Knowledge about protein dynamics is crucial for the understanding of protein function.[1] NMR spectroscopy can characterize the amplitudes and rates of motions that are either faster than the rotational correlation time t c (sub-t c motion) with heteronuclear relaxation experiments or between approximately 50 ms and 10 ms (ms/ms motion) with relaxation dispersion. [2, 3] The extent of motions occurring in folded proteins on the time scale between the rotational correlation time t c and the ms/ms range (supra-t c motion) has been a matter of debate.[4] However, the functional relevance of such motions has recently been shown for the aggregation rate of natively unfolded proteins involved in neurodegenerative diseases.[5] Very recently, motions on this supra-t c time scale have been observed in a 0.2 ms molecular dynamics simulation of ubiquitin. [6] Residual dipolar couplings (rdcs) were recognized early on as an ideal tool to widen the time window of dynamics that can be characterized by NMR spectroscopy, since they are sensitive to motional averaging occurring over the sub-and supra-t c time scales (ps to ms).[4] We recently analyzed NH rdcs of ubiquitin measured in 31 different alignment conditions and derived the order parameters , 7] To differentiate between the sub-and supra-t c time scale, these order parameters were compared to Lipari-Szabo order parameters S 2 LS that are derived from conventional relaxation time measurements and that are only sensitive to the sub-t c time scale. [8,9] Interestingly, a periodic variation of the S 2 rdc value can be observed with a periodicity of two residues in the b strands of ubiquitin (amino acids 2-6, 12-16, 41-45, 66-71), while it is largely absent from the S 2 LS and exchange-rate data. Most prominently, the S 2 rdc values are larger for residues Gln 41, Leu 43, and Phe 45, and smaller for residues Arg 42 and Ile 44 in the b strand 41-45 (Figure 1). Correspondingly, the side chains of residues Gln 41, Leu 43, and Phe 45 point towards the hydrophobic core (core residues), whereas the side chains of Arg 42 and Ile 44 are exposed to solvent (exposed residues). Thus, the alternating pattern of the S 2 rdc values seems to correlate with the orientation of the side chains towards the solvent or away from it (Figure 2).The observation that solvent-exposed residues exhibit reduced S 2 rdc values relative to core residues holds not only for the b strands but also for the rest of the protein. The amino acids of ubiquitin (1d3z) are color coded in Figure 3 according to the S 2 rdc value of the backbone amide groups. Residues with less-mobile NH vectors (blue and green) have side chains predominantly pointing towards the hydrophobic core (Figure 3 a), while for those with more-mobile NH vectors (yellow, orange, and red), the side chains are solvent-exposed (Figure 3 b).To investigate this effect quantitatively, an order parameter for only the supra-t c time scale is derived:. For residues with supra-t c motions, we expect a value for S