Temperature-dependent con formational transitions of deoxyoligonucleotides have been monitored by measuring 3 1 P chemical shifts, spin-lattice relaxation times ( T I ) , and :31P-IHI nuclear Overhauser enhancements (NOES). The measured NOE ranged from 30 to 80% compared to the theoretical maximum of 124'X1 fur a dipolar relaxation mediated by rapid isotropic rotation. The observed 3'3' phosphate diester :'lP T I showed a similar temperature dependence over the range 2-75Β°C for both double-and single-stranded oligonucleotides, and for dinucleotides. T h e results show that dipole-dipole interactions dominate the internucleotide phosphate relaxation rate in oligonucleotides. The same is true of terminal phosphate groups a t low temperature; but a t higher temperature another process, possibly due to contamination by paramagnetic ions, becomes dominant. The rotational correlation time T R calculated from the dipole-dipole relaxation rate of the internucleotide phosphate in d(pA), a t 16Β°C is T R = 5.0 X 10-lOsec, implying a Stokes radius for isotropic rotation of 7.6 A. T h e TI and NOE values for the double-helical octanucleotide d(pA):'pGpC(pT):j are consistent with dominance of dipole-dipole relaxation and isotropic rotation of a sphere of radius 14 A, a reasonable dimension for the double helix. Activation energies for the rotation of dinucleotides range from 4 to 6 kcalimol, close to the value of 4 kcal/mol expected for isotropic rotation. In order t o test the possible effect of internal motion of correlation time T(; on the results, we considered a model in which the nucleotide chain rotates about the P -0 bonds. Comparison of the calculation with our experimental results shows that internal motion with rg z sec, as found from other studies to be present for large nucleic acids, would not influence our TI and NOE values enough to be distinguished from isotropic rotation.
However, we can conclude that T(; cannot be as fast as sec, even for dinucleotides.