Structure-activity relationships on the odor detectability of homologous carboxylic acids by humans

Abstract

We measured concentration detection functions for the odor detectability of the homologs: formic, acetic, butyric, hexanoic, and octanoic acids. Subjects (14 ≤ n ≤ 18) comprised young (19-37 years), healthy, nonsmoker, and normosmic participants from both genders. Vapors were delivered by air dilution olfactometry, using a three-alternative forced-choice procedure against carbon-filtered air, and an ascending concentration approach. Delivered concentrations were established by gas chromatography (flame ionization detector) in parallel with testing. Group and individual olfactory functions were modeled by a sigmoid (logistic) equation from which two parameters are calculated: C, the odor detection threshold (ODT) and D, the steepness of the function. Thresholds declined with carbon chain length along formic, acetic, and butyric acid where they reached a minimum (ODTs = 514, 5.2, and 0.26 ppb by volume, respectively). Then, they increased for hexanoic (1.0 ppb) and octanoic (0.86 ppb) acid. Odor thresholds and interindividual differences in olfactory acuity among these young, normosmic participants were lower than traditionally thought and reported. No significant effects of gender on odor detectability were observed. The finding of an optimum molecular size for odor potency along homologs confirms a prediction made by a model of ODTs based on a solvation equation. We discuss the mechanistic implications of this model for the process of olfactory detection.

Fig. 1

Left Group detectability functions for the odor of the five carboxylic acids. Each symbol represents the outcome of 630 judgments in the cases of formic and hexanoic acids, 560 judgments in the case of acetic acid, and 490 judgments in the cases of butyric and octanoic acids. Right Plots of group average confidence rating as a function of concentration for each acid. In all cases, bars indicate standard error (SE) of the mean

Fig. 2

Showing the close similarity between odor detection functions obtained from all subjects tested with each acid, and those obtained form the common subjects tested with all five acids. Bars represent SE

Fig. 3

Individual plots of odor detection thresholds as a function of carbon chain length for the 14 subjects tested with butyric acid, which have also been tested with one or more of the other acids. All participants showed an odor detection threshold minimum for butyric acid

Fig. 4

Plot of C, i.e., odor detection threshold in log ppb by volume, against the maximum molecular length (in Angstroms, Å) of carboxylic acids

Fig. 5

Trends in odor detection thresholds as a function of the variable carbon chain length for the acids and five other homologous series (Cometto-Muñiz and Abraham , , , ; Cometto-Muñiz et al. ; see text). Bars, sometimes hidden by the symbol, indicate SE

Fig. 6

Left Showing the wide spread of odor detection thresholds reported for each carboxylic acid, as listed in the compilations by van Gemert (2003) (squares) and by Devos et al. (1990) (circles). Right Showing the similarity of odor detection thresholds for each carboxylic acid among three recent sources: the present study (crosses), Nagata (2003) (triangles), and Wise et al. (2007) (diamonds). Also note that the odor detection thresholds from these three studies appear at the low end of the range of values for each acid listed in the compilations (and shown on the left)