Solvent-exposed tryptophans probe the dynamics at protein surfaces.

Area of Science:

• Biophysics
• Protein Dynamics
• Fluorescence Spectroscopy

Background:

• Proteins possess unique surface properties influencing their interactions with solvents.
• Tryptophan (W) residues are often used as intrinsic probes for protein structure and dynamics.

Purpose of the Study:

• To investigate the local dynamics of tryptophan side chains on protein surfaces.
• To probe the influence of solvent viscosity on protein side chain motion.
• To understand water's role at protein-solvent interfaces.

Main Methods:

• Time-resolved fluorescence anisotropy measurements.
• Fluorescence quenching by potassium iodide (KI) to assess solvent exposure.
• Use of glycerol-water mixtures to vary bulk solvent viscosity (eta).

Main Results:

• Tryptophan side chains in subtilisin Carlsberg (SC) and myelin basic protein (MBP) are solvent-exposed.
• Rotational motion of W in SC is unhindered, while partially hindered in MBP.
• Local motion of W on protein surfaces deviates from Stokes-Einstein predictions, unlike denatured proteins or free W.

Conclusions:

• Protein surface water exhibits specific partitioning behavior in glycerol-water mixtures.
• This specific water partitioning affects the local dynamics of W side chains.
• Deviations from Stokes-Einstein relationship highlight non-bulk solvent behavior at protein interfaces.