ESR behaviour of spin-labelled oligoDNAs

Oligodeoxyribonucleotides (oligoDNAs) were spin-labelled at the 5'-end internucleotide linkage, directly or through the spacers of different lengths, with TEMPO and the labelled oligoDNAs were explored for the ESR spectroscopical behaviour in solution. Oxidation of H-phosphonate intermediates in CCl4 solutions of TEMPO and diamines was taken for the introduction of TEMPO and spacers, respectively. ESR lines of TEMPO attached to oligoDNAs, directly or through spacers, were already broadened compared to those of free TEMPO, and the broadening decreased with increasing chain length of the spacer and increased with increase in the chain length of the oligoDNAs. The ESR lines of the oligoDNA labelled directly were further broadened in the presence of a DNA complementary to the labelled oligoDNA. In addition, the broadened lines were further broadened in the presence of a larger DNA. This observation implies that spin labelling at the internucleotide linkage is useful in the DNA probe method to avoid B/F separation.