Dynamics of bases in hydrated [d(CGCGAATTCGCG)]2

Solid-state 2H NMR spectroscopy has been used to investigate the dynamics of a DNA oligonucleotide with a defined sequence, [d(CGCGAATTCGCG)]2, which contains the EcoRI binding site. Quadrupole echo line shapes and spin-lattice relaxation times were obtained as a function of hydration on two different deuterated samples, both in the form of the Na salt. In one sample, the C8 protons of all purines in the self-complementary dodecamer were exchanged for deuterons. In the other sample, a specifically labeled thymidine (C6 deuterated) was synthetically incorporated at the seventh position (counting 5' to 3') in the sequence. The general trends for both samples were quite similar. At all levels of hydration, the data reveal the presence of a rapid, small-amplitude libration of the bases (tau c less than or equal to 1 ns, 6 degrees-10 degrees amplitude). At the higher hydration levels (80% relative humidity or higher), the results indicate the presence of a much slower motion (tau c approximately 10-100 microseconds), which at 80% relative humidity is of small amplitude (approximately 5 degrees) and at higher hydration levels may be of larger amplitude. There is no evidence for large-amplitude (greater than +/- 10 degrees) motion on a nanosecond or faster time scale under any hydration condition. The 2H NMR results were analyzed with a dynamical model which treats the oligonucleotide as a deformable filament and which can include collective torsional fluctuations. The slow motion observed at high hydration levels is attributed to the uniform twisting mode (of the entire helix).