In order to examine the internal dynamic processes of the dodecamer the 13C-enriched d(CGCAAATTTGCG) 2 , oligonucleotide was synthesized. The C3, A4 and A6 residues were selectively 13C labeled at the C1º and C5º positions and their and relaxation rates were measured. Data variations were observed
for the three residues. The analysis of the relaxation rates in the context of the model-free formalism of Lipari and Szabo indicates other pathways than those described by S2 and Direct evidence for the presence of a s g , s i . conformational exchange process was obtained for the residue A4 from rotating frame relaxation measurements. Taking into account the exchange process contribution, analysis of these data gave a value of 3.7 ns at 32 ÄC, in s g agreement with those computed from independent 13C relaxation rate measurements on the three thymines or from the H5-H6 NOE measurements. The frequency di †erence between the exchanging sites was computed in the range 130-250 Hz. This additional relaxation process involves large amplitude and slow motions ls) which can (s ex = 130 reÑect special dynamics of the tract AAATTT which supports the spine of hydration. Analysis of 13C relaxation rates indicated smaller order parameters S2 for C5º (0.40) than for C1º (0.80). These data reÑected additional motions undergone by the C5ºH5º or C5/H5/ vectors such as repuckering and motions around the C4ºÈC5º bond.