BACKBONE DYNAMICS OF A HIGHLY DISORDERED 131-RESIDUE FRAGMENT OF STAPHYLOCOCCAL NUCLEASE

In order to characterize the dynamic properties of the denatured state of staphylococcal nuclease, R(1), R(2), and NOE relaxation parameters have been measured for the backbone N-15 nuclei of a 131 residue frag ment that serves as a model of the denatured state under non-denaturin g conditions. The relaxation data indicate a wide range of amplitudes for segmental motion and are inconsistent with a random coil conformat ion. An optimal value of 7.8 ns was obtained for the molecular rotatio nal correlation time tau(m) based on the analysis of the 79 residues f or which R(1), R(2), and NOE relaxation data could be obtained. This v alue corresponds roughly to the slowest detectable motion on the nanos econd time scale and is of a magnitude consistent with global tumbling of a large portion of the molecule. For the majority of residues, exp erimental data could be described most adequately in terms of a modifi ed ''model-free'' formalism which includes contributions from internal motions on both an intermediate (tau(e)) and a fast time scale (tau(i )) in the context of slow overall tumbling (tau(m)). The generalized o rder parameter S-2, which gives the amplitude of motions on time scale s faster than tau(m), correlates with sequence hydrophobicity and sugg ests a relationship between chain flexibility and sequence propensity for hydrophobic collapse. The fractional populations of three alpha-he lices in the protein show a stronger correlation with S-2 values and h ydrophobicities than with intrinsic helix propensities. These observat ions suggest that secondary structure may be preferentially stabilized in hydrophobic segments of the sequence.