In chemistry,protonation (orhydronation) is the adding of aproton (orhydron, or hydrogen cation), usually denoted by H+, to anatom,molecule, orion, forming aconjugate acid.[1] (The complementary process, when a proton is removed from aBrønsted–Lowry acid, isdeprotonation.) Some examples include
Protonation is a fundamental chemical reaction and is a step in manystoichiometric andcatalytic processes. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biologicalmacromolecules. Protonation and deprotonation (removal of a proton) occur in mostacid–base reactions; they are the core of most acid–base reaction theories. ABrønsted–Lowry acid is defined as achemical substance that protonates another substance. Upon protonating a substrate, the mass and the charge of the species each increase by one unit, making it an essential step in certain analytical procedures such as electrospraymass spectrometry. Protonating or deprotonating a molecule or ion can change many other chemical properties, not just the charge and mass, for examplesolubility,hydrophilicity,reduction potential oroxidation potential, andoptical properties can change.
Protonations are often rapid, partly because of the high mobility of protons in many solvents. Therate of protonation is related to theacidity of the protonating species: protonation byweak acids is slower than protonation of the same base bystrong acids. The rates of protonation anddeprotonation can be especially slow when protonation induces significant structural changes.[2]
Enantioselective protonations are under kinetic control, are of considerable interest inorganic synthesis. They are also relevant to various biological processes.[3]
Protonation is usually reversible, and the structure and bonding of the conjugate base are normally unchanged on protonation. In some cases, however, protonation inducesisomerization, for examplecis-alkenes can be converted totrans-alkenes using a catalytic amount of protonating agent. Many enzymes, such as theserine hydrolases, operate by mechanisms that involve reversible protonation of substrates.