Quantifying internal friction in unfolded and intrinsically disordered proteins with single-molecule spectroscopy

Area of Science:

• Biophysics
• Protein Dynamics
• Polymer Physics

Background:

• Internal friction, a measure of energy landscape roughness, influences protein folding and conformational changes.
• Experimental quantification of internal friction and its role in protein dynamics remains a significant challenge.

Purpose of the Study:

• To experimentally determine reconfiguration times of unfolded proteins.
• To investigate the mechanisms and contributions of internal friction to protein dynamics.
• To quantify internal friction using multiple, independent approaches.

Main Methods:

• Single-molecule Förster resonance energy transfer (smFRET)
• Nanosecond fluorescence correlation spectroscopy (nfCS)
• Microfluidic mixing for rapid sample manipulation

Main Results:

• Internal friction was quantified as an additive contribution to unfolded protein reconfiguration time.
• Internal friction magnitude correlates with the compactness of the unfolded protein chain.
• Contribution of internal friction is significant for compact unfolded states (approx. 100 ns) but diminishes for expanded chains.

Conclusions:

• Internal friction is particularly relevant for proteins folding on microsecond or faster timescales.
• Low internal friction in expanded intrinsically disordered proteins may influence their interactions with cellular partners.
• The study provides a quantitative framework for understanding internal friction in protein dynamics.