The study of protein internal motions from analysis of 13C and 15N NMR relaxation data and auto- and cross-correlation spectral densities is being pursued in many labs. Model-free approaches and derived motional order parameters are normally used to interpret NMR relaxation data and internal mobility in proteins and peptides. Correlated motions can substantially modify the behavior of NMR auto- and cross-correlation spectral density functions and the values of derived motional order parameters. Here, a simple model is proposed to describe small amplitude (less than about 60° or 1 rad), internally restricted correlated rotations (IRCR) in peptides and proteins in order to analyze order parameters. Bond rotations are represented by vectors whose motions are correlated by a correlation coefficient, cij, which is the cosine of the angle between these vectors. Order parameters for NH, CαH and CβH bond motions have been calculated from molecular dynamics simulations performed on short peptides with well-defined α-helix and β-sheet structures in order to derive values for cij. General equations relating dipolar auto- and cross-correlation order parameters for CαH, CβH, and NH bonds to cij have been derived. The sign of cij depends on the specific motional correlation within a particular molecular conformation. For glycine φ, ψ rotations, the sign of cij can be derived from analysis of dipolar auto- and cross-correlation order parameters. Long-range motional correlations are observed with hydrogen bonds modulating internal mobility. In general, backbone NH order parameters, S2NH, are more sensitive to structure than are CαH order parameters, S2CH. S2CH is increased and S2NH is decreased when correlation coefficients cψ′φ and cφψ are decreased and increased, respectively.
|Original language||English (US)|
|Number of pages||11|
|Journal||Journal of physical chemistry|
|State||Published - May 16 1996|