Hydrophilic modifications in peptide nucleic acid — Synthesis and properties of PNA possessing 5-hydroxymethyluracil and 5-hydroxymethylcytosine

We have investigated the chemistry for the incorporation of C5-hydroxymethyl-uracil and -cytosine in peptide nucleic acid (PNA) and the subsequent effect of this modification on PNA hybridization behavior. Largely based on literature precedent, we prepared a peptide nucleic acid monomer, possessing 5-hydroxymethyuracil, which was compatible with Fmoc-based oligopeptide synthesis. An improved, large-scale synthesis of 5-hydroxymethylcytosine was developed, as a starting point for the synthesis of a monomer containing this nucleobase. In each case, the hydroxyl group was blocked as a t-butyldiphenylsilyl ether, and the exocyclic amino group of cytosine was additionally blocked with the benzoyl-group. The modified monomers were incorporated into isolated positions in the oligomer sequence using standard protocols. The modified oligomers showed that the 5-hydroxymethyl group is compatible with triplex and duplex formation.