H center dot center dot center dot 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 N-15-H-1 dipolar coupling provides a measure for the N . . .H/H -N distance ratio. Measurements were carried out for a dodecamer, d(CGCGAAT TCGCG)(2), in which a C-G and an A-T basepair were uniformly enriched in N- 15. When assuming H-bonds to be perfectly linear, dipolar data indicate tim e-averaged hydrogen bond lengths of 1.80 +/-0.03 Angstrom for A-T and 1.86 +/-0.02 Angstrom for C-G. When using H-bond orientations from high resoluti on X-ray data, H-bond lengths are about 0.1 Angstrom shorter.