170 DAVID R. EVANS 



Conformational analysis has been extensively employed in the study of 

 the requirements of bacterial enzymes that act on cyclitols ; and the cyclitols 

 which occur naturally, it has been pointed out, all possess a stable chair 

 form (cf. Angyal and Anderson, 1959). 



So far evidence has been given to the effect that all carbohydrates that 

 stimulate the blowfly chemoreceptor do not do so by binding to a single 

 combining site. Nothing, however, has been said as to whether the sites 

 are associated with a single sugar receptor cell or more than one. For most 

 of what follows the point is probably irrelevant, but there is evidence that 

 the combining sites belong to the same neuron. Dethier and Rhoades 

 (1954) and Dethier et al. (1956) found that flies could not distinguish one 

 sugar from another. Hodgson (1957) tested a number of sugars electro- 

 physiologically and reported that they all activated one receptor cell. 

 Evans (unpublished) assayed solutions of mixtures of glucose and fructose 

 as well as these sugars individually, using the electrophysiological method 

 of Morita (1959), and obtained in each case only the impulses of the water 

 receptor (Evans and Mellon, 1962a) and a single sugar receptor. Thus, 

 the evidence points to a single sensory neuron with two or more types of 

 combining sites for sugars. 



Structural Requirements of the Receptor Site for Glucose 



In considering the combination of glucose with its receptor site, attention 

 immediately focuses on the polar groups (the hydroxyls, the Cg carbinol, 

 and the ring oxygen). While almost all of the molecules of glucose in 

 solution are in the pyranose form and CI conformation, this by itself does 

 not prove that that is the form which reacts with the receptor ; but other 

 considerations do. Dethier (1955) could detect no difference between a 

 and /i glucose. These experiments were repeated under conditions where 

 less than 2 per cent of the original stereoisomer had mutarotated ; again 

 no difference was found in stimulating effectiveness (cpds. 1,2 and 3; 

 Table 1). On the other hand, the respective glucosides with small aglycons 

 (cpds. 4,5,6,7) were for all intents and purposes qualitatively different. 

 The a-methyl and a-phosphate derivatives were slightly more stimulating 

 than glucose, while the /i-methyl and /?-gold-thioglucose were ineffective 

 at any concentration. These effective compounds are all confined in the 

 a-pyranose form, so clearly the 6-membered ring was as effective as glucose. 

 Consequently, small amounts of the free aldehyde or furanose form were 

 not responsible for the stimulation by glucose solutions. 



The foregoing results could be interpreted either as that the a-oxygen 

 atom was required for stimulation or that /i substituents of a certain size 

 hindered stimulation. Compound 8, lacking the C^ hydroxyl group, was as 

 effective as glucose, indicating y^ steric hindrance was the case. Further- 

 more, it showed that a 6-membered ring containing the very stable ether 



