Backbone dynamics of trp repressor, a 25 kDa DNA binding protein, have been studied using 15N relaxation data measured by proton-detected two-dimensional 1H-15N NMR spectroscopy. 15N spin-lattice relaxation time (T1), spin-spin relaxation time (T2), and heteronuclear NOEs were determined for all visible backbone amide 15N nuclei. Monte Carlo simulations of the amplitudes of backbone motions led to the conclusion that a wobbling in a cone model with consideration of the anisotropic reorientation of the molecule was appropriate to describe the underlying motions, allowing us to derive the semiangle of the cone (alpha) and the effective correlation time for internal motions (tau e) for each N-H bond vector. The final optimized rotational diffusion coefficients parallel (D parallel) and perpendicular (D perpendicular) to the unique axis of the molecule were found to be 1.48 +/- 0.06 x 10(7) and 1.15 +/- 0.05 x 10(7) s-1, respectively. The average semiangle of the cone (alpha) describing the amplitude of NH vector motions on the picosecond time scale was found to be 20.9 +/- 5.7 degrees. Large amplitude motions on the picosecond time scale are found at both the N and C termini but are restricted in both the hydrophobic core and DNA-binding regions.