Figure 1.

(a) Various reference frames which define the SRLS model: L – laboratory frame, C – global diffusion frame associated with protein shape, C’ – local director frame fixed in the protein, M – local ordering/local diffusion frame fixed in the C-D bond, Q – quadrupolar tensor frame along the C-D bond. (b) Application to methyl dynamics. The simple case of motion about the rotation axis of the methyl group is illustrated. The equilibrium orientation of the CH₂D−C bond (C^α−C^β for alanine, C^β−C^γ1 and C^β−C^γ2 for valine, etc.) is taken as the local director, C’. The local ordering frame, M, is assumed in this illustration for simplicity to be axially symmetric. Z_M orients preferentially parallel to C’. It also represents the methyl rotation axis, and it is tilted relative to Z_Q, i.e., the C−D bond, at β_MQ = 110.5° (when r_CH = r_CD is set equal to 1.115Å − ref. 23). The angle β_C'M is the stochastic angle between the instantaneous orientation of the M frame, Z_M, and its equilibrium orientation, C’.