5-Nitroindole as an universal base analogue

4-, 5- and 6-Nitroindole have been investigated and compared with 3-nitropyrrole as universal Bases in oligodeoxynucleotides. Of these the 5-nitroindole derivative was found to be superior giving higher duplex stability, and behaving indiscriminately towards each of the four natural Bases in duplex melting experiments. 3-Nitropyrrole, whilst not discriminating between the natural Bases, was found to lead to considerable destabilisation of the duplexes, particularly when multiple substitutions were made, in contrast to the 5-nitroindole nucleoside.