Denaturation means rendering a substance ineffective for some purpose without changing its chemical composition. The term has a number of more specific meanings, but is most commonly used in connection with proteins and nucleic acids. These consist of chain-like molecules that can fold over in various ways to form complex three-dimensional shapes. The links in the chains are held together by strong covalent bonds, but the folds result from a variety of bond types that are usually weaker, and that can be broken by heat, and by various chemical agents. The molecules are said to have been denatured when some or all of these bonds have been broken, causing them to lose their shape, but leaving the chains intact and the chemical composition unchanged.
Protein and Nucleic Acid Structure
Proteins are made up of amino acids and are organized on several different structural levels. The primary structure is simply the sequence of amino acid building blocks that defines the protein. These building blocks are held together by covalent bonds known as peptide bonds. The secondary, tertiary, and quaternary structures describe the three-dimensional arrangements of protein subunits, whole proteins, and protein complexes. These structures result from chains of amino acid building blocks folding over on themselves, due to the formation of various types of relatively weak bonds between units at different parts of the chain.
The secondary structure results from hydrogen bonding between a hydrogen atom in one amino acid unit and an oxygen atom from another. This can produce a coiled or a sheet-like formation, or a combination of both. The tertiary structure results from bonds forming between these coils and sheets, giving a three-dimensional protein unit. The quaternary structure is formed by the bonding of two or more of these units.
Tertiary and quaternary structures are held together by a variety of bond types, including hydrogen bonds. Covalent disulfide bonds between the sulfur atoms in two amino acid units can also form. “Salt bridges” form when oppositely charged parts of molecules attract one another in a similar way to the ionic bonds that are found in salts.
Denaturation generally does not affect the primary structure, but it does cause degradation of the complex three-dimensional arrangements of the proteins. Most protein functions result from chemical traits arising from the three-dimensional arrangements of the amino acid chains, so degradation of such structures generally results in a loss of protein function. Enzymes are an important class of proteins where the shapes of the molecules are crucial to their functions.
Nucleic acids, such as DNA and RNA, have two strands built out of units known as bases. The strands are bound together into a double helix shape by hydrogen bonds between bases on opposite sides. During denaturation, the strands are separated by the breaking of these bonds.
Causes of Denaturation
A number of factors can cause proteins and nucleic acids to denature. Heating causes molecules to vibrate more vigorously, which can lead to the breaking of bonds, especially weaker ones. Many proteins will be denatured if they are heated to temperatures above 105.8°F (41°C), due to the breaking of hydrogen bonds. A familiar example is the change that occurs in egg white when it is heated: the protein albumin is denatured and turns from a transparent gel into a white solid. Proteins are also denatured when food is cooked, a process that kills harmful microorganisms.
Denaturation can also be caused by various chemical agents. Strong acids and bases, because of their ionic nature, interact with the salt bridges that help hold together the tertiary structures of proteins. Positively and negatively charged parts of these compounds are attracted to the oppositely charged parts of a protein salt bridge, breaking the link between the different parts of the protein chain. The salts of some metals can also have this effect.
Covalent disulfide bonds can also be broken, leading to denaturation. Compounds of some heavy metals, such as lead, mercury, and cadmium, can do this, because they bond easily with sulfur. The sulfur-sulfur bond can also be broken when each sulfur atom bonds with a hydrogen atom. Some reducing agents will produce this effect.
Various organic solvents can also have a denaturing effect by breaking the hydrogen bonds between amino acids that maintain the tertiary structure. One example is ethanol, commonly called alcohol. It forms hydrogen bonds of its own with parts of protein molecules, replacing the original ones.
The term "denaturation" is sometimes used to refer to the process of rendering food or drink inedible but still useful for some function aside from consumption. The most common example of this is denatured alcohol, also referred to as methylated spirits. The product is often used as a solvent or fuel, and taxes imposed on alcohol for drinking can be avoided when it is used for other purposes if it is made undrinkable. The alcohol itself is not chemically altered, but additives, usually methanol, render it toxic.