Ionization

Ionization Definition:

• Ionization refers to the process by which a molecule gains or loses protons, leading to the formation of charged species (ions).
• The extent of ionization depends on the pKa of the compound and the pH of the surrounding environment.

Importance in Biological Action:

1. Drug Absorption:

   • Non-ionized (neutral) drugs cross biological membranes (lipid bilayers) more easily.
   • Ionized drugs tend to dissolve in water and remain in the aqueous phase.

2. Bioavailability:

   • The ratio of ionized to non-ionized form determines oral bioavailability.
   • Weak acids are better absorbed in the stomach (low pH), while weak bases are absorbed in the intestines (higher pH).

3. Distribution:

   • Ionized drugs are more soluble in blood and less likely to cross the blood-brain barrier.

4. Excretion:

   • Ionized drugs are more easily excreted by the kidneys since they are less likely to be reabsorbed in renal tubules.

Example:

• Aspirin (Acetylsalicylic Acid): A weak acid (pKa ~3.5), mostly non-ionized in the acidic stomach, enabling absorption. However, in the alkaline intestine, it ionizes and absorption is reduced.

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