Sometimes, people just don't smell good – and not only in the figurative sense. The sense of smell plays an important role in interpersonal relations.
The human armpit contains chemical substances that – at least in premodern times – probably provided direction for the selection of a genetically compatible partner for reproduction.
The average modern nose is offended by strong bodily odors, and the smell of perspiration is scorned by our society.
The typical smell associated with the human armpit is caused by a cocktail of odorous substances, including various androstenone derivatives and certain volatile fatty acids, as well as sulfur-containing compounds about which little has been previously known.
Two Swiss research teams have now identified several of the main sulfur-containing components, as well as a bacterial enzyme that seems to be responsible for the release of these compounds.
A team headed by Anthony J. Clark at the Swiss company Firmenich SA enlisted 30 volunteers to work up a heavy sweat on exercise machines and in a sauna. The sweat from their armpits was collected and sterilized. Normally, the secretions released by the many sweat glands in the underarm region are initially odorless. The human armpit, however, is the home of a diverse population of bacteria, mostly strains of Corynebacteria and Staphylococcus, that transform the components of sweat into strongly odorous substances.
Through experiments with bacteria from human underarm skin, the researchers determined that different strains of bacteria produce different scents. "Of the micro-organisms we tested, Staphylococcus haemolyticus produced the most sulfurous scent," reports Clark. Isolation and analysis of these decomposition products by gas chromatography, mass spectroscopy, and the trained noses of perfumers revealed the presence of at least eight sulfanyl alcohols – compounds containing both OH and SH groups. One of these was identified as 3-methyl-3- sulfanylhexan-1-ol, which is present at a concentration of 4 ppb (one particle per every billion particles of fluid). The atoms in this molecule can be arranged in two ways, as two mirror-image forms of the molecule. While the (S)-form (S from the Latin, sinister = left) smells of sweat and onion, the scent of the mirror-image (R)-form (R from the Latin, rectus = right) is described as fruity and grapefruit-like. "The sweat broken down by S. haemolyticus contained the (S)- and (R)-forms in a 3:1 ratio," says Clark.