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 vect ors from N-15 relaxation parameters alone. Assuming that the spectral density is given by the sum of Lorentzian functions, the approach allo ws values of J(omega) to be obtained at omega=0, omega(N), and 0.870 o mega(H), where omega(N) and omega(H) are Larmor frequencies of nitroge n 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 frequenc y. Alternatively, when measurements are performed at two different spe ctrometer 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, whe re omega Ni, for example, is the N-15 Larmor frequency for a spectrome ter operating at i MHz. Additionally, measurements made at two differe nt spectrometer frequencies enable contributions to transverse relaxat ion from motions on millisecond-microsecond time scales to be evaluate d and permit assessment of whether a description of the internal dynam ics is consistent with a correlation function describing the dynamics of the N-15-NH bond vector are necessary, provided that dJ(omega)/d om ega 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 experi mental data establish the validity of the methodology. Results are pre sented for a folded and an unfolded form of the N-terminal SH3 domain of the protein drk.