168 DAVID R. EVANS 



conclusions of Dethier (1955) and von Frisch (1935) that chemical struc- 

 ture could not be related to stimulating effectiveness. These authors had 

 considered only configurational structure. However, even when the con- 

 formation of the molecules was taken into account, it was apparent that a 

 single combining site could not account for the data. There had to be 

 multiple combining sites with different structural requirements to account 

 for the extreme structural specificity on the one hand and the odd array of 

 effective molecules on the other. 



The first suggestive observation was that mixtures of the very weakly 

 stimulating sugar, mannose, with fructose markedly reduced stimulation 

 by the fructose. Similar mixtures of glucose and mannose were as effective 

 as if the glucose were present alone (Dethier, 1955). Subsequently, 

 Dethier et al (1956) reported that mixtures of glucose and fructose were 

 more effective than their concentrations accounted for, i.e. that their 

 action in mixtures was synergistic rather than merely additive. And the 

 inhibition of fructose stimulation by mannose was competitive. The sim- 

 plest interpretation of these data is that glucose and fructose stimulate by 

 combining with different sites and that mannose combines ineffectively with 

 the fructose site, competing with fructose for that site. 



Confirmation of this viewpoint came from an unusual source. It was 

 found that the sensitivity of the adult receptors either to fructose or 

 to glucose could be altered drastically by rearing the developing larva in 

 the presence of one or the other sugar (Evans, 1961). The sensitivity to the 

 two sugars was clearly dissociable experimentally. 



This postulation of multiple combining sites explains simply why for- 

 merly it had not been possible to find a structure that was common to the 

 stimulating sugars and absent in the tasteless ones. Now two possible 

 experimental approaches were to determine from studies of mixtures which 

 sugars or derivatives were inhibitory, synergistic, or merely additive in their 

 effects or to introduce single small structural changes into one molecule 

 and assay the effect. The latter, by reason of directness and simplicity was 

 the method of choice and could show what chemical structure was required 

 of a molecule for stimulation. 



A number of considerations determined which molecule was the most 

 appropriate for study. In recent years, it has become increasingly evident 

 that the conformation of sugars, in addition to configuration, plays a fun- 

 damental role in their chemical and biological reactions. For example, 

 cis and trans in a configurational sense is an almost useless concept in the 

 reactions of almost all sugars. As has been shown both theoretically and 

 experimentally (cf. Reeves, 1950, 1958), almost without exception sugars 

 and their cyclical derivatives (6-membered) assume either one or both of 

 the two chair forms rather than the boat forms ; and it is only in the boat 

 forms that true cis orientation is possible. The geometry assumed by a 



