Capping B2 containing 40% pyridine in acetonitrile is used as a capping reagent component during the chemical synthesis of DNA and RNA oligonucleotides by the phosphoramidite method.

The purpose of Capping B2 is to provide pyridine as a base and reaction-promoting component during the acetylation of unreacted hydroxyl groups on the growing oligonucleotide chain.

Capping B2 is a solution containing approximately 40% pyridine in acetonitrile with acetonitrile serving as the solvent. Pyridine functions primarily as a base and helps facilitate the capping reaction when used with acetic anhydride.