Explain how amino-acid side chains determine major chemical classes, and give examples of each class.

### What a good answer covers - The **R group** is the variable side chain attached to the alpha carbon. - Nonpolar side chains tend to avoid water; examples include alanine and valine. - Polar uncharged side chains can interact with water without carrying a full charge; examples include serine and glutamine. - Acidic and basic side chains can carry charges, while aromatic and sulfur-containing amino acids have distinctive structures. ### The concept Side-chain classes explain why amino acids behave differently in proteins. A protein’s sequence places these side chains in particular environments, where they influence folding and chemical activity. Histidine is often useful in acid-base roles because its side chain can gain or lose a proton near physiological conditions. ### Terms - **R group**: the variable side chain of an amino acid. - **Alpha carbon**: the central carbon to which the amino group, carboxyl group, hydrogen, and side chain attach.