Single-molecule lactonization of octadecylrhodamine B at a liquid-liquid interface

Area of Science:

• Interface Science
• Physical Chemistry
• Fluorescence Microscopy

Background:

• Understanding molecular behavior at interfaces is crucial for various chemical and biological processes.
• Octadecylrhodamine B (C(18)RB) is a fluorescent probe used to study interfacial phenomena.
• The chemical state of C(18)RB can change depending on environmental conditions like pH.

Purpose of the Study:

• To investigate the lateral diffusion of single C(18)RB molecules at the toluene-water interface.
• To determine the effect of pH on the interfacial diffusion and chemical state of C(18)RB.
• To quantify the lactonization rate of C(18)RB at different interfaces.

Main Methods:

• Total internal reflection laser fluorescence microscopy was employed to track single C(18)RB molecules.
• Interfacial diffusion constants were determined by analyzing the maximum residence time of fluorescent C(18)RB cations in a defined observation area.
• The conversion rate to the nonfluorescent lactone form was studied by monitoring changes in fluorescence over time.

Main Results:

• At low pH (<2), single fluorescent C(18)RB cations exhibited measurable interfacial diffusion.
• At higher pH (>3), the maximum residence time significantly decreased, indicating rapid conversion to a nonfluorescent lactone form.
• The lactonization rate of C(18)RB was found to be very fast (0.13 ms) at the toluene-water interface, but considerably slower (67 ms) at a dimyristoylphosphatidylcholine-saturated interface.

Conclusions:

• The chemical state of C(18)RB is highly pH-dependent at the toluene-water interface.
• Rapid lactonization of C(18)RB at higher pH interferes with diffusion measurements of the fluorescent cation.
• The study highlights the importance of considering molecular transformations when using fluorescent probes at interfaces, especially in the presence of lipids.