Solution structure of a selectively 13C-labeled intramolecular DNA triplex.

The three-dimensional structure of an intramolecular triple helix whose three strands have been linked by a hexaethylene glycol chain, and selectively 13C-enriched in position C1' on the third strand was investigated by NMR spectroscopy and constrained molecular mechanics calculations. Starting from different initial conformations, we show that the NOE constraints determined by the complete relaxation matrix calculation and iterative back-calculations allowed us to reach the same final restrained triple helix, taking into account implicitly the solvent effect. We conclude that this triplex adopted a B-type conformation rather than a A-type. The sugar pucker was found predominantly in the S-type conformation, in the range of C2'-endo geometry.