The Exonuclease I (Exo I)-resistance Quality Control (QC) is an analytical assay used to verify the structural integrity, modification efficiency, or purity of synthetic single-stranded DNA (ssDNA) oligonucleotides.
Exonuclease I is an enzyme that specifically degrades linear single-stranded DNA in a 3'- to 5'-direction, releasing deoxyribonucleoside 5'-monophosphates. It requires a free, unmodified 3'-hydroxyl (3'-OH) terminus to initiate and continue digestion.
If an oligonucleotide is engineered to resist this degradation, the Exo I assay serves as a strict pass/fail or quantitative test to ensure the structural features are correct.
During the assay, an oligonucleotide sample is incubated with Exonuclease I under optimal buffer conditions, followed by analysis to see if the molecule survived.
Exo I (3'→5')
Linear ssDNA:
5'—[N1]—[N2]—[N3]—[N4]—OH 3' ——————> Degraded into single nucleotides
Modified or circular ssDNA:
5'—[N1]—[N2]—[N3]—[N4]—X 3' ——————> Protected (Intact Oligo)
Protocol
[1] Incubation: The oligonucleotide is treated with Exo I at 37°C.
[2] Quenching: The reaction is stopped (typically by heat inactivation at 80°C or by adding a chelating agent such as EDTA.
[3] Analysis: The remaining intact DNA is quantified or visualized using methods such as:
a) Capillary Electrophoresis (CE) or HPLC, for precise quantification.
b) PAGE (Polyacrylamide Gel Electrophoresis), for visual verification.
c) Fluorescence spectroscopy, if using a fluorophore-quencher pair or intercalating dyes.