1614 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



inhibition was produced by 5% carbitol, 10% dioxane or 0.2 M CaCl2; 

 50% inhibition was produced in strong light by 0.18 M formaldehyde, 0.4 

 M sodium sulfate, 0.7 M acetone, 1AM ethanol, etc. No stimulation by 

 small quantities of formaldehyde (as reported by Bose for photosynthesis, 

 cf. Vol. 1, page 343) was found at concentration down to 10"^*^%. 



The suspensions used in Macdowall's experiments contained between 

 2 X 10~^ and 4 X 10~^ mole/liter chlorophyll. In checks with o-phenan- 

 throline (2 X 10"^ mole/liter) and thymol (2.5 X 10"^ mole/liter) the 

 inhibition proved to be approximately independent of chlorophyll concen- 

 tration in these limits. 



Arnon et al. (1954) reported that with whole chloroplasts capable of 

 (1) Hill reaction with quinone, (^) P* fixation as ATP* in light, and (3) 

 C*02 fixation in Hght: 2,4-dinitrophenol (8 X 10-^ M) inhibits (5) and (2) 

 much stronger than (1); iodoacetamide (2 X 10"^ M) inhibits (3) by 97% 

 and has no effect on (2) ; and p-chloromercuribenzoate inhibits (3) strongly, 

 (£) less strongly, and (1) hardly at all. 



Ultraviolet Light. Irradiation by ultraviolet light (253.6 mix) was found 

 by Holt, Brooks and Arnold (1951) to inhibit the Hill reaction in chloro- 

 plast fragments in the same way, as it does the Hill reaction of whole cells 

 (cf. part C below) and photosynthesis (page 344). In this case, too, the 

 logarithm of the residual rate is a linear function of the duration of irradia- 

 tion, indicating a "first order" deactivation process (i. e., deactivation by 

 single quanta of ultraviolet light, and not by combined effect of two or 

 more quanta on a single molecule). The fact that the Hill reaction — 

 in chloroplasts or whole cells — is inhibited by ultraviolet light to about the 

 same extent as photosynthesis is an argument against the hypothesis, made 

 in chapter 13 (on the basis of observations of inhibition of C* uptake in 

 the dark by ultraviolet light), that the 253.6 m.ix sensitive factor in photo- 

 synthesis is the carbon dioxide fixing enzyme. 



Activity was tested with the complete Hill's mixture and with ferricyan- 

 ide alone, and the effect proved to be the same in both cases. This proves 

 that photochemical decomposition of oxalate — which is known to occur in 

 ultraviolet light — is not the cause of the inhibition of the reaction with 

 Hill's mixture. 



Similarly to whole Chlorella cells, the irradiation of chloroplast prepara- 

 tions with the line 253.6 mn does not cause any significant changes in the 

 absorption spectrum, even if it leads to complete deactivation. This has 

 been checked spectrophotometrically for the region 220-400 m/^; no 

 change of color was noticeable after irradiation. 



The proportion of the photochemical activity which "survives" a cer- 

 tain irradiation dose is the same whether the activity is tested in strong or in 

 weak light. Ultraviolet light thus belongs to the group of agents (in- 



