Receptor-drug interactions bond types

In the broadest sense, these "bonds would include covalent, ionic, hydrogen, dipole-dipole, van der Waals, and hydrophobic interactions. Most drug-receptor interactions constitute a combination of the bond types listed in Table 1.1, most of which are reversible under physiological conditions. 

Electrostatic forces are due to the ionic charges residing on the molecules, which attract or repel each other. The macromolecular structures of the receptors and enzymes mean that there are a number of ionic charges to attract the oppositely charged drug molecules. The forces of electrostatic interactions are weaker than covalent bonding. Electrostatic interactions are more common in drug-receptor interactions. There are two types of electrostatic interactions ... 

Noncovalent bond. There is no formation of a shared electron pair. The bond is reversible and typical of most drug-receptor interactions. Since a drug usually attaches to its site of action by multiple contacts, several of the types of bonds described below may participate. 

It seems likely that biochemists will continue to use a more pragmatic and less comprehensive approach. In biochemical processes, two important features are the structures of substrate and product. The overall steric structure of the substrate (and not just the possession of some structural feature such as a double bond) is important in terms of binding to an enzyme or receptor. Since many enzymatic reactions are readily reversible, overall product structure is important for the same reason. Furthermore, since many enzymes make more than one type of stereodifferentiation, the use of the stereo-differentiating terminology of Izumi and Tai would be somewhat cumbersome. The overall steric structure of molecules (as opposed to isolated structural features) is also important in the area of drug-receptor interactions. 

According to the equation (21.2), ligand-receptor interactions are characterized by enthalpy-entropy compensation in which one term favors and the other disfavors binding. While enthalpic contributions include electrostatic, hydrogen bond, and Van der Waals interactions, entropic contributions arise from several sources. On one hand, the loss of flexibility upon binding has an important entropic cost, counterbalanced on the other hand by the displacement of ordered water molecules. This will be discussed in the next section as well as the various types of drug-receptor interactions. 

Covalent bonds are strong bonds. Actual bonds are formed between the interacting molecules via the sharing of electrons. Hence, this type of interaction is expected to provide long lasting effects although not many drug-receptor bonds are of this nature. 

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