Cap A

Cap A solution consists of acetic anhydrideanhydride, lutidine, and tetrahydrofuran (THF).

Cap A solution is used after the phosphoramidite reaction/coupling step.

Cap A 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.