Solution conformation and dynamics of the octadeoxy-nucleotide d(CACTAGTG)₂: a multinuclear n.m.r. relaxation study

The conformation and internal dynamics of the octadeoxynucleotide d(CACTAGTG)₂ have been examined by ¹H- and ¹³C-n.m.r. relaxation. All the non-exchangeable protons, the seven phosphate resonances, and most of the ¹³C resonances of the proton-bearing carbons have been assigned by conventional two-dimensional n.m.r. methods. The average conformations of each nucleotide have been determined using time-dependent one-dimensional n.O.e.'s and ³J_HH values derived from both NOESY and 2-quantum-filtered COSY experiments. All glycosidic torsion angles are anti, and in the range -95 to 125°, in which the pyrimidines have a significantly larger angle than the purines. All sugars were found mainly (> 80%) in the conformation range C-2′ endo to C-3′exo. The DNA fragment is within the B-family of conformations. The cytosine H-6-H-5 vectors move with an apparent correlation time of 3 ns at 25°. Cross-relaxation rate constants for the H-1′-H-2b vectors and some H-2a-H-2b and H-2a-H-3′ vectors were measured, from which order parameters were determined. The order parameters are all in the range 0.7-0.9, which is consistent with only moderate internal mobility on the sub-ns time scale. The {¹H}-¹³C n.O.e. and the spin-lattice relaxation rate constant show that the terminal residues are relatively more mobile than the internal residues, and that the C-2′-H and C-3′%z.sbnd;-H vectors move with order parameters of 0.6-0.75.