They each contain sulfur—hydrogen (-SH) groups. In chemistry, thio
refers to the element sulfur and thiol to the -SH group. These three
molecules are examples of organic thiols, the -SH group in a thiol
being analogous to the -OH group in an alcohol. (Sulfur occurs just
below oxygen in the periodic table of the elements, reflecting the fact
that oxygen and sulfur have some similar chemical properties.) Thiols
often smell stinky. There is something about the shape of the -SH
group attached to carbons that promotes fitting into a constellation
of GPCR olfactory receptors that give rise to signals interpreted by the
brain as stinky. The three molecules above are major chemical con-
stituents of the stinky secretion of skunks.
Here are two more molecules that have a stinky quality to them:
methanethiol and dimethylsulfide. These molecules are found in the
urine of many people who have eaten asparagus and impart a unique
stinky aroma to the urine. However, methanethiol and dimethyl-
sulfide are not found in asparagus, either fresh or cooked. They are
apparently produced during the digestive chemical transformation of
molecules that are found in asparagus, such as asparagusic acid.
H3C—SH H;C—S—CH,
Methanethiol DimethylsulfideOo OH
$-$
Asparagusic
acid