Which amino acid contains an unbranched alkyl side chain?

The amino acid with an unbranched alkyl side chain is alanine. Alanine has a side chain that is a simple methyl group (–CH3), making it the smallest non-polar amino acid. This unbranched structure allows alanine to easily fit into various protein environments and plays a significant role in protein stability and structure.

Proline features a unique cyclic structure which introduces additional complexity and can affect protein folding and function, but it does not have a straightforward alkyl side chain like alanine. Serine, on the other hand, contains a hydroxymethyl side chain (–CH2OH) that includes a functional group, thus it is not purely alkyl. Valine has a branched isopropyl group as its side chain, which means it does not meet the criteria for an unbranched alkyl structure.

Overall, alanine's simple, linear methyl side chain categorizes it distinctly as the amino acid with an unbranched alkyl side chain.