What is an amino acid: the r group defines the property of each amino acid residue, the accumulated r groups define the properties of polypeptide/protein. Side chains of amino acids: interactions between side chains determine how a protein will fold, side chains differ in, size, shape, charge, hydrophobicity, reactivity. Solubility: the ability of a molecule to interact with water through hydrogen bonding - thermodynamically favourable, a molecule is soluble if it can form bonds with hydrogen and is charge polarized. Hydrophobic amino acids: non-polar side chains: two categories, aromatic - phenylalanine, tyrosine, tryptophan. Tyrosine has a hydroxyl group, so can be either hydrophobic or hydrophilic: aliphatic - alanine, valine, isoleucine, leucine, methionine. Hydrophilic amino acids: water-soluble, and are found on the exterior of proteins, charge-polarized, and able to form hydrogen bonds, have hydroxyl groups and charged oxygens at a ph of 7.