The backbone dynamics of uniformly 15N-labeled interleukin-1 beta are investigated by using two-dimensional inverse detected heteronuclear 15N-1H NMR spectroscopy. 15N T1, T2, and NOE data at a spectrometer frequency of 600 MHz are obtained for 90% of the backbone amide groups. The data provide evidence for motions on three time scales. All the residues exhibit very fast motions on a time scale of approximately less than 20-50 ps that can be characterized by a single-order parameter with an average value of 0.82 +/- 0.05. For a model comprising free diffusion within a cone, these residue-specific order parameters translate to an average cone semiangle of 20.7 +/- 3.3 degrees. Thirty-two residues also display motions on a time scale of 0.5-4 ns, slightly less than the overall rotational correlation time of the protein (8.3 ns). These additional motions must be invoked to account for the discrepancy between experiment and the simplest theoretical formulation in which the internal motions are described by only two parameters, a generalized order parameter and an effective correlation time [Lipari, G., & Szabo, A. (1982a) J. Am. Chem. Soc. 104, 4546-4559]. In particular, while the simple formulation can account for the 15N T1 and T2 data, it fails to account for the 15N-1H NOE data and yields calculated values for the NOEs that are either too small or negative, whereas the observed NOEs are positive. With the introduction of two internal motions that are faster than the rotational correlation time and differ in time scales by at least 1-2 orders of magnitude [Clore, G. M., Szabo, A., Bax, A., Kay, L. E., Driscoll, P. C., & Gronenborn, A. M. (1990) J. Am. Chem. Soc. 112, 4989-4991], all the relaxation data for these 32 residues can be fitted by two order parameters and an effective correlation time for the slower of the two internal motions. A simple model for these two motions is one in which the very fast motion involves axially symmetric diffusion within a cone, while the slower motion comprises jumps between two different orientations of the NH vector. For such a model the jump angle (excluding the C-terminal residue) ranges from 15 degrees to 69 degrees with a mean value of 28.6 +/- 14.0 degrees. Another 42 residues are characterized by some sort of motion on the 30-ns-10-ms time scale, which results in 15N line broadening due to chemical exchange between different conformational substates with distinct 15N chemical shifts.(ABSTRACT TRUNCATED AT 400 WORDS)