Stochastic pH Oscillations in a Model of the Urea-Urease Reaction Confined to Lipid Vesicles.

Area of Science:

• Biochemical reactions
• Chemical kinetics
• Physical chemistry

Background:

• The urea-urease reaction exhibits a pH switch, capable of forming pH oscillators in open reactors.
• Lipid vesicles allow differential transport, enabling pH recovery and sustained urea supply for sustained reactions.

Purpose of the Study:

• To numerically investigate the confinement of the urea-urease clock reaction within lipid vesicles.
• To analyze the impact of vesicle size and molecular discreteness on pH oscillation dynamics and periodicity.

Main Methods:

• Numerical simulations of the urea-urease reaction dynamics within lipid vesicles.
• Stochastic treatment of reaction dynamics for microscopically small vesicles.
• Analysis of oscillation period variation and robustness across different vesicle sizes.

Main Results:

• Intrinsic noise in small vesicles causes significant statistical variation in oscillation periods, increasing as vesicle size decreases.
• The mean oscillation period demonstrates robustness for vesicle sizes down to approximately 200 nm.
• Periodicity is progressively lost in vesicles smaller than 200 nm.

Conclusions:

• The confinement of the urea-urease clock reaction in lipid vesicles leads to noise-induced period variations.
• Vesicle size critically affects oscillation stability, with a robustness limit around 200 nm.
• The observed oscillations represent a canard-like limit cycle, distinct from conventional feedback oscillators.