MOLECULAR MOTIONS IN RNA AND DNA INVESTIGATED BY P-31 AND C-13 NMR RELAXATION

The NMR relaxation behavior of carbon-13 and phosphorus-31 has been investigated to elucidate the conformational fluctuations of RNA and DNA. Measurements of the spin-lattice relaxation time (T1), 31P {1H} nuclear Overhauser effect, line width, and rotating frame spin-lattice relaxation in an off-resonance radiofrequency field (T1ρoff) were performed for phosphorus-31 in poly(A), poly(I)-poly(C), and calf thymus DNA. A two correlation time model entailing anisotropic rotational wobbling about phosphorus-oxygen bonds in the nucleic acid backbone and long-range bending motions was capable of simultaneously fitting all four 31P relaxation parameters quite well at all temperatures in the range examined, 6-40°C. The rotational wobbling motion has a correlation time of 0.3-0.5 ns for all three nucleic acids and is independent of temperature or salt concentration. The long-range bending motion, however, does depend on temperature, presence or absence of magnesium ion, and single- or double-stranded character of the nucleic acid; the correlation time is on the order of microseconds. Carbon-13 spin-lattice relaxation time and 13C {1H} nuclear Overhauser effect measurements were made for calf thymus DNA. Although the base moiety appears to be immobilized, the ribose moiety possesses a considerable amount of internal mobility. The internal motion correlation times for the ribose carbons are on the order of nanoseconds with motional freedom increasing from 1′, 3′, 4′ < 2′ < 5′, which displays nearly the mobility of the phosphate moiety. © 1979 American Chemical Society.