SOLUTION STRUCTURE OF A SELECTIVELY C-13-LABELED INTRAMOLECULAR DNA TRIPLEX

The three-dimensional structure of an intramolecular triple helix whos e three strands have been linked by a hexaethylene glycol chain, and s electively C-13-enriched in position C1' on the third strand was inves tigated by NMR spectroscopy and constrained molecular mechanics calcul ations. Starting from different initial conformations, we show that th e NOE constraints determined by the complete relaxation matrix calcula tion and iterative back-calculations allowed us to reach the same fina l restrained triple helix, taking into account implicitly the solvent effect. We conclude that this tripler adopted a B-type conformation ra ther than a A-type. The sugar pucker was found predominantly in the S- type conformation in the range of C2'-endo geometry.