A convenient synthesis of DNA fragments nitrogen‐15 labeled at the exocyclic cytosine amino group

A convenient method is described for the synthesis of oligodeoxynucleotides ¹⁵N labeled at the exocyclic amino group of cytosine. The 4‐triazolide derivative of deoxyuridine is used as a building block in the solid phase synthesis of the oligodeoxynucleotide. Deprotection and cleavage from the solid support is achieved by treatment with ¹⁵N labeled NH₄OH. This treatment also substitutes the 4‐triazolide group of the deoxyuridine by an ¹⁵N labeled amino group to yield 4‐¹⁵N cytosine. The method was applied to a DNA fragment corresponding to a glucocorticoid response element. The oligomer was characterised by ¹H‐¹⁵N coherence spectroscopy.