Theophylline acts as a Bronsted base. This is because it can donate a pair of electrons to a proton (H+). The nitrogen atom in the imidazole ring of theophylline is the most basic site and is responsible for its Bronsted basicity. When theophylline donates a pair of electrons to a proton, it becomes protonated and forms a positively charged species called a conjugate acid.

The Bronsted basicity of theophylline is important for its pharmacological activity. Theophylline is used as a bronchodilator, which means that it relaxes the muscles in the airways and makes it easier to breathe. This effect is thought to be due to the ability of theophylline to inhibit the action of an enzyme called phosphodiesterase, which breaks down cyclic adenosine monophosphate (cAMP). cAMP is a second messenger that is involved in the regulation of many cellular processes, including muscle contraction. By inhibiting the breakdown of cAMP, theophylline can increase the levels of cAMP in cells and this can lead to relaxation of the airway muscles.

The Bronsted basicity of theophylline is also important for its pharmacokinetics. Theophylline is absorbed from the gastrointestinal tract and distributed throughout the body. It is extensively metabolized by the liver and excreted in the urine. The Bronsted basicity of theophylline affects its absorption, distribution, and excretion. For example, the Bronsted basicity of theophylline makes it more likely to bind to proteins in the blood, which can reduce its distribution to tissues.