Poly(d2NH2A-dT): Two-Dimensional NMR Shows a B to A Conversion in High Salt

Poly(d2NH2A-dT) forms a structure in high salt that is clearly distinct from the B form present in low salt. Two-dimensional nuclear Overhauser effect (2D NOE) NMR spectra establish that the conformation of the high-salt form is not Z. Correlations of observed cross peaks in the 2D NOE spectra and estimated interproton distances of the common DNA conformations are consistent only with an A form or closely related structure. This interpretation is also consistent with the negative circular dichroic band observed in the high-salt form of poly(d2NH2A-dT) and in A-form ribohomopolymer helices containing 2NH2A.