Deuterium relaxation and internal motion in solid Li-DNA

As part of an effort to explore the nature of the internal motion in solid polynucleotides, the spectral densities of motion J1(omega 0) and J2(2 omega 0) have been measured for oriented, partially hydrated samples of calf thymus Li-DNA deuterated in the guanine and adenine 8-positions. Both spectral densities increase with increasing hydration level, J1 is found to be 2-5 times larger than J2, and their frequency dependence appears to be omega-1 and omega-3/2, respectively. The large values of the ratio J1/J2 rule out any in-plane torsional motion as the dominant relaxation mechanism in these samples, but a drop in this ratio at high hydration levels (G13 H2O/nucleotide) may indicate increasing contributions from such torsional motion. Although a satisfactory fit to a particular motional model has yet to be achieved, our findings show that the librational motion of the C8-D bond at or below a hydration level of 10 H2O/nucleotide is approximately uniaxial, with correlation times for the motion in the range 0.2-3.0 microseconds.