346 PHOTOSYNTHESIS 



b. The Physical Properties of the Leaf Pigments. 



Chlorophyll a and b precipitated from ether by means of petroleum 

 ether are microcrystalline. On slow evaporation of solution in an ether- 

 petroleum ether mixture the a component crystallizes in bundles of nar- 

 row, thin plates. Dry chlorophyll a, as usually obtained, is a blue-black, 

 soft powder, chlorophyll /' is a dark green, almost black powder. 



Chlorophyll a, according to Willstatter and StoU, softens and runs to 

 a tough mass when heated to 117-120^; the b component softens be- 

 tween 86-92", becomes molten at 120-130" and swells. 



Mention has already been made of the difiference between the solubil- 

 ity and the ease of extraction of the pigments from the leaf. Chloro- 

 phyll a is very soluble in absolute alcohol, ether, acetone, chloroform, 

 carbon bisulfide and benzol. In methyl alcohol, in the cold, it is not very 

 soluble, though easily when the reagent is warm. In 95 per cent ethyl 

 alcohol it dissolves readily, in 80 per cent with difficulty ; in 90 per 

 cent methyl alcohol, even when warm, it is only slightly soluble ; in 80 

 per cent methyl alcohol it is almost insoluble. In petroleum ether it is 

 only slightly soluble, in ligroin it is more soluble, easily when warm. A 

 very small amount of alcohols in the petroleum ether greatly increases 

 the solubility in the latter solvent. 



The solubility of chlorophyll b is in general somewhat less than that 

 of the a component ; this is especially noticeable in petroleum ether in 

 which chlorophyll b is insoluble in the cold and the solution becomes but 

 only slightly colored on boiling. Chlorophyll b is easily soluble in abso- 

 lute alcohol, ether and the reagents which dissolve the a component. 



Chlorophyll a, in ethyl alcohol solution, is blue-green, with a deep red 

 fluorescence. In concentrated ether solution the color is almost blue 

 which takes on a green hue on dilution. A colloidal solution of chloro- 

 phyll a, which can be kept for weeks, is produced by diluting rapidly 

 a concentrated solution in alcohol or acetone with much water. This 

 solution is green and does not fluoresce, but has a blue-green opalescence. 

 This solution can be extracted with ether only by prolonged shaking 

 with the solvent; however, on addition of a little calcium chloride it goes 

 into the ether immediately. 



The color of solutions of chlorophyll b does not vary as that of the a 

 component in different solvents. The ether solution is bright green. 

 In comparison to solutions of chlorophyll a, those of chlorophyll b have 

 a yellow hue ; in carbon bisulfide the latter are yellow-green. The fluores- 

 cence of chlorophyll b solutions is brown-red. 



The absorption spectra of solutions of chlorophyll have been ex- 

 amined very extensively, and these have been used to some extent to 

 determine the purity of the chlorophyll preparations. Tswett '' was able 

 to isolate individual pigments from his chromatograms and study the 



"Tswett, Ber. hot. Ges., 25, 388 (1907). Englemann, Zcit. zmss. Mikrosk., 5, 

 289 (1888). Brdlik, Conipt. rend., 147, 990 (1908). 



