Many odorants are complex structures involving combinations of multiple functional groups and hydrocarbon elements. On straight-chained hydrocarbon backbones, distinct functional groups appear to be coded as distinct molecular features (Johnson and Leon, 2000a; Johnson et al., 2002, 2004). Aspects of hydrocarbon structure also appear to represent distinct molecular features when the functional group remains constant (Johnson et al., 1998, 1999, 2004; Johnson and Leon, 2000b). Recently, we have begun to use more complex odorants involving combinations of individually characterized features to ask whether these features are coded independently or whether there are interactions between the features.  

We found that carboxylic acids and high concentrations of ketones activate particular glomerular modules independently of hydrocarbon structure (Johnson and Leon, 2000b; Johnson et al., 2005b). However, most glomerular modules appear to respond only to particular combinations of functional group and hydrocarbon structure (Johnson et al., 2005b). For example, modules responding to methyl esters and alcohols on a straight-chained backbone do not respond to methyl esters or alcohols containing a cyclohexyl hydrocarbon structure (Johnson et al., 2005b). Interactions of functional groups with aromatic, branched, and both branched and double-bonded hydrocarbon structures also were observed (Johnson et al., 2005b). Finally, aromatic compounds appear to stimulate a glomerular module that previously had been associated specifically with the ketone functional group (Johnson et al., 2005b).  

Alcohol functional group on different hydrocarbon structure backgrounds.  

We also found that there are interactions between functional groups and their positions within a molecule. Whereas positional isomers of ketones evoke nearly identical activity patterns, the activity patterns evoked by alcohols depend heavily on the position of the hydroxyl group within a straight-chained hydrocarbon backbone (Johnson et al., 2005a). Intermediate effects were observed for positional isomers of esters (Johnson et al., 2005a).  

Effects of substitution position on patterns of 2-DG uptake evoked by seven-carbon alcohols.

As another example of interactions between functional group features and hydrocarbon structural features, we found that responses to carboxylic acid and methyl ester functional groups can be disrupted by unsaturated bonds (Johnson et al., 2006b) or ring structures (Johnson et al., 2006c) located nearby in the same odorant molecule. Similar effects have been reported in detailed studies of the receptive range of an individual odorant receptor (Araneda et al., 2000). Also, when there is more than one oxygenic functional group in an individual odorant molecule, the pattern of activity appears to be determined by the overall high water solubility of the odorant, rather than by the combination of responses to the separate functional groups (Johnson et al., 2006c).