A view of dynamics changes in the molten globule-native folding step by quasielastic neutron scattering

In order to understand the changes in protein dynamics that occur in the final stages of protein folding, we have used neutron scattering to probe the differences between a protein in its folded state and the molten globule states. The internal dynamics of bovine alpha-lactalbumin (BLA) and its molten globules (MBLA) have been examined using incoherent, quasielastic neutron scattering (IQNS). The IQNS results show length scale dependent, pico-second dynamics changes on length scales from 3.3 to 60 A studied. On shorter-length scales, the non-exchangeable protons undergo jump motions over potential barriers, as those involved in side-chain rotamer changes. The mean potential barrier to local jump motions is higher in BLA than in MBLA, as might be expected. On longer length scales, the protons undergo spatially restricted diffusive motions with the diffusive motions being more restricted in BLA than in MBLA. Both BLA and MBLA have similar mean square amplitudes of high frequency motions comparable to the chemical bond vibrational motions. Bond vibrational motions thus do not change significantly upon folding. Interestingly, the quasielastic scattering intensities show pronounced maxima for both BLA and MBLA, suggesting that "clusters" of atoms are moving collectively within the proteins on picosecond time scales. The correlation length, or "the cluster size", of such atom clusters moving collectively is dramatically reduced in the molten globules with the correlation length being 6.9 A in MBLA shorter than that of 18 A in BLA. Such collective motions may be important for the stability of the folded state, and may influence the protein folding pathways from the molten globules.