650 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



A colloidal solution prepared by Meyer by rapid addition of 3 volumes 

 of water to 1 volume of a chlorophyll solution in ethanol was clear, trans- 

 parent and nonfluorescent. Its particles were 0.5 to 3 /i in diameter. 

 The band maximum was at 670 m/x, and the shape of the extinction curve 

 was similar to that of the original solution in ethanol. On the other hand, 

 a colloidal solution prepared by adding quickly 0.6 volume of water to 1 

 volume of ethanol and then diluting by 6.4 volumes of water, was turbid 

 and opalescent. Its particles had a diameter of 1-3 n, i. e., were not sub- 

 stantially larger than those of the first, transparent colloid, but they con- 

 tained more pigment. The absorption maximum of this colloid was sit- 

 uated further toward the red, at 673 m^, and the whole shape of the extinc- 

 tion curve was more like that of the leaves, as shown by figure 21.27 

 (Meyer described the spectrum of this colloidal preparation as "identical" 

 with that of the leaves, but figure 21.27 does not justify this statement). 

 By counting the particles of the colloid, Meyer found that the concentra- 



48 



400 



450 



650 



700 



Fig. 21.27. 



500 550 600 



WAVE LENGTH, m/i 



Transmission curves of leaves (1 and 2) and of colloidal chlorophyll 

 solutions (3) (after K. P. Meyer, 1939). 



tion of chlorophyll in the particles of the turbid solution was of the order of 

 0.13 mole/1., i. e., similar to that in the chlorophyll grana in the leaves (cf. 

 Vol. I, chapter 15, page 411). Even these ''densely packed" colloid par- 

 ticles are still far from "sohd," but contain up to 90% solvent. 



Because of the intensity of the absorption bands, the extinction curves 

 of dyestuffs usually are measured with concentrations of the order of 10"^ 

 to 10~'* mole/I. No deviations from Beer's law (i. e., no changes in the ex- 

 tinction curve with concentration) were observed in chlorophyll solutions 

 in this range of concentrations. With very thin glass cells (~0.1 mm. 

 deep), dyestuff solutions containing 10 ~^ mole/1, can be investigated, but 

 even this is a hundred times more dilute than the 0.1 mole/1, reached in 

 Meyer's colloidal particles and also present in the chlorophyll grana in 



