Thymidine-Inosine Dimer Building Block for Reversible Modification of Synthetic Oligonucleotides.

Modification of synthetic oligonucleotides and DNA is widely used in many applications in the life sciences. However, in most cases, modified DNA cannot be restored to its native state. Here, we report the preparation of a thymidine-inosine dimer building block (TID) for oligonucleotide synthesis. The TID modification supports the functionalization of synthetic oligonucleotides, which can later be removed to restore the DNA strand to its native state. The TID unit allows for a wide spectrum of postsynthetic modifications of oligonucleotides through click chemistry, including conjugation with fluorescent tags and small molecules, preparation of branched oligonucleotide scaffolds, and anchoring to a solid support. Due to the modification of the thymine base, the TID unit reduces the stability of the DNA duplex. We found that the negative effect of internal TID modification on duplex stability does not exceed the same for a single base mismatch. As long as the TID modification is present in the DNA strand, it disrupts its natural functionality. The "caging" effect of TID in the template strand with respect to DNA polymerase was demonstrated in primer extension experiments. Traceless removal of the temporary functional group occurs through oxidative cleavage of the inosine subunit, resulting in the formation of a native DNA strand with the thymine base left at the cleavage site. An anthracene-modified dodecamer oligonucleotide and a branched oligonucleotide scaffold were used to study the cleavage of the reporter group or the oligonucleotide side strand, respectively. It was shown that aqueous tetramethylguanidine efficiently cleaves the oxidized inosine subunit of TID at 37 °C, forming the native DNA strand.