Evidence that pyrene excimer formation in membranes is not diffusion-controlled

Area of Science:

• Photophysics
• Biomembrane science
• Fluorescence spectroscopy

Background:

• Pyrene fluorescence is a common tool for probing membrane environments.
• The theory of collisional excimer formation is often applied to interpret pyrene fluorescence data in solution and membranes.
• Previous studies have not fully elucidated the mechanism of excimer formation in complex membrane systems.

Purpose of the Study:

• To evaluate pyrene fluorescence in model membranes using the theory of collisional excimer formation.
• To investigate the concentration dependence of pyrene fluorescence and its implications for pyrene aggregation.
• To determine the mechanism of excimer formation in membranes.

Main Methods:

• Kinetic and steady-state fluorescence measurements of pyrene in model membranes.
• Analysis of molecular fluorescence decay kinetics and lifetimes.
• Evaluation of concentration-dependent fluorescence intensity ratios.

Main Results:

• Pyrene fluorescence decay in membranes is biphasic, with lifetimes independent of concentration.
• Rate constants for collisional excimer formation are estimated to be low (≤10^6 M⁻¹s⁻¹).
• Pyrene exists in both slowly diffusing monomeric and aggregated forms within membranes, influencing fluorescence.

Conclusions:

• Excimer fluorescence in membranes likely arises from aggregated pyrene via a static mechanism, not collisional formation.
• The concentration dependence of excimer/monomer fluorescence ratios is linked to monomer-aggregate exchange equilibrium, not collisional rates.
• This finding refines the understanding of pyrene photophysics in lipid bilayers.