denaturation, term used to describe the loss of native, higher-order structure of protein molecules in solution. Most globular proteins exhibit complicated three-dimensional folding described as secondary, tertiary, and quarternary structures. These conformations of the protein molecule are rather fragile, and any factor that alters the precise geometry is said to cause denaturation. Extensive unfolding sometimes causes precipitation of the protein from solution. Denaturation is defined as a major change from the original native state without alteration of the molecule's primary structure, i.e., without cleavage of any of the primary chemical bonds that link one amino acid to another. Treatment of proteins with strong acids or bases, high concentrations of inorganic salts or organic solvents (e.g., alcohol or chloroform), heat, or irradiation all produce denaturation to a variable degree. Loss of three-dimmensional structure usually produces a loss of biological activity. Thus, the denatured enzyme is often without catalytic function. Renaturation is accomplished with varying success, and occasionally with a return of biological function, by exposing the denatured protein to a solution that approximates normal physiological conditions. Denaturation may be studied in the laboratory in any number of ways that monitor the physical properties of protein. Thus measurements of changing viscosity, density, light-scattering ability, and movement in an electrical field all record slight changes in molecular architecture. Denaturing is also used to describe the unrelated process of adding a poisonous substance to ethanol to make it unsuitable for human consumption.