Backbone dynamics of the natively unfolded pro-peptide of subtilisin by heteronuclear NMR relaxation studies

The dynamics of the natively unfolded form of the pro-peptide of subtilisin (PPS) have been characterized at two different pHs (6.0 and 3.0) by N-15 r elaxation experiments. N-15 relaxation data is obtained at multiple field s trengths and a detailed comparison of spectral density mapping, the model f ree approach and the recently proposed Cole-Cole model free (CC-MF) analysi s is presented. The CC-MF analysis provides a better fit to the observed ma gnetic field dependence of N-15 relaxation data of unfolded PPS than conven tional model free approaches and shows that fluctuations in R-2 may be acco unted for by a distribution of correlation times on the nanosecond timescal e. A new parameter epsilon derives from the analysis and represents the wid th of the distribution function and the heterogeneity of the dynamics on th e nanosecond timescale at a particular site. Particularly interesting is th e observation that epsilon is sensitive to pH changes and that PPS samples a wider distribution of nanosecond time scale motions at less acidic pHs th an at more acidic pHs. These results suggest that PPS experiences a higher degree of correlated motion at pH 6.0 and that electrostatic interactions m ay be important for inducing correlated motions on the nanosecond timescale in unfolded PPS.