Proton and phosphorus nmr investigation of the conformational states of acid polyadenylic double helix

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Abstract
Proton and phosphorus nmr have been used to investigate the double‐helical structures of polyriboadenylic acid [poly(A)] formed in acidic solutions (pH < 6). The results obtained at low pH (∼4.5) are consistent with the model for the acid poly(A) double helix proposed by Rich [Rich, A., Davies, D. R., Crick, F. H. C. & Watson, J. D. (1961) J. Mol. Biol. 3, 71–86]. Other models that have been proposed are inconsistent with the nmr data. The nmr measurements have also been used to examine the conformation of poly(A) helix in the half‐protonated state. Although the base‐stacking arrangement of this state is similar to that observed in the more extensively protonated low‐pH state, the phosphate backbone conformation is different from that found in either the neutral or low‐pH structures.