which is formed when two atoms share their electrons. Covalent bonds vary in strength and properties according to the elements involved in the bond. One method of classifying the elements, which join together to form bonds, is according to their electronegativity. The electronegativity value for an element provides a measure of its affinity for electrons and hence its ability to attract a negative charge or conversely to give up some electron density and hold a positive charge. The electronegativity of some of the more common elements in biology is shown in Table 1.1; the derivation of electronegativity values is discussed later in the chapter. Covalent bonds are usually formed between non-metals, with the more electronegative element in the bond attracting negative charge and creating a dipole (see Box 1.1). Bonds formed between metals and non-metals are ionic because of their large differences in electronegativity. In an ionic bond the electrons are not shared, the less electronegative element gives the more electronegative element its electron(s) and then they are held together in the bond by the attraction of the resultant opposite charges. A third type of bond is due to weak van der Waals forces which are strongest between bulky chemical groups. Figure 1.1 shows some examples of bond types using an example which approximates to the binding of adrenaline to a b2-adrenergic receptor where it interacts with several amino acid side chains on the G-protein. Collectively, the making and breaking of various bond types drives the machinery of living organisms. Thus, for instance, the binding of calcium ions to certain proteins causes a change in their shape which results in muscle contraction. Similarly, the binding of a biologically active molecule such as acetylcholine to its receptor protein on muscle cell changes the shape of the pores in the muscle cell membrane, allowing sodium ions to enter the cell.
Bond type and bond strength