Correlated motions of C'-N and C α -C β pairs in protonated and per-deuterated GB3.

We investigated correlated µs-ms time scale motions of neighboring 13 C'-15 N and 13 Cα -13 Cβ nuclei in both protonated and perdeuterated samples of GB3. The techniques employed, NMR relaxation due to cross-correlated chemical shift modulations, specifically target concerted changes in the isotropic chemical shifts of the two nuclei associated with spatial fluctuations. Field-dependence of the relaxation rates permits identification of the parameters defining the chemical exchange rate constant under the assumption of a two-site exchange. The time scale of motions falls into the intermediate to fast regime (with respect to the chemical shift time scale, 100-400 s-1 range) for the 13 C'-15 N pairs and into the slow to intermediate regime for the 13 Cα -13 Cβ pairs (about 150 s-1 ). Comparison of the results obtained for protonated and deuterated GB3 suggests that deuteration has a tendency to reduce these slow scale correlated motions, especially for the 13 Cα -13 Cβ pairs.