An odor reminiscent of sweat and onions, and one reminiscent of grapefruit. These impressions seem far apart, yet a study of underarm odor found this difference between two mirror-image forms of a single component.
In 2004, Myriam Troccaz and colleagues were investigating an unidentified sulfur-containing component of underarm odor. By recreating the odor with bacteria, then isolating and studying its components, they uncovered an unexpected relationship between molecular configuration and smell.
Recreating underarm odor in the laboratory
The research began with a problem: reconstructions of underarm odor made by mixing known components did not fully reproduce its characteristic sharpness. The researchers focused on trace sulfur compounds as possible candidates for the missing note.
They collected underarm sweat produced during exercise or sauna use from 30 men, removed bacteria by filtration, and prepared odorless samples. They then added bacteria isolated from the axilla and examined the development of odor.
The intensity of sweaty, onion-like, buttery, and other impressions varied with the bacterium. One perfumer and 30 trained assessors compared the whole incubated sweat samples. The researchers selected Staphylococcus haemolyticus, a strain that produced a strong sulfurous underarm odor, and obtained odor components for analysis from 500 mL of pooled sweat.
Separating compounds with an instrument and locating them by smell
To search for the components, the researchers used gas chromatography, an instrumental method that separates the constituents of a mixture in sequence. By having people smell the output, they could narrow down where the target odor appeared among the many compounds.
They combined this with sulfur-selective detection and mass spectrometry, which provides clues to molecular identity. They also synthesized a candidate substance for comparison, identifying the unknown component as 3-methyl-3-sulfanylhexan-1-ol, or 3M3SH. Identification relied not merely on a similar smell, but also on matching analytical results.
Mirror-image molecules change the words used to describe an odor
3M3SH has two forms that correspond as mirror images but cannot be superimposed, like right and left hands. These are called enantiomers, distinguished here as the S and R forms. They have the same types of atoms and the same connectivity, but different spatial arrangements.
The researchers used a narrow tube called a column that could separate the two forms, and smelled each as it emerged from the instrument.
- S form: sweaty, onion-like, and animal-like odors
- R form: fruity, grapefruit-like, and sulfurous odors
One interesting aspect is that the R form retained a sulfurous impression. Grapefruit-like was one of the descriptors used for its odor. These differences in smell were hidden within the same chemical name.
Techniques for separating mirror-image forms are also used in quality testing of orange essential oil. Looking beyond a component’s name to identify which of its forms is present gives us further clues to understanding aroma.
The sweat sample contained more of the sweaty-smelling S form
In the analyzed sweat extract, the S form was more abundant than the R form, with an estimated ratio of approximately 3:1. This was an approximation derived from small signals for a trace component, not a determination that every person’s underarm odor always contains 75% S form.
Taking its odor character and relative abundance together, the authors considered the S form important to typical underarm odor. The study identified a previously unknown sulfur compound and went further by showing that its two spatial configurations had different smells.
The comparison here was between enantiomers. It was not an experiment showing that dilution turns a sweaty odor into a citrus aroma. Nor did it investigate whether grapefruit fruit contains 3M3SH. The material studied was sweat in which bacteria had generated odor; the research did not explain the entire medical condition of axillary bromhidrosis through this one component.
Summary
The 3M3SH identified in underarm odor had an S form reminiscent of sweat and onions and an R form that also carried a grapefruit-like impression. The analyzed sweat sample contained more of the S form, which the authors considered an important contributor to the character of underarm odor.
Impressions that we usually place in broad categories such as sweaty and fruity can reveal surprising connections when traced down to molecular configuration. This is an example of separating compounds and smelling them to capture a relationship that odor labels alone would not suggest.
References
Troccaz, M., Starkenmann, C., Niclass, Y., van de Waal, M., & Clark, A. J. (2004). 3-Methyl-3-sulfanylhexan-1-ol as a Major Descriptor for the Human Axilla-Sweat Odour Profile. Chemistry & Biodiversity, 1(7), 1022–1035. Original article / Author-shared full text.
