Structures, relations between odors

An ultimate objective in the resolution of relations between odors and odorant structures is to predict odor from chemical identities and concentrations of odorants in air containing mixtures of odorants. 

III. STRUCTURE-ACTIVITY RELATIONS BETWEEN ODORS AND RECEPTORS TRANSDUCING ODOR SPACE INTO BRAIN SPACE... 

It is not yet possible to design a molecule with specific odor (or taste) characteristics because the relations between sensory properties of flavor compounds and their molecular properties are not well understood. As a consequence, the development of compounds with desired flavor qualities has had to rely on relatively tedious synthetic approaches. Recent advances, however, in computer-based methods developed by the pharmaceutical industry to study QSAR (quantitative structure-activity relationships) may ultimately be helpful in the rational design of new flavor-structures with predictable sensory attributes. Results from QSAR studies may also provide insight into the mechanism of the molecule-receptor interaction. 

The difference between citronellol and citronellal is typical of that between an alcohol and an aldehyde. Although they are clearly related in odor, the aldehyde is, as we would expect from quite a small molecular structure, very much more powerful and harsh. In hydroxycitronellal the addition of an—OH and—H across the double bond (in effect by adding a molecule of water, a process known as hydration) produces an alcohol group near to the other end of the molecule, away from the aldehydic group. The material still maintains some of its aldehydic character but now, in addition, has some of the softness and floral character associated with an alcohol. The presence of more than... 

The properties of perfume materials are intimately related to their chemical constitution, but the mechanisms whereby chemical structure leads to odor perception involve, in crucial ways, a physical phenomenon the mutual attraction forces between molecules. These forces determine the rate of evaporation of odor materials from solutions or surfaces, they are the basis of fixation and substantivity, they explain why the odor quality of mixtures varies depending upon the solvent or base in which they are incorporated. They are also involved in the very process of odor perception, in the contact between the odorant molecule and the receptor cell. Moreover they are at the heart of distillation, extraction, solubility, and the mechanism of chromatography. In this chapter, we will briefly discuss the physical basis of some of these phenomena, showing also how the attraction forces between molecules are related to their chemical structure. 

Proteins are chiral, so they should interact differently with the two enantiomeric forms of a chiral molecule, which should eventually translate into a difference of the odor impression of these mirror images of the molecules. A more detailed knowledge of the relations between the chemical structure of a molecule, including its absolute configuration, and its odor properties will contribute to the elucidation of the receptor mechanism. 

Reviews of the association of chemical and sensory aspects were published as early as 1965 by Wick and later, for instance, by Vernin (1981), who gave examples of odor-structure relationship. Different structures can be related to similar odors, similar structures to different odors, and similar structures to similar odors. A statistical treatment of data became necessary to correlate analytical results with those obtained by sensory analysis, as stated by Adda and Jounela-Eriksson (1979). Since then, correlation between sensory analysis and instrumental analysis has been the subject of important sessions of the Weurman Flavor Research Symposia, for instance in the chapters on Sensory science in flavor research (Weurman 5th Meeting, 1987) or Correlation between sensory and instrumental analysis (Weurman 7th meeting, 1993). 

This book presents contributions from a diverse group of researchers interested in the relation between chemical structure and both odor quality and odor intensity. As such, it presents one of the first volmnes devoted solely to research in structure-activity relationships, and is a key resource for serious investigators and other interested individuals. 

The relationship between the molecular structure of an aroma compound and its threshold is still unclear. Volatility of a compound may not relate to its threshold. For example, the threshold of ethanol (boiling point is 78°C) is much higher than octanol (boiling point is 195°C) or other homologous alcohol. Ethanol has high volatility but low odor intensity. It is often used as a solvent in compounded flavors. 

The physicochemical approach to odor detection involves instrumental analysis of the chemical structure and concentration of an odorant. The relationship between the sensation of an odor as perceived by a human and the physical or chemical properties of the odor is not as simple as other sensations such as color or sound, which can be related to known physical forms such as intensity and wavelength. Measurement of odorant structure and concentration is possible through quantitative chemical analytical techniques. The ideal situation is to develop an instrument that is capable of replicating the human olfactory system without the added problems of human variability, sensitivity, and perception. 

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