SPECTRA OF CRYSTALLINE AND COLLOIDAL PIGMENTS 



1817 



represents the transmission spectrum of still larger microcrystals, obtained 

 by using a more concentrated pigment solution. 



Electron microphotographs of some of these crystalline preparations 

 were reproduced in fig. 37B.8. In the case of the 6-compound, only 

 relatively large microcrystals could be obtained. 



The spectrum of the microcrystals was measured in the collimated trans- 

 mitted beam, in aqueous suspension. The presence of a Tyndall cone indi- 

 cated that all suspensions produced marked scattering. For crystals with 

 dimensions small compared to the wave length of light, this scattering is not 

 too disturbing, as shown by the dotted line in fig. 37C.166. Although the 



z 



O 



< 



o 

 p 



Ql 

 O 



1.00 



.50 - 



.00 



350 



450 550 650 750 850 



WAVE LENGTH (M^a) 



950 



Fig. 37C.17. Spectrum of methyl bacteriochloroplij^llide in solution (dashed line) 

 and in large microcrystals (solid line). (Jacobs ei al. 1953.) 



scattering is predominantly of the "selective" type, with peaks on the red 

 side of the transmission minima, it is not strong enough to shift the peaks ; 

 nor does it affect significantly the shape of the bands. 



For crystals Avith linear dimensions >0.5/x, (c/. figs. 37B.8and37C.16c), 

 the shape of the transmission curves is more strongly influenced by scat- 

 tering. The experimental scattering curve of a suspension of such crystals 

 is shown by the dashed line in fig. 37C.16c. It has a sharp peak at 765 

 m/x- — considerably on the long-wave side of the transmission minimum 

 at 745 m^ — and a minor peak at 715 m/i. The scattering dechnes only 

 slowly in the infrared; this must be largely responsible for the similar 

 behavior of the transmission curves of the larger microcrystals. With 

 such crystals, correcting the transmission curve for scattering— to trans- 

 form it into true absorption curve — does change the position of the peak 

 significantly, shifting it by about 5 m^ and making its slope on the long- 

 wave side much steeper. 



Fig. 37C.17 shows the effect of crystallization on the spectrum of 

 methyl bacteriochlorophyllide. 



As described in chapter 37B, section 5, similar spectroscopic shifts 



