One of the most useful bits of information that can be obtained from structure–activity relationship studies is the types of atoms and functional groups that are important in binding a drug to its target binding site. Ionic bonds are formed between groups of opposite charge. Many of the body’s natural chemical messengers contain an amine functional group, which is ionized at body pH. Hydrogen bonding involves an interaction between two molecules, where one of the molecules acts as a hydrogen bond donor and the other acts as a hydrogen bond acceptor. The hydrogen bond donor contains a functional group which has a proton attached to an electronegative atom. The dipole moment of a molecule represents the relative orientation of electron density. Drugs having a dipole moment are likely to align with localized dipole moments in the binding site such that the dipole moments are parallel and in opposite directions.