier by Brody (l) at 77''K and (b) that 0705 was identical 

 to the long wave chlorophyll "P7OO" functioning as the trap- 

 ping center in the long wave photoact (system I) of photosyn- 

 thesis. 



Identity of P7OO and C705 appeared unlikely because the 

 data indicated a concentration of C705 as high as 2-5% of 

 total chlorophyll, whereas we never observed P700 in a concen- 

 tration higher than 1 per 3OO or 400 chlorophylls (5). An 

 identification of P7OO with F73O did not appear likely either: 

 The photochemical bleaching of P700 is irreversible at 77°K 

 and a fluorescence emission would have to come from its oxi- 

 dized form. The possibility is not excluded, however, that 

 P700 ox still does absorb, viz . , at 69O mu (6). The follow- 

 ing experiments bear on these questions: 



Expt. J^ig. 2 shows emission spectra (77°K) of chloroplasts 

 briefly treated v/ith increasing concentrations of acetone in 

 water (7). The data show that a low concentration of acetone 

 (20%) decreases the 730 mu fluorescence relatively less than 

 the short wave bands. Higher concentrations produce band 



CHLOROPLASTS TREATED WITH INDICATED PERCENTAGES ACETONE TT-K 



700 720 



WAVELENGTH mfi 



Figure 2 

 shifts and additional emissions in the long wave region, and 

 also lower the yield severely. (The numbers in brackets indi- 

 cate the factors applied to match the curves.) 75% Acetone 



