18 PHOTOCHEMICAL PRINCIPLES 



was added to a large volume of water, which does not dissolve chlo- 

 rophyll. The chlorophyll precipitated as small colloidal particles that 

 gave the curve shown below that of the alcohol extract. This curve 

 was measured 1 min after the alcoholic solution of chlorophyll had 

 been added to water. The flattening effect can be seen by comparing 

 the curve of the colloidal particles with that for the solution. The 

 bottom curve, measured 25 min later, shows the further aggregation 

 of the chlorophyll into larger colloidal particles. Such solutions when 

 first prepared are clear for a few minutes, then become cloudy as the 

 aggregates grow. Another part of the flattening effect in the solid 

 state comes from the fact that molecules tangle with each other, thus 

 cramping each other's freedom to vibrate at their own characteristic 

 frequencies. Thus, internal interaction between molecules, as well as 

 the purely optical shading of one particle by another, may result in a 

 broadening of the absorption spectrum. 



An interesting effect shows up in the blue part of the spectrum of 

 the two colloidal chlorophyll solutions. The 436-m^ peak of the 

 freshly precipitated material is at first higher than the 415-mM peak. 

 However, after 25 min the 415-m/x peak has risen while that at 436 

 mfi has lowered. The exact explanation for this effect is not obvious, 

 but there is evidently a slowly occurring combination of chlorophyll 

 molecules with each other which differently influences their relative 

 freedom to vibrate at the two frequencies. Similar shifts of peak posi- 

 tion and broadening of chlorophyll bands of chlorophyll adsorbed on 

 filter paper have been studied by Smith, Shibata, and Hart (1957). 

 Here again the 415-m/A peak is much more prominent than that at 

 436 mix. 



So far we have talked about chlorophyll in the amorphous, non- 

 crystalline state. The work of Jacobs, Vatter, and Holt (1954) has 

 shown the appearance of another absorption band at about 740 m/x 

 characteristic of chlorophyll crystals. The height of this band increases 

 with the size of the crystal. There is much interest in the possibility 

 of such microcrystalline chlorophyll units occurring in living cells, 

 but absorption of light in this region has not yet been detected by 

 chlorophyll in vivo. 



We have seen the spectral changes that occur when a single pure 



