106 MACROMOLECULAR COMPLEXES 



these periodic precipitations if the precipitated molecules are light- 

 sensitive. Many experiments on the Liesegang ring formation have 

 been performed; data on the concentration of reactants, tempera- 

 ture, interspace distances between ring formations, and the time of 

 the reactions have been collected. How much the experimental data 

 can tell us about the chlorophvll-protein specificity or the orienta- 

 tion of chlorophyll in the chloroplast is at present questionable, but 

 it has led us to some interesting considerations on periodic crystal- 

 lization in colloids and proteins. 



Periodic Crystallization. Molecules in solution take up con- 

 figurations of lowest energy. This leads to crystallization when the 

 number of molecules in the solution exceeds a certain minimum 

 value characteristic of those particular molecules. Such molecular 

 interaction described in terms of crystallization can perhaps be 

 extended to the kinds of molecules that are present in lamellar sys- 

 tems of the chloroplast. An example of such periodic crystallization 

 is that of potassium dichromate in gelatin. The procedure is to place 

 a drop of saturated potassium dichromate in gelatin solution on a 

 microscope slide, warm gently, and quickly transfer the slide to 

 the microscope. Crystallization begins around the periphery of the 

 drop and proceeds in a periodic manner, bv alternating periods of 

 rapid and slow growth during which a few relatively large geo- 

 metrical crystals grow. The distance between the rings decreases 

 with the thickness of the film and with increasing rate of crystalliza- 

 tion. (Fig. 8c). The crystallization of sodium chloride is also 

 affected by extremely small concentrations of most colloids (e.g., 

 serum ) ; as the ions are absorbed on the surface, the sodium chloride 

 molecules are deposited in fine crystals on the glass surface, and 

 periodic rings are formed at certain salt concentrations (Du Noiiy, 

 1926). Digitonin, when it evaporates as a drop on a glass surface, 

 forms periodic rings; however, when a digitonin solution of chloro- 

 plastin evaporates as a drop, similar rings develop, but now the 

 chlorophyll concentrates in these rings ( Fig. 8a and b ) . The chloro- 

 phyll molecules are carried with the digitonin and become oriented 

 within these rings. When the rings were scanned from the center 

 out with the microspectrophotometer at 675 mix, the major absorp- 

 tion peak for chloroplastin, and at 550 m/x, the minimum absorption 

 peak, it was found that the maximum chlorophyll absorption occurs 

 in the rings, and although the concentration of chlorophyll decreases 

 as it moves out, its concentration remains relatively constant within 



