NMR relaxation-derived spectral densities provide information on molecular and internal motions occurring on the picosecond to nanosecond time scales. Using 13C and 15N NMR relaxation parameters [T1, T2, and NOE] acquired at four Larmor frequencies (for 13C: 62.5, 125, 150, and 200 MHz), spectral densities J(0), J(ωC), J(ωH), J(ωH + ωC), J(ωH - ωC), J(ωN), J(ωH + ωN), and J(ωH - ωN) were derived as a function of frequency for 15NH, 13CαH, and 13CβH3 groups of an alanine residue in an α-helixforming peptide. This extensive relaxation data set has allowed derivation of highly defined 13C and 15N spectral density maps. Using Monte Carlo minimization, these maps were fit to a spectral density function of three Lorentzian terms having six motional parameters: τ0, τ1, τ2, c0, c1, and c2, where τ0, τ1 and τ2 are correlation tunes for overall tumbling and for slower and faster internal motions, and c0, c1, and c2 are their weighting coefficients. Analysis of the high-frequency portion of these maps was particularly informative, especially when deriving motional parameters of the side-chain methyl group for which the order parameter is very small and overall tumbling motions do not dominate the spectral density function. Overall correlation times, τ0, are found to be in nanosecond range, consistent with values determined using the Lipari-Szabo model-free approach. Internal motional correlation times range from picoseconds for methyl group rotation to nanoseconds for backbone N-H, Cα-H, and Cα-Cβ bond motions. General application of this approach will allow greater insight into the internal motions in peptides and proteins.
Bibliographical noteFunding Information:
This work was supported by research grants from the National Science Foundation (MCB-9729539) and the National Institutes of Health (GM-58005). NMR instrumentation was made possible with funds from the NSF (BIR-961477) and the University of Minnesota Medical School.
- C and N relaxation
- Internal motions
- Spectral densities