Dynamic Transition Associated with the Thermal Denaturation of a Small Beta Protein

Abstract

We studied the temperature dependence of the picosecond internal dynamics of an all- β protein, neocarzinostatin, by incoherent quasielastic neutron scattering. Measurements were made between 20°C and 71°C in heavy water solution. At 20°C, only 33% of the nonexchanged hydrogen atoms show detectable dynamics, a number very close to the fraction of protons involved in the side chains of random coil structures, therefore suggesting a rigid structure in which the only detectable diffusive movements are those involving the side chains of random coil structures. At 61.8°C, although the protein structure is still native, slight dynamic changes are detected that could reflect enhanced backbone and β-sheet side-chain motions at this higher temperature. Conversely, all internal dynamics parameters (amplitude of diffusive motions, fraction of immobile scatterers, mean-squared vibration amplitude) rapidly change during heat-induced unfolding, indicating a major loss of rigidity of the β-sandwich structure. The number of protons with diffusive motion increases markedly, whereas the volume occupied by the diffusive motion of protons is reduced. At the half-transition temperature ( T = 71°C) most of backbone and β-sheet side-chain hydrogen atoms are involved in picosecond dynamics.