Diels-Alder cycloadditions in water for the straightforward preparation of peptide–oligonucleotide conjugates

The Diels-Alder reaction between diene-modified oligonucleotides and maleimide-derivatized peptides afforded peptide-oligonucleotide conjugates with high purity and yield. Synthesis of the reagents was easily accomplished by on-column derivatization of the corresponding peptides and oligonucleotides. The cycloaddition reaction was carried out in mild conditions, in aqueous solution at 37 degrees C. The speed of the reaction was found to vary depending on the size of the reagents, but it can be completed in 8-10 h by reacting the diene-oligonucleotide with a small excess of maleimide-peptide.