reprotonated

Reprotonated is a chemical descriptor applied to a species that has regained a proton after previously losing one, i.e., the reverse of deprotonation. The term is commonly used in acid–base chemistry and biochemistry to describe the protonation state of molecules.

In solution, reprotonation occurs when a conjugate base accepts a proton from an acid or solvent. The extent of reprotonation depends on pH and the pKa of the functional group. For bases, the equilibrium B + H+ ⇌ BH+ governs reprotonation; the ratio [BH+]/[B] is determined by pH relative to pKa, with lower pH favoring the protonated form. For acids with a removable proton, HA ⇌ A− + H+, reprotonation of A− restores the neutral acid. The protonation state affects charge, reactivity, and hydrogen bonding.

In analytical techniques such as mass spectrometry, reprotonation can occur during ionization or ion transfer, producing protonated species [M+H]+. In proteomics and metabolomics, protonation state influences ionization efficiency and fragmentation patterns. In biochemistry, protein active sites and ligand binding can be sensitive to reprotonation of key residues like histidine, lysine, or aspartate.

Common examples include amines (R3N + H+ → R3NH+), carboxylates (RCOO− + H+ → RCOOH), and phosphate groups (e.g., phosphate diester or phosphate monoester). Reprotonation is a dynamic process that contributes to the pH-dependent behavior of many molecules and to the function of biological systems and pharmaceutical agents.