How are amino acids classified based on their side chains?

Amino acids are classified by side chain (R group) properties. Nonpolar/hydrophobic amino acids (glycine, alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine, tryptophan) tend to cluster in protein interiors. Polar uncharged amino acids (serine, threonine, cysteine, tyrosine, asparagine, glutamine) form hydrogen bonds. Charged amino acids include acidic/negative (aspartate, glutamate) and basic/positive (lysine, arginine, histidine). These properties determine protein folding, enzyme active site chemistry, and protein-protein interactions. Essential amino acids (9 in humans) cannot be synthesized and must be obtained from diet.