Dynamic Exchange Incorporated into the Slowly Relaxing Local Structure Model

Recent multifrequency ESR studies of spin labeled mutants of T4-lysozyme have shown the existence of two spectral components that have been tentatively ascribed to the existence of more than one stable conformer of the nitroxide tether of the MTSSL spin label. These multifrequency experiments have previously been analyzed by the slowly-relaxing local structure (SRLS) model that includes as distinct entities the overall motion of the protein and the local motion sensed by the spin label. Results at the different frequencies have suggested that there may be a dynamic exchange occurring between the two conformers, such that the spectral effects of the exchange depends on the ESR frequency studied.

Dynamic exchange has now been incorporated into the SRLS model, wherein each conformer has its own overall and local rotational dynamics and orienting potential. The exchange process is characterized by the probability of each site and the mean exchange rate. We illustrate the effects of dynamic exchange for the case of two sites, wherein both sites exhibit very slow overall tumbling, but Site 1 exhibits relatively fast local motion and low local ordering, whereas Site 2 exhibits slow local motion and high local ordering. Due to their different time scales, ESR spectra taken at the higher (lower) frequencies are sensitive to faster (slower) exchange processes, as is illustrated for the cases of 9, 95, and 250 GHz.