Which oligonucleotide modification results in decreased stability at lower pH levels?

• A. Methylation
• B. Hydroxymethylation
• C. Both methylation and hydroxymethylation
• D. No modification

Answer: (B)

The correct answer is based on the chemical properties of hydroxymethylation and its effect on oligonucleotide stability at lower pH levels. Hydroxymethylation introduces a hydroxymethyl group (-CH2OH) to the oligonucleotide, which can interact with protons in a more significant way than a simple methyl group would.

At lower pH, the increased concentration of protons can lead to the protonation of hydroxymethyl groups, making the oligonucleotide more susceptible to hydrolysis. This alteration in the oligonucleotide structure can lead to instability under acidic conditions, ultimately causing the oligonucleotide to break down more quickly compared to those that are solely methylated.

Methylation does not introduce a -OH group and does not exhibit the same sensitivity to lower pH levels. The stability of methylated oligonucleotides is generally retained across a broader pH range due to the lack of functional groups that can form hydrogen bonds or be protonated.

Thus, while hydroxymethylation increases susceptibility to destabilization under low pH conditions, methylation maintains stability, resulting in the conclusion that hydroxymethylation leads to decreased stability at lower pH levels.