Which amino acid side chains are polar and can form hydrogen bonds?

The correct choice is founded on the nature of the side chains of serine and threonine. Both of these amino acids possess hydroxyl (-OH) groups in their side chains, making them polar. This polarity arises because oxygen is more electronegative than hydrogen, allowing serine and threonine to engage in hydrogen bonding with other molecules, including other polar molecules or water. This characteristic contributes significantly to the structure and function of proteins, as it influences their solubility and interactions within biological systems.

In contrast, the other amino acids listed do not have similarly polar side chains. For instance, phenylalanine and methionine are primarily non-polar; phenylalanine has a large hydrocarbon side chain while methionine contains a thioether, which does not lend to forming hydrogen bonds effectively. Likewise, isoleucine and valine are branched-chain amino acids that are also hydrophobic, possessing non-polar characteristics. Finally, leucine and proline have non-polar side chains as well, with proline being unique in that its side chain forms a cyclic structure, which further limits its ability to form hydrogen bonds.

Thus, the side chains of serine and threonine are clearly the most