Selectively 13C-enriched DNA: 13C and 1H assignments of a triple helix by two-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 homo-pyrimidine strands, the carbon Cl' of the pyrimidines were selectively 13C-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 triplex formation is easily carried out in solution by observing the changes of the 1Hl'-13Cl' 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.