SELECTIVELY C-13-ENRICHED DNA - C-13 AND H-1 ASSIGNMENTS OF A TRIPLE-HELIX BY 2-DIMENSIONAL RELAYED HMQC EXPERIMENTS

We present NMR studies of an intramolecular triple helix, the three strands of which have been linked by a hexaethylene glycol chain. To overcome the generally encountered difficulties of assignment in the homopyrimidine strands, the carbon C1' of the pyrimidines were selectively C-13-enriched. Assignments of the aromatic and sugar protons were obtained from NOESY-HMQC and TOCSY-HMQC spectra. We show that the recognition of a DNA duplex by a third strand via tripler formation is easily carried out in solution by observing the changes of the (1)H1'-(13)C1' connectivities as a function of pH. Furthermore, the conformation of the sugars has been found to be C2'-endo, on the basis of the coupling constant values directly measured in an HSQC spectrum.