Deuterium NMR in the solid-state and in solution of the molecular motion of the bases in poly(I) and poly(I)·poly(C)

Abstract

To provide information regarding the confirmational flexibility of nucleic acids, in particular the rate and amplitude of base motions, we have observed the deuterium NMR from single-stranded and double-stranded polynucleotides. Poly(I) was deuterated at the 8-position of the base, and the deuterium NMR was examined in solution (at 23.0 and 55.4 MHz) and for hydrated and dry fibers (at 23.0 MHz). In the solid state, the deuterium signal of dry poly(I) exhibits a powder pattern with the maximal expected quadrupolar splitting, while the relatively short spin-lattice relaxation time indicates the prescence of a rapid internal reorientation of the C-D bond with an amplitude of that motion of at least ±2.4 degrees. Hydrating the poly(I) fibers to the extent of eight molecules of water per nucleotide results in the disappearance of the deuterium signal, apparently due to a decreased spin-spin relaxation time shorter than the instrumental dead-time (even using the quadrupolar echo technique); this could occur if conformational fluctuations are occuring at a rate comparable to the deuterium quadrupole interaction strength, i.e., 175 kHz. In solution, a theoretical fit to the measured Lorentzian linewidths and spin-lattice relaxation times necessitates the inclusion of at least two motional correlation times, with a subnanosecond internal motion. Double-stranded poly(I)·poly(C) yielded a solid state spectrum similar to poly(I), albeit with a longer T 1, which reduced the lower limit for the amplitude of an internal motion to ±1.9 degrees. The 2H signal from the poly(I)·poly(C), hydrated to a degree of approx. eight molecules of water per base pair, retained its solid-state lineshape (with a reduced T 1 value, indicating increased internal mobility of the bases with a lower limit on amplitude of ±4.7°). In solution, however, the 2H-NMR signal from poly(I)·poly(C) became virtually undetectable, even in solid-echo experiments, when the echo was observed after 52 μs. This indicates that the spin-spin relaxation time of the deuterium nucleus must be close to its theoretical minimum of about 9 μs, and the correlation time for an isotropic reorientation of the C-D vector can be estimated to be between 0.2 and 200μs.