Denaturation

Denaturation is a change in the three-dimensional structure of a large biological molecule, most often a protein, that leaves its chain of building blocks intact. A protein keeps its working shape through many weak forces, such as hydrogen bonds, ionic attractions and hydrophobic interactions, helped in some proteins by disulfide bonds. High temperature, strong acids or bases, detergents, organic solvents and chemicals such as urea disrupt these forces, and the folded protein loosens into a disordered chain. Its primary structure, the order of its amino acids, stays the same because the peptide bonds are not broken.

Since shape determines function, a denatured enzyme stops working, which is why each enzyme performs best within a certain range of temperature and pH. Denaturation is sometimes reversible: if the denaturing agent is removed gently, some small proteins fold back up on their own. Often it is permanent, as when egg white turns solid and white during cooking. Stomach acid denatures the proteins in food, opening them up so digestive enzymes can reach their bonds.

Nucleic acids denature too. Heating double-stranded DNA breaks the hydrogen bonds between paired bases and separates the two strands, the first step of every cycle of PCR. On cooling, complementary strands can find each other and pair again, a process called annealing or renaturation.