1552 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



Milner, Koenig and Lawrence (1950) investigated the effect of added 

 salts on colloidal chloroplast dispersions ; they found that in the presence of 

 methanol, addition of certain salts markedly increased the photochemical ef- 

 ficiency of the material, but that no such activation was noticeable in the 

 absence of methanol. Figure 35.12 shows that in water, quantities of KCl 

 up to 0.2 mole/liter had no effect. (If [KCl] was >0.2 mole/liter, de- 

 activation resulted; precipitation followed at about 1 mole/liter.) When 







80 



20 40 60 



7o INCREASE IN ACTIVITY WITH 0.05 M SALT 

 Fig. 35.13. Activation of chloroplast suspensions by different salts (after Milner, 



Koenig and Lawrence 1950). 



25% methanol was present, addition of 0.1 M KCl doubled the photo- 

 chemical efficiency. However, it must be taken into account that the 

 chloroplast dispersion used had been only \i as active as the suspension 

 from which it was prepared ; thus, KCl merely restored a part of activity 

 lost by dispersion. Reactivation was accompanied by turbidity; after- 

 wards, the active material could be separated by mild centrifugation. In 

 other words, activation was associated with visible coagulation of par- 

 ticles. The activation effect was strongest in the most concentrated dis- 

 persions. It was not permanent : after 24 hrs. the activity was back to the 

 level it had before the addition of KCl. The loss of activation was even 

 faster when NH4CI had been used instead of KCl. Phosphate buffer 

 (0.01 M) produced a passing activation similar to that caused by other 

 salts. 



