Visualization of the reaction layer in the immediate membrane vicinity

Area of Science:

• Biophysical Chemistry
• Chemical Kinetics
• Membrane Transport

Background:

• Mass transfer across unstirred layers (USL) near membranes is often diffusion-limited.
• Reaction rates can also limit overall process kinetics.
• Quantifying the interplay between diffusion and reaction is crucial for understanding membrane-associated processes.

Purpose of the Study:

• To quantitatively analyze coupled diffusion and chemical reaction processes near a membrane.
• To define and measure the size of the reaction layer (RL).
• To investigate how enzyme concentration affects reaction-limited versus diffusion-limited kinetics.

Main Methods:

• Proton concentration profile measurements in the membrane vicinity.
• Utilizing a theoretical model for reaction and diffusion kinetics.
• Employing the alcohol dehydrogenase-catalyzed oxidation of acetaldehyde as a model system due to measurable pH changes.

Main Results:

• The size of the unstirred layer (USL) was used to scale diffusion restrictions.
• The reaction layer (RL) size was calculated from pH profiles within the USL.
• Increased enzyme concentration led to a dramatic shortening of the RL, shifting kinetics from reaction-limited to diffusion-limited.

Conclusions:

• The reaction layer (RL) size effectively characterizes limitations due to finite reaction rates.
• The interplay between diffusion and reaction kinetics near membranes can be quantitatively modeled.
• Enzyme concentration is a critical factor in determining whether membrane transport is reaction- or diffusion-limited.