Figure 1.

Schematic of a molecular system undergoing inter-conversion between two conformations A and B with rate constants k_BA and k_AB for the forward and backward processes, respectively. L denotes a laboratory-fixed reference frame, M a reference frame rotating with diffusional tumbling, and I the frame of a vector of interest giving rise to an observable signal. Ω_LM specifies the orientation of the tumbling molecular frame relative to the laboratory-fixed frame. ${\mathfrak{D}}^{\epsilon }$ denotes the diffusion tensor of conformation ε, while Ω_εI specifies the orientation of the vector of interest relative to the associated principal axes, and Ω_AB specifies the rotation required to transform between the principal axis frames of states A and B. The orientation of the vector of interest in state ε relative to the molecular frame is given by ${\Omega }_{\text{MI}}^{\epsilon }$. Note that the resulting vector autocorrelation function depends only on ${\mathfrak{D}}^{\epsilon }$, Ω_AB, Ω_εI, and the rate constants for inter-conversion between the states.