Proton and phosphorus magnetic relaxation studies on the dynamic structure of single-stranded polyriboadenylic acid.

Abstract

AbstractProton and phosphorus‐31 nuclear spin‐lattice relaxation times (T1) have been measured with the Fourier‐transform method at 100 and 40.5 MHz, respectively, on single‐stranded polyriboadenylic acid (poly(A)) in a neutral D2O solution in the temperature range of 14–82°C. T1 minimum is observed around 35–45°C for H(8), H(1′), and phosphorus resonances. Rotational correlation times have been deduced from the T1 data, which indicate that the sugar–phosphate backbone as well as the base–sugar segment is undergoing rapid internal motion of 10−8–10−10 sec range. The molecular motion of the sugar–phosphate backbone as deduced from the phosphorus relaxation is well‐characterized by a single activation enthalpy of 8.1 kal/mole for the whole temperature range of 14–82°C. Activation enthalpies of similar magnitude have been obtained for the motion of the adenine–ribose moiety from H(8) and H(1′) relaxation. The relative magnitude of T1 for H(8) and H(1′) infers that the poly(A) nucleotide exists on the average as anti in the single‐stranded form. The phosphorus T1 value is consistent with a conformation such that both C(4′)–C(5′) and C(4′)–C(3′) bonds are nearly trans to their connected O–P bonds.