SPECTRAL DENSITY-FUNCTION MAPPING USING N-15 RELAXATION DATA EXCLUSIVELY

A method is presented for the determination of values of the spectral density function, J(omega), describing the dynamics of amide bond vectors from N-15 relaxation parameters alone. Assuming that the spectral density is given by the sum of Lorentzian functions, the approach allows values of J(omega) to be obtained at omega=0, omega(N), and 0.870 omega(H), where omega(N) and omega(H) are Larmor frequencies of nitrogen and proton nuclei, respectively, from measurements of N-15 T-1, T-2 and H-1-N-15 steady-state NOE values at a single spectrometer frequency. Alternatively, when measurements are performed at two different spectrometer frequencies of i and j MHz, J(omega) can be mapped at omega=0, omega(N)(i), omega(N)(j), 0.870 omega(H)(i) and 0.870 omega Hj, where omega Ni, for example, is the N-15 Larmor frequency for a spectrometer operating at i MHz. Additionally, measurements made at two different spectrometer frequencies enable contributions to transverse relaxation from motions on millisecond-microsecond time scales to be evaluated and permit assessment of whether a description of the internal dynamics is consistent with a correlation function describing the dynamics of the N-15-NH bond vector are necessary, provided that dJ(omega)/d omega is relatively constant between omega=omega(H) + omega(N) to omega=omega(H)-omega(N). Simulations demonstrate that the method is accurate for a wide range of protein motions and correlation times, and experimental data establish the validity of the methodology. Results are presented for a folded and an unfolded form of the N-terminal SH3 domain of the protein drk.