Comparison of Solid-State Dipolar Couplings and Solution Relaxation Data Provides Insight into Protein Backbone Dynamics

Internal motions on the scale of 10 ns-10 µs fall in a “blind spot ” of solution NMR experiments traditionally used for studies of protein dynamics.1 Recently, a new experimental strategy has emerged to probe these motions. In this new approach, the generalized order parameters derived from conventional 15N relaxation data are compared with their counterparts extracted from 1HN-15N residual dipolar couplings.2,3 This method, however, is rather demanding in the part involving residual dipolar couplings (rDCs). The complete analysis of rDC requires a separation of structural and dynamic variables, as well as the variables pertaining to protein alignment in the orienting media. The success is dependent on the availability of accurate data from multiple aligned samples which need to be significantly different (orthogonal). Furthermore, the degree of alignment Aa proves to be “entangled” with the amplitude of the (axially symmetric) internal motion, whic