H...N hydrogen bond lengths in double stranded DNA from internucleotide dipolar couplings.

The ratio of the internucleotide dipolar coupling and the corresponding one-bond imino 15N-1H dipolar coupling provides a measure for the N...H/H-N distance ratio. Measurements were carried out for a dodecamer, d(CGCGAATTCGCG)2, in which a C-G and an A-T basepair were uniformly enriched in 15N. When assuming H-bonds to be perfectly linear, dipolar data indicate time-averaged hydrogen bond lengths of 1.80 +/- 0.03 A for A-T and 1.86 +/- 0.02 A for C-G. When using H-bond orientations from high resolution X-ray data, H-bond lengths are about 0.1 A shorter.