Fast Diffusion Characterization by Multiphoton Excited Fluorescence Recovery while Photobleaching.

Area of Science:

• Biophysics
• Materials Science
• Pharmacology

Background:

• Molecular diffusion is critical for understanding cellular processes, material properties, and drug interactions.
• Existing fluorescence recovery after photobleaching (FRAP) methods struggle with rapid diffusion ( > 100 μm²/s) and concurrent recovery during photobleaching.
• Multiphoton excitation presents challenges due to lower efficiency and photothermal effects.

Purpose of the Study:

• To introduce and validate beam-scanning fluorescence recovery while photobleaching (FRWP) for quantitative measurement of rapid molecular diffusion.
• To address the limitations of FRAP, particularly for systems with diffusion coefficients exceeding current measurement capabilities.
• To extend the applicability of fluorescence microscopy techniques to faster diffusion events.

Main Methods:

• Developed beam-scanning FRWP using patterned line-bleach illumination and a fast-scanning mirror.
• Introduced a theoretical model for fluorescence intensity fluctuations due to photobleaching and photothermal effects.
• Established a mathematical framework to quantify temporal fluorescence curves and recover diffusion coefficients.

Main Results:

• FRWP successfully measured rapid molecular diffusion over microsecond to millisecond timescales.
• The upper limit of measurable diffusion rates is determined by the scanning mirror frequency.
• Demonstrated FRWP's capability by characterizing the diffusion of rhodamine-labeled BSA, GFP, and IgG in varying viscosity solutions.

Conclusions:

• FRWP is a powerful technique for quantitative analysis of rapid molecular diffusion, surpassing FRAP's limitations.
• The method provides accurate diffusion coefficient measurements even with competing photobleaching and photothermal effects.
• FRWP significantly advances the study of molecular mobility in diverse scientific fields.