Direct evidence for a bimorphic structure of a DNA-RNA hybrid, poly(rA).poly(dT), at high relative humidity

31P solid state nuclear magnetic resonances of highly oriented poly(rA).poly(dT) fibers demonstrated that this hybrid undergoes a conformational transition induced by hydration at about 90% relative humidity, namely, the hybrid has a single backbone conformation of the A family below 87% and two distinct conformations above 92% relative humidity. The structural model (of the B-like form) proposed on the basis of the x-ray diffraction data of this hybrid by Zimmerman and Pheiffer (Zimmerman, S.B., and Pheiffer, B.H. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 78-82) was proved to be fully consistent with our NMR data.