Capping B1 containing 40% acetic anhydride in acetonitrile is used as a capping reagent during the chemical synthesis of DNA and RNA oligonucleotides by the phosphoramidite method. It is typically used in automated oligonucleotide synthesizers together with Capping A.

The purpose of Capping B1 is to acetylate and permanently block unreacted hydroxyl groups on oligonucleotide chains after the coupling step. This prevents incompletely coupled sequences from continuing into subsequent synthesis cycles thereby reducing th

Capping B1 is a solution of approximately 40% acetic anhydride in acetonitrile with acetonitrile serving as the solvent. Acetic anhydride provides the acetylating agent that reacts with unreacted hydroxyl groups while Capping A containing N-methylimidazole facilitates the capping reaction. The reagent is a clear flammable liquid and must be handled according to the applicable safety and storage requirements.