THE VISUAL PIGMENTS 



Digitonin and saponin are usually used in 1-2 per cent concentra- 

 tion, the bile salts and CTAB are satisfactory in 4 per cent concentra- 

 tion. 



Mode of action of the extractants. All the extractants have amphi- 

 pathic properties, that is their molecules are in part water-attracting 

 (hydrophilic) and in part oil-attracting (oleophilic or hydrophobic). 

 Such molecules tend to align themselves at an oil/water interface so 

 that the hydrophiUc end is in the water phase and the oleophilic end 

 in the oil phase. This interfacial activity makes them useful in 

 stabilizing oil-in-water or water-in-oil emulsions. 



Amphipathic substances may be non-electrolytes, which dissolve 

 in water without producing ions (e.g. digitonin), or they may be 

 electrolytes and dissolve to give a simple inorganic ion and an oppo- 

 sitely-charged amphipathic ion. The amphipathic ion may be 

 negatively charged (e.g. the cholate ion of sodium cholate) or 

 positively charged (e.g. the cetyltrimethylammonium ion of CTAB 

 or CTAC). Visual pigments can be efficiently extracted by represen- 

 tatives of all these types. 



The mechanism of extraction is still not completely understood but 

 recently our knowledge has been advanced through an interesting 

 study of an homologous series of positively charged amphipathic 

 ions (bridges, 1955). bridges examined the extracting efficiency 

 of the substances, 



CH3-(CH2)„-NH3}cf 



from « = 4 to 71 = 15. He found that the molecular ratio of extract- 

 ant to visual purple (the conditions of extraction being such that the 

 retinal visual purple was in excess of the extracting agent) varied 

 from c. 50,000 in the case of amylammonium chloride (« = 4), the 

 least effective substance, to 330 for cetylammonium chloride (/I = 15), 

 the most effective. 



BRIDGES considers that, with these compounds, the positively- 

 charged nitrogen combines electrostatically with the negatively- 

 charged free carboxyl groups of the visual purple protein. This 

 means that the hydrophobic parts of the extractant molecules are 

 oriented outwards to give a complex (Fig. 1 .4 {a)) which would be 

 insoluble in water. However, a second molecular layer of extractant 

 with hke part to like (Fig. \A{b)) would reverse this and confer 

 solubiHty in water. The probability that this is what happens is 

 suggested by the fact that dodecamethylene l:12-diammonium 



10 



