For each synthesis cycle up to 1 to 2 % of free 5-hydroxy groups remain after the phosphoramidite coupling step has been completed. By running a subsequent Capping step using an anhydride these free hydroxyl groups are converted to acetates and are hindered from further chain elongation and formation of long oligonucleotides with incorrect sequences. For optimal acetylation a solution of acetic anhydride in Tetrahydrofuran or acetonitrile (Capping A) will be mixed in situ during reaction with a catalytic acting solution of N-methylimidazole (Capping B). Additives such as pyridine and lutidine function as mild bases to enhance the efficiency of the capping reaction.

it is used as a mild capping reagent in oligonucleotide synthesis particularly when protecting sensitive nucleobase or other labile functionalities is important.

The main purpose of Capping A is to acetylate or otherwise chemically block oligonucleotide chains that failed to undergo the preceding coupling reaction.

The mixture is referred to as Ultramild because it provides effective capping under relatively mild conditions and is suitable for synthesis protocols in which harsh capping conditions could cause undesirable side reactions.