Combined analysis of (15)N relaxation data from solid- and solution-state NMR spectroscopy.

Nanosecond time-scale backbone dynamics in proteins has been a subject of much interest. While these motions can be detected by solution N relaxation methods, they tend to be masked by the overall protein tumbling. As pointed out by Chen et al., “sometimes nanosecond time scale motions with a corresponding squared order parameter as low as 0.9 can go undetected, even with good quality data available at two magnetic fields”. The situation can be improved if relaxation data are augmented by residual dipolar couplings (RDCs). This approach, however, is experimentally demanding and the interpretation is complicated by ‘structural noise’, lack of absolute reference, and possible coupling between internal dynamics and alignment. Solid-state methods, on the other hand, are well-suited to detect nanosecond motions. In the absence of overall tumbling, slower forms of internal dynamics provide the most efficient channel of spin relaxation. However, because of experimental limitations it has been difficult to obtain a definitive picture of protein dynamics from the solid-state data alone. For instance, a wide range of effective correlation times, from hundreds of picoseconds to hundreds of nanoseconds, have been reported in the solid-state studies of proteins. In this communication we undertake a combined analysis of the solidand solution-state relaxation data from a small globular protein, α-spectrin SH3 domain (spc SH3). It is common knowledge that structures of globular proteins as determined in solids and in solution are essentially identical. In fact, crystallographic structures provide the best models for analyzing solution NMR data. Crystal contacts, which involve fluid-like layers formed by outward-pointing side chains, have only limited impact. Taking this notion a step further, we suggest that internal protein dynamics in solids and in solution are also similar (assuming that the solid sample is well hydrated and the measurements are conducted at the same temperature). Note the parallel with the RDC studies where it is also postulated that the interaction with environment does not alter native protein dynamics. The combined analysis uses N 1 R , 2 R , and NOE data (500, 600 MHz) measured in solution, as well as N 1 R rates (600, 900 MHz) measured in solid, see Figure 1. The solution data were recorded using well-established experiments; the solid-state data were obtained using an HSQC-style sequence (Figure S1, Supporting Information (SI)) applied to the deuterated sample with a 10% content of amide protons. In the latter case, deuteration allows for H detection at high resolution, alleviates problems arising from proton-driven spin diffusion, and avoids extensive probe heating caused by proton decoupling. As a first step toward data interpretation, the solution data at two fields were analyzed by means of the 2 1 / R R approach 11