SPECTRA OF CRYSTALLINE AND COLLOIDAL PIGMENTS 1821 



in table 37C.III, the absolute optical density of the chlorophyll monolayers 

 of this type (in the peak of the red band), is smaller than that of the chloro- 

 phyllide monolayers, in the ratio of 11 : 19 = 0.58 — which is not much less 

 than the ratio of molecular surface densities (70: 106 = 0.6G). 



The optical density of the chlorophyll monolayers of crystalline type 

 is about 2.4 times that of colloidal ones, and 1.4 times that of ethyl chloro- 

 phyllide monolayers. It seems probable (but needs experimental con- 

 firmation) that its molecular surface density is correspondingly higher. 

 The "red shifts" of the absorption band — which may be considered as in- 

 dices of the closeness of the chromophore packing — follow the same trend 

 (table 37C.II). That it should be possible to rearrange a monolayer of the 

 type shown in fig. 37B.9, into a 30% denser pattern would be startling 

 enough even if we were dealing with chlorophyllide itself; it is even less 

 likely if we are, at the same time, substituting, for chlorophyllide, chloro- 

 phyll, whose phytol "tail" must interfere with close packing. 



One could suggest that an increase in optical density could be achieved 

 without a proportional increase in molecular density. For this, the vibra- 

 tion planes of the optical electrons should be re-oriented so as to enhance 

 the absorption of light passing through the monolayer normal to its plane. 

 However, in Hanson's model of the monolayer, the short axis of the por- 

 phin ring is oriented parallel to the water surface; and, according to Piatt 

 (table 37C.I), this is the plane of vibration of the main red absorption band. 

 If these assignments are correct, one does not see how any re-orientation of 

 molecules could enhance the optical density of the monolayer in the red 

 band. 



A more plausible interpretation of the high density surface layer of 

 chlorophyll is that it is a two-molecular layer. If this is so, the shift of the 

 red absorption band to 735 m/x may be due either to an increased inter- 

 action within each layer, or to interaction between the adjacent pigment 

 molecules in the two layers. 



The blue-violet absorption band also shifts toward the longer waves in 

 crystals and monolayers, but its behavior is more complex, because of its 

 double structure. According to Kromhout (1952) the observed changes in 

 this band can be explained by taking into account the different polarizations 

 of the two components (c/. table 37C.I) and the preferential growth of the 

 microcrystals in one plane: when crystal particles grow mainly in one 

 plane, transmitted light consists increasingly of rays that have passed 

 through the crystals under the right angle to this plane. 



The strong scattering of light by the larger crystals — already mentioned 

 in connection with the red band — adds complication to the shape of the 

 blue-violet doublet. According to fig. 37C.16c, the selective scattering of 

 large microcrystals has its peak at 485 m/x, on the long-wave side of the 



