Merck, L040050-500ML, Cap A

Application
Cap A solution is used after the phosphoramidite reaction/coupling step. This solution aborts chains bearing unreacted 5′hydroxyl groups due to failure of reaction with activated phosphoramidite (typically, 1 to 2% of growing oligonucleotide chains in each synthesis cycle). These unreacted 5′hydroxyl groups are capped with an acetyl group and thus rendered unreactive for subsequent synthesis steps.
Preparation Note
Acetonitrile / N-Methylimidazole 8/2 (v/v)
Other Notes
Cap A solution, containing acetonitrile (ACN) and N-methylimidazole (NMI), is mixed in situ with Cap B1 and Cap B2 solution during the capping (acetylation) reaction. Pyridine is applied as a mild base while NMI provides a powerful acylation catalyst.
Legal Information
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