They would have made sculptors drop their chisels. The two women were preparing lunch for us when my friend suddenly became self-conscious of his armpits’ stench. There was no deodorant in sight, but I assured him we could easily concoct something before they returned from the kitchen. A bit of ashes and soap should do the trick, I told him.
When I woke up from the dream, I dismissed the ashes recipe, but I was reminded of an idea I had when I was in college. The nauseating smell of armpits comes from organic acids produced by bacteria. I reasoned that if an enzyme could play the role of a strong acid usually required to catalyze the conversion of organic acids and alcohols to esters, we would end up with pleasant smells. The alcohol, if in excess, would also inhibit the growth of bacteria. In those days I was under the false impression that butyric acid was the main culprit of armpit odor. If that was the case, ethanol and a catalyst, butyric acid would be converted into one of the sweet smells of pineapple, ethyl butyrate.
Of course, throughout the day I could not stop sneaking a peak at the current knowledge of deodorant chemistry. One of the compounds secreted by armpit sweat glands is 3-hydroxy-3-methylhexanoyl-glutamine (C12H22N2O5). This compound has no odor; however, it’s not left intact by the most abundant armpit bacteria Corynebacterium jeikeium, a harmless member of a genus that also includes a species responsible for diphtheria. With the help of zinc-dependent enzyme, C. jeikeium cleaves off the glutamine component and leaves behind the cheesy and rancid compound 3-hydroxy-3-methylhexanoic acid:
They don’t have exclusive control over this semi-closed and moist environment— prime real estate for bacteria. Staphylococcus haemolyticus also hangs out here and converts a different precursor into 3-methyl-3 sulfanylhexan-1-ol. This molecule is not as repulsive as the C. jeikeium’s byproduct. It has a fruity, onion-like smell. Not surprisingly, female armpits produce more of the latter. They have, on average, lower ratios of C. jeikeium to S. haemolyticus bacteria. If you look closely at the data of the ratio of the will-turn-to-cheesy-smell to will-turn-to-onion-smell secretions, you can see a wide variety of compositions in men, but none of the women tested showed the high peaks that appear in more than half the male samples.
Most deodorants use the right strategy: their ingredients curb the growth of bacteria. Speedstick, for example, uses propylene glycol, soap(sodium stearate), salt and stearyl alcohol. Some more innovative deodorants include pleasant-smelling molecules similar in shape to the organic acids so that they compete for spots on nasal receptors. Unfortunately for women, their discriminating noses aren’t as easily fooled as those of men. Other additives in some preparations attempt to block active sites on enzymes that bacteria use to generate the offensive smells.
Regardless of the precise formulation, rub-on deodorants and anti-perspirants are often dissolved in clear, odorless silicones known as cyclomethicones. Their advantage is that these non-irritants apply smoothly, evaporate quickly, leaving the active residue as their only trace.
Cyclomethicones are also used in antiperspirants, whose active ingredient is commonly aluminum zirconium tetrachlorohydrex with glycine hydrated in its structure. Some investigators were under the impression that the compound combines with intraductal keratin fibers to temporarily block sweat pores, which starves bacteria. What’s more likely is a precipitation reaction involving the complex salt, and the insoluble solid blocks the pore.
Eventually, with a deodorant additional sweat dilutes and washes away soap and other soluble ingredients. With an antiperspirant, a pressure buildup eventually forces insoluble products out of the duct. Either way, monsters of the armpits eventually regain control.
Gautschi, Markus; Natsch, Andreas; Schröder, Fridtjof Biochemistry of Human Axilla Malodor and Chemistry of Deodorant Ingredients Volume 61, Numbers 1-2, February 2007