Orientational exchange approach to fluorescence anisotropy decay

Area of Science:

• Molecular Biophysics
• Physical Chemistry
• Computational Chemistry

Background:

• Fluorescence depolarization is crucial for studying molecular dynamics.
• Current models often rely on the rotational diffusion equation, with implicit assumptions about jump models.
• Previous studies suggested equivalence between 90-degree jump models and diffusion equations for symmetric cases.

Purpose of the Study:

• To derive a general result for fluorescence depolarization using a compartmental formalism for an exchange model with arbitrary dipole orientations.
• To compare this result with the standard rotational diffusion equation approach.
• To define the limits of validity for using 90-degree jump models to represent rotational diffusion.

Main Methods:

• Derivation of a general exchange model using compartmental formalism.
• Monte Carlo simulations to verify the derived model.
• Analysis of temperature dependence of exchange rates in relation to Kramer's theory.

Main Results:

• The derived exchange model yields results different from the rotational diffusion equation, even for fluorescence depolarization.
• Monte Carlo simulations confirmed this discrepancy.
• The study defines conditions under which 90-degree jump models are not equivalent to continuous diffusion.

Conclusions:

• The compartmental formalism provides a framework to combine rotational motion with discrete jumps or other kinetics.
• The developed model offers a more accurate representation for systems with jumps between preferred orientations.
• The study highlights the limitations of the diffusion equation and provides a method to distinguish between diffusion and finite step-size random walks.