584 



Jerome J, Wolken 



and then dehydrated. This can be considered a kind of Liesegang 

 Phenomenon (in which the fixa-tion is not continuous but forms a 

 series of precipitated concentric rings separated by clear spaces 

 in the protein). Light can modify these periodic structures if 

 the precipitated molecules are light sensitive. If a 2'/j aqueous 

 digitonin solution (the concentration for extracting the pigment 

 complex) is evaporated from a layer, periodic ring structures 

 will be formed. In addition, digitonin exhibits flow birefring- 

 ence, and has properties of a liquid crystal. The digitonin mi- 

 celles have a ver;;' strong attraction for the d;-e molecules, chl- 

 orophyll and carotenes. If clrLloroplastin is alla\red to stand, no 

 birefringence is observed; ha/ever, a liquid-crystalline phase 

 will separate out, and under certain conditions birefringent rod- 

 like fibers (tactoids) form. These experiments would indicate 

 that the pigment molecules become aligned in the digitonin mi- 

 celles. 



Tlie rapid evaporation of ivater from chlorop.l^stin w.lll produce 

 period.ic ring structures in which the chlorophyll molccioles be- 

 come oriented (Fig. 93-). V/hen these structiures are analyzed by 

 scanning the rings and interspaces with the microspectrophotome- 

 ter at 675 mn and 550 m)a near the maximun and minimum absorption 

 peaks for chlorophyll in tlie chloroplast, and the pigments are 

 found in the rings, not in tJic interspaces (Fig, 9^j). Therefore, 

 in the chloroplastin raicelles, we not only have clilorophyll, caro- 

 tenoids, c^-tociiromes, lipids, and protein in the sane relative con- 

 centration, but in similar orientation as in the chloroplast. The 

 lamellar structiore for the orientation of the pigment molecules 

 must be an efficiency mech.anism for light capture, but it may al- 

 so be a critical fiinctioning device. 



It has been suggested that the role of the detergent micelle 

 in reacting with one of the substi'ates simultaneoujsly attracting 

 the other substrate to the ss-me vicinity, parallels t:ic beliavior 

 of an enzyme in bringing the reactants together; the porph:vTln- 

 micelle being analogous to an enzyme-substrate comx^lex'^'^''''', CM-- 

 oroplastin then Its.s featices in comaon with an enzyme -catal^'zed 

 system. 



Finally, it is necessary to develop raicroo-nal^'tical methods to 

 study the in vivo ctLloroxJlast chemistry and molecular structure, 

 Clxl.oroplastin micelles offer for the present an excellent exper- 

 imental model system to further pursue the mechanisms of the 

 photoreceptor chemistry to its molecular structure. 



