Capping A consisting of 20% N-methylimidazole (NMI) in acetonitrile (ACN) is primarily used as a reagent in the chemical synthesis of DNA and RNA oligonucleotides. It is commonly used in automated phosphoramidite-based oligonucleotide synthesizers during the capping step.

The purpose of Capping A is to facilitate the capping of unreacted hydroxyl groups on the growing oligonucleotide chain after the coupling reaction. N-methylimidazole acts as a base and catalyst promoting the acetylation reaction that prevents incomplete

Capping A is a clear liquid solution containing approximately 20% N-methylimidazole in acetonitrile which serves as the solvent. N-methylimidazole facilitates the reaction with an acetic anhydride-containing capping reagent while acetonitrile provides a suitable low-water reaction medium. The reagent is generally supplied for use as part of a capping reagent system in oligonucleotide manufacturing.