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 fi nal stages of protein folding, we have used neutron scattering to probe the differences between a protein in its folded state and the molten globule s tates. The internal dynamics of bovine alpha-lactalbumin (BLA) and its molt en globules (MBLA) have been examined using incoherent, quasielastic neutro n scattering (IQNS). The IQNS results show length scale dependent, pico-sec ond dynamics changes on length scales from 3.3 to 60 Angstrom studied. On s horter-length scales, the non-exchangeable protons undergo jump motions ove r potential barriers, as those involved in sidechain rotamer changes. The m ean potential barrier to local jump motions is higher in BLA than in MBLA, as might be expected. On longer length scales, the protons undergo spatiall y restricted diffusive motions with the diffusive motions being more restri cted in BLA than in MBLA. Both BLA and MBLA have similar mean square amplit udes of high frequency motions comparable to the chemical bond vibrational motions. Bond vibrational motions thus do not change significantly upon fol ding. Interestingly, the quasielastic scattering intensities show pronounce d maxima for both BLA and MBLA, suggesting that "clusters" of atoms are mov ing collectively within the proteins on picosecond time scales. The correla tion length, or "the cluster size", of such atom clusters moving collective ly is dramatically reduced in the molten globules with the correlation leng th being 6.9 Angstrom in MBLA shorter than that of 18 Angstrom 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. ( C) 2000 Academic Press.