Strange kinetic phase in the extremely early folding process of β-lactoglobulin

A continuous-wave probed laser-induced temperature jump system was constructed and applied to monitor the changes in tryptophan fluorescence of the β-lactoglobulin during its folding; the kinetic phases were traced from 300 ns to 10 ms after a temperature jump. Notably, an early phase with typical squeezed-exponential characteristics, [exp{−(kt)β}, β > 1.0], was observed around several tens of microseconds after the temperature jump, which is actually the earliest phase ever observed for β-lactoglobulin. This process can be explained by conformational shift occurring within the unfolded ensemble (U → U′), which is followed by the non-native intermediate (I) formation of this protein.