The amino acid you are describing is cysteine. Amino acids are the building blocks of proteins. They consist of a carboxylic acid, an amine, and a side chain often designated as R. Most proteins are composed of various combinations of about 20 common amino acids. All of these amino acids differ in the nature of their side chains. Of these 20, only two contain a sulfur atom, methionine and cysteine. Cysteine has a side chain that consists of a methylene group (CH2) attached to a thiol group (SH) as described in the question. This makes it a polar amino acid.