Two-state expansion and collapse of a polypeptide

Area of Science:

• Biophysics
• Protein Dynamics
• Biochemistry

Background:

• Protein folding is crucial for biological function.
• The initial protein folding step is often a rapid collapse from an expanded to a compact state.
• Theoretical models suggest protein collapse may be a two-state transition or a continuous process.

Purpose of the Study:

• To experimentally determine the kinetic mechanism of protein collapse.
• To resolve the time scale and nature of the collapse transition for a model protein.

Main Methods:

• Utilized laser temperature-jump (T-jump) technique.
• Employed fluorescence spectroscopy for real-time monitoring.
• Achieved nanosecond time resolution to capture fast folding events.

Main Results:

• Observed an exponential time course for cytochrome c collapse.
• Demonstrated the process is thermally activated with a significant free energy barrier.
• Measured an apparent activation energy of approximately 9 k(B)T.

Conclusions:

• Polypeptide collapse is kinetically a two-state transition.
• The free energy barrier significantly slows protein collapse compared to homopolymer models.
• Provides critical experimental evidence for the kinetics of the initial protein folding phase.