Physicochemical Properties in Relation to Biological Action - Ionization, Solubility, Partition Coefficient, Hydrogen bonding : Pharmaguideline -->

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Physicochemical Properties in Relation to Biological Action - Ionization, Solubility, Partition Coefficient, Hydrogen bonding

Chemical compounds exert their therapeutic effect in response to the physical and chemical properties of biomolecules that are in contact with them.
  • Chemical compounds exert their pharmacological or therapeutic effect in response to the physical and chemical properties of biomolecules that are in contact with them.
  • Pharmacologically active organic medicinal agents possess the following physiochemical properties.


The ionized form gives the medication strong water solubility, which is necessary for effective drug-receptor binding interactions, whereas the non-ionized form allows the drug to traverse cell membranes. As a result, a suitable mix of ionized and non-ionized forms is required for improved pharmacodynamic and pharmacokinetic properties.


Because water makes up a large portion of all biological structures, all biochemical reactions rely on the dissolution of tiny molecules or the dispersion of macromolecular fragments in an aqueous phase. Cells are composed primarily of non-aqueous lipid structures, such as plasma membranes and organelle membranes, which can dissolve either polar or non-polar hydrophobic molecules. All physiologically and pharmacologically significant small molecules have one major physical feature in common: they are all soluble, since only in solution can they attach to cellular and subcellular structures that carry drug receptors, thus triggering pharmacological action.

In equilibrium with a solid solute, the concentration of dissolved solute determines the solubility of a substance at a certain temperature. The atoms and molecules of all organic substances are held together by various types of bonds (e.g., hydrogen bond, dipole-dipole, Vander Waals). These forces are important in solubility because solubility is governed by solvent-solvent, solute-solute, and solvent-solute interactions.

Improving solubility – methods
  • Complexation
  • Use of co – solvents
  • Employing surfactants
  • Structural modification

Partition coefficient

  • Lipophilicity refers to a drug's capacity to dissolve liquid phase.
  • Highly water-soluble medications cannot permeate lipid-rich organs such as the brain and other neural tissue, whereas compounds that are particularly lipophilic will be held in the initial site of loss, such as fat tissue, and will be unable to leave this site rapidly to reach the target.
  • The partition coefficient is among the numerous physiochemical factors that determine how drugs are transported and distributed as well as how they reach their targets.
  • The partition coefficient is the drug concentration equilibrium constant for a two-phase molecule.
  • Because partition coefficients in living systems are difficult to quantify. As a lipid phase, 1-octanol is typically used, and a pH 7.4 phosphate buffer is typically used as an aqueous phase. A measure of lipophilicity frequently relies on the logarithm of the partition coefficient P.

Hydrogen bonding

In polar bonds such as N-H, O-H, or F-H, the hydrogen atom interacts with an electronegative atom such as O, N, or F through dipole-dipole interaction. It is represented by O-H...B. Each molecule (or part of a molecule) is A-H, and B represents a part of a second molecule; the dotted line indicates the hydrogen bond. These three atoms are commonly found in a straight line, however, the angle AHB can depart from linearity by up to 20 degrees.

Hydrogen bonding is divided into two types: A. intermolecular hydrogen bonding and B. intramolecular hydrogen bonding.

Hydrogen bonding between molecules
Hydrogen bonding between two or more molecules of the same substance happens in this kind, resulting in the creation of a polymeric aggregate. Intermolecular hydrogen bonding raises the compound's boiling point and makes it more water soluble. Solubility of molecules may be improved by intermolecular hydrogen bonding with water. While ethanol and dimethyl ether have the same molecular weight, ethanol has a higher boiling point and is more soluble in water than dimethyl ether.

Intramolecular hydrogen bonding
Hydrogen bonding happens between two atoms of the same molecule in this kind. This sort of hydrogen bonding is known as chelation and is widespread in organic molecules. A ring with six or five members is sometimes formed by intramolecular hydrogen bonding.

Intramolecular hydrogen bonding reduces the compound's boiling point and solubility in water. Due to the intermolecular hydrogen bonding between ortho substituted groups, the melting and boiling points of the molecules are inhibited, since the chelation between them limits the possibility of intermolecular hydrogen bonding.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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