Structure-odour correlations

Odour has three properties, viz. character, intensity and persistence. All three are subjective and can only be measured in sensory terms. Most correlation work has been done on character since it is, superficially, the easiest to measure. However, the description of an odour is associative, since we have no hard reference points. Difficulties in finding correlations can arise from the use of odour classification systems. For instance, in our fragrance work, we found that the use of the term fruity to describe an odour family led to confusion, since the criteria for a molecule to possess an apple odour are not the same as those for pear. We classify the two together because of the similarity of the botanical sources, but this is not necessarily related to the odour properties. To study structure-odour correlations, we must therefore ensure that we are using meaningful parameters. 

Structure-odour correlations rely on the measurement of odour. It must be remembered that odour is not a physical property. Odour exists in the mind. We must not assume that it exists in the receptor also. The fact that cross-adaptation can occur between chemically different materials with similar odours, and also between chemically similar materials with different odours, suggests that it does not. There is no evidence that the receptors are tuned to specific odours or even odour types. It is quite likely that the recognition process is based on physical and/or chemical properties of odorants and that odour only exists when the brain puts an interpretation onto a pattern of signals from the olfactory nerves. 

Equally, what we term odour is, in fact, the end result of a number of discrete processes. In consequence, we cannot assume that any correlation between agent and effect arises from only one of these. All too often it is assumed that only the interaction between the odorant and the receptor protein is of significance and that structure-odour correlations give mechanistic information about this specific event. 

The correlations obtained with the intensity of the odour of fatty acids (the reference structure is the isovaleric acid) were U) not nearly as good as one would hope for . 

Another common failing in the logic behind odour theories is the inability to distinguish between cause and effect. All too often, someone finds a correlation between two parameters and assumes a causal relationship, without asking whether this correlation could simply be between two effects of a common cause. An example of this in the field of olfaction is the assumption that a correlation between the infrared spectra of a set of odorants and their odours demonstrates that the odour was caused by those specific vibrations. The odours and the spectra are both effects of a common cause, that is, the molecular structure. 

ADAPT has been developed and used by Jurs in a wide range of SAR applications. In the field of olfaction these include the correlation of odour intensities for 58 structurally and organoleptically diverse odor-... 

In the field of odorants, we find examples of structurally different compounds with similar odour qualities and structurally similar compounds with different odour qualities. A notable example is the well-defined musk odour that can be found within five very different structural families (Figure 14.10). However, within each family there are very tight structural criteria. A very small structural change, such as the addition of one methyl group, can cause complete destruction of the musk odour. This creates a problem for the SAR worker. Some have searched for correlations within a family, while others have tried to speculate on molecular parameters common to all groups. 

ADAPT has been developed and used by Jurs in a wide range of S AR applications. In the field of olfaction, these include the correlation of odour intensities for 58 structurally and organoleptically diverse odorants (Edwards and Jurs, 1989), and the investigation of the relationship between molecular structure and musk odour (Jurs and Ham, 1977 Ham and Jurs, 1985 Narvaez et al., 1985). To date, no one has used pattern-recognition techniques in the study of muguet odorants. 

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