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In chemistry, protonation, is the addition of a proton (H+) to an atom, molecule, or ion. Protonation is possibly the most fundamental chemical reaction and is a step in many stoichiometric and catalytic processes. Some ions and molecules can undergo more than one protonations or deprotonations; such species are labeled polybasic. Most biological macromolecules are polybasic.
Upon protonating a substrate, the mass and the charge of the species each increase by one unit. Protonating or deprotonating a molecule or ion alters many chemical properties beyond the change in the charge and mass: hydrophilicity, reduction potential, optical properties, among others. Protonation is also an essential step in certain analytical procedures such as electrospray mass spectrometry.
Protonation and deprotonation occur in most acid-base reactions; they are the core of most acid-base reaction theories. A Bronsted-Lowry acid is defined as a chemical substance that protonates another substance.
Rates of protonation and deprotonation
Protonations are often rapid, in part because of the high mobility of the proton in water. The rate of protonation is related to the acidity of the protonated species. Weak acids deprotonate more slowly than strong acids. The rates of protonation and deprotonation can be especially slow when protonation induces significant structural changes.
Reversibility and catalysis
Usually, protonations are reversible and the conjugate base is unchanged by being protonated. In some cases, however, protonation causes isomerization. Cis-alkenes can be converted to trans-alkenes using a catalytic amount of protonating agent. Many enzymes, such as the serine hydrolases operate by mechanisms that involve reversible protonations of substrates.
Formally, the term "hydronation" is recommended by the IUPAC, just as "hydron" is recommended in place of "proton" because only 99.999% of natural hydrogen nuclei are protons; the rest are deuterons and rare tritons. However, the use of the terms "hydrons" and "hydronation" remains obscure.