NITROGEN-15-LABELED OLIGODEOXYNUCLEOTIDES .8. USE OF N-15 NMR TO PROBE HOOGSTEEN HYDROGEN-BONDING AT GUANINE AND ADENINE N7 ATOMS OF A DNA TRIPLEX

We have used [7-N-15]-labeled deoxyguanosine and deoxyadenosine to pro be the Hoogsteen hydrogen bonding in C+. GC and T . AT DNA triplexes. The tripler used in this work consists of a 21 base pair duplex with a 15 base third strand. Use of a tripler that is formed intermolecularl y, rather than one formed intramolecularly, enabled us to examine sepa rately the single strand, the duplex, and the tripler under the same c onditions of temperature and pH. The single strands and the duplexes e ach show only a single resonance, while the tripler samples show two r esonances at each intermediate temperature, one identical to that of t he duplex and another 6 to 9 ppm upfield, which we assign to the tripl er. At lower temperature or lower pH the intensity of the upfield (tri pler) resonance is increased, while at higher temperature or higher pH it is decreased. These results provide the first direct evidence for the Hoogsteen hydrogen bonding to the purine N7 atoms postulated for C +. GC and T . AT triplets. Moreover, the magnitude of the chemical shi ft changes seen for this DNA tripler suggests that N-15 NMR of appropr iately labeled DNA and RNA molecules may be able to identify the prese nce of Hoogsteen hydrogen bonding to a purine N7 in less well defined systems. This may be of particular importance for RNA structures, wher e a wider variety of tertiary interactions is present.