Step bunching in a diffusion-controlled system: phase-shifting interferometry investigation of ferritin

Area of Science:

• Crystallization Science
• Biophysics
• Materials Science

Background:

• Protein crystallization is crucial for structural biology and drug development.
• Understanding crystal growth kinetics and pattern formation is key to optimizing crystallization processes.
• Ferritin crystallization serves as a model system for studying protein crystal growth dynamics.

Purpose of the Study:

• To introduce a novel phase-shifting interferometry technique for analyzing unsteady kinetics during protein crystallization.
• To investigate the formation of spatio-temporal patterns in ferritin crystal growth.
• To elucidate the relationship between interfacial growth processes and bulk solute transport.

Main Methods:

• Application of a novel phase-shifting interferometry technique.
• In-situ monitoring of ferritin crystal growth.
• Analysis of growth rate, step density, and step velocity fluctuations.
• Investigation of the influence of supersaturation, crystal size, and distance from step sources.

Main Results:

• Observed strong fluctuations in growth rate, step density, and step velocity attributed to step bunch passage.
• Demonstrated that fluctuation amplitudes decrease with increased supersaturation, crystal size, and distance from step sources.
• Identified weak interaction between steps through analysis of step velocity dependence on local slope.

Conclusions:

• Fluctuations in ferritin crystal growth are rooted in the coupling of interfacial growth processes with bulk solute transport.
• In diffusion-controlled systems with weakly interacting steps, stable growth occurs via equidistant step trains.
• Randomly arising step bunches tend to decay, supporting the stable growth mode.