Poly(dA-dT)•Poly(dA-dT) in Low Salt Appears to be a Left-Handed B-Helix Combined Use of Chemical Theory, Fiber Diffraction and NMR Spectroscopy

Abstract

Poly(dA-dT)•poly(dA-dT) can adopt the B- and D- forms in the fibrous state. Theoretical energy calculations and fiber diffraction analyses suggest that there can be three structural models of poly(dA-dT)•poly(dA-dT) in each of these two forms viz right and left-handed Watson Crick models and left-handed Hoogsteen—a total of six possible models. Fiber data for the polymer in the B- or the D-form or energy calculations cannot distinguish any one model from the other. However, a comparison of observed proton chemical shifts with the theoretically computed ones and the NOE studies on exchangeable and nonexchangeable protons suggest that poly(dA-dT)•poly(dA-dT) in low salt solution exists predominantly in the left-handed B-conformation.