monoprotonated

Monoprotonated refers to a chemical species that has gained exactly one proton (H+) relative to its neutral form. In this state, the molecule typically carries a single positive charge, resulting from protonation of a basic site such as an amine, a heteroaromatic nitrogen, or other ionizable groups. The term describes the first protonation step in molecules that can undergo multiple protonations, and it is distinct from diprotonated or polyprotonated forms, which have two or more added protons.

Common examples include the monoprotonated amine, R-NH3+, formed when an amine (R-NH2) accepts a proton. Similarly, protonation of a heteroaromatic nitrogen, as in pyridine to form the pyridinium ion (C5H5NH+), is a typical monoprotonation. In biochemistry and pharmacology, many amino groups in proteins and drugs exist in their monoprotonated forms at physiological pH, influencing charge, solubility, and intermolecular interactions. In mass spectrometry, monoprotonated species are frequently observed as the primary ions, described as [M+H]+ for a neutral molecule M.

The extent of monoprotonation depends on pH and the pKa value of the conjugate acid. At pH values below the pKa, the monoprotonated form is favored; above the pKa, the neutral form predominates. Some molecules can repeatedly protonate at multiple sites, leading to diprotonated or higher-charge species under strongly acidic conditions.

Monoprotonation is a central concept in acid–base chemistry, ionization processes, and molecular modeling, providing a simple, common state to describe proton-addition at a single reactive site.