350 



Messrs. I. Jorgensen and F. Kidd. 



(a) Nitrogen. 



On exposure of chlorophyll sol to light in presence of nitrogen no apparent 

 change takes place to the chromogen complex of the chlorophyll molecule, 

 even after several hundred hours' exposure to the light of a mercury vapour 

 lamp, and no formaldehyde is produced. 



(b) Carbon Dioxide. 



Colloidal solutions of chlorophyll of different concentrations were exposed 

 to light in presence of carbon dioxide. In all cases there was a rapid 

 change of the pigment to yellow-green or brownish-green, the colour change 

 being preceded in the sols of highest concentrations by precipitation of the 

 colloid. The same change takes place if the tubes are kept in the dark, but 

 the action is considerably slower. The colour is not further changed by 

 prolonged exposure to light, nor by any further keeping in the dark. 



It may be recalled here that the action of acids on chlorophyll is to 

 produce the magnesium-free derivative phaeophytin, which possesses this 

 same yellow-green or brownish-green colour. As carbon dioxide in water 

 forms a weak acid, it at once suggests that the action of carbon dioxide on a 

 colloidal solution of chlorophyll is simply that of an acid. That this is 

 indeed the explanation of the change observed in the chromogen complex of 

 chlorophyll becomes evident from the following experiments : — 



1. An exactly similar colour change is produced when the chlorophyll soi- 

 ls treated with other weak acids, such as boric acid or acetic acid. If, for 

 instance, some chlorophyll sol is sealed off in an atmosphere of nitrogen with 

 boric acid (for instance, 5 c.c. of sol + 1 e.c. N. boric acid) and exposed to' 

 light, the colour change is exactly similar to that in the tube containing the 

 chlorophyll sol and carbon dioxide, and takes place in about the same time. 



2. If the sol is made up in water from a solution of chlorophyll in 

 pyridine, no change in colour takes place in an atmosphere of carbon dioxide. 

 The sol contains a little pyridine, which is a strong base, so the acid is- 

 neutralised and the change to phseophytin prevented. 



If a sol made up from an alcohol solution is kept in an ordinary loosely 

 stoppered bottle in the dark it will, in the course of a few months, gradually 

 turn yellow-green. A sol made up from a pyridine solution will under the 

 same conditions remain quite unchanged in colour. 



3. It is possible to maintain approximate neutrality in experiments with 

 carbon dioxide by using sodium bicarbonate in the solution. A solution 

 containing H 2 C0 3 and NaHC0 3 in the proportions of 1 to 375 is approxi- 

 mately neutral, but to avoid precipitation of the colloid the concentration 

 of the bicarbonate must be kept low, consequently full atmospheric pressure- 



