INTRODUCTION AND METHODS 



+ + _ _ 



dichloride ({NHg — (CH2)i2 — NHgjCla) has an unexpectedly high 

 extraction efficiency. This molecule is hydrophihc at both ends and 

 consequently only a single layer is required (Fig. 1.4 (c)). 



bridges' work suggests that negatively-charged amphipathic ions 

 (e.g. cholate, palmityl sulphonate) may function as solubilizers in a 

 converse manner, namely through electrostatic bonding with the 

 positively-charged ammonium groups of the visual purple protein. 

 The mode of action of the non-ionic extractants is unknown. 



OoC 



(a) (b) (c) 



Fig. 1.4. Diagram to illustrate electrostatic combination between 



extractant molecules and the carboxyl groups ( — CO2) of visual purple 



protein. Filled circles represent the hydrophilic part, and rectangles the 



hydrophobic part of the extractant molecule. 



(After Bridges, 1955) 



It is an open question whether visual purple — in its native state — 

 is water soluble or not. Water alone will not extract it. But this 

 may be because the visual purple is incorporated in the structure 

 of the rod by electrostatic bonding and hence requires a counter- 

 attraction to take it from its contexture. If visual purple could 

 be freed by physical means, e.g. by the action of ultra-sonic 

 vibrations (sidman, private communication) it would, perhaps, be 

 water-soluble. 



EXTRACTION PROCEDURES 



When whole retinae are treated with extractants, substances in 

 addition to the visual pigment pass into solution. The principal 

 contaminants are the red haemoglobins and cytochromes, the colour- 

 less, water-soluble proteins and the yellow, fat-soluble lipids. Little 

 can be done to purify such an extract. However, by first bathing the 



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