Amino acid side chain
= Amino acid side chain
{title2=$R$}
An <amino acid side chain> is the variable group attached to an <amino acid alpha carbon>, conventionally denoted $R$. For <alanine> it is $\mathrm{CH_3}$: a <carbon> bonded to three <hydrogen> <chemical atoms>. In a <beta strand>, successive <amino acid side chains> alternate between the two faces in the idealized arrangement. See https://openstax.org/books/biology-2e/pages/3-4-proteins[Clark, Douglas and Choi: amino-acid R groups].