Water Distribution and Clustering on the Lyophilized IgG1 Surface: Insight from Molecular Dynamics Simulations

Area of Science:

• Protein science
• Physical chemistry
• Biophysics

Background:

• Water significantly impacts protein structure and stability, especially in freeze-dried formulations.
• Water vapor sorption isotherms and Brunauer-Emmett-Teller (BET) analysis are common methods to study protein-water interactions.
• The BET monolayer parameter (Wm) is typically 6-8% for most proteins.

Purpose of the Study:

• To investigate water distribution on an IgG1 surface using molecular dynamics (MD) simulations.
• To develop a mechanistic model for water vapor sorption isotherms based on hydrogen bonding.
• To redefine the physical meaning of the BET monolayer and propose a new calculation model for Wm.

Main Methods:

• Molecular dynamics (MD) simulations to analyze water distribution on an IgG1 surface at various hydration levels.
• Development of a mechanistic model based on hydrogen bonding networks derived from MD simulations.
• Comparison of the model with experimental Type-II isotherms observed for proteins.

Main Results:

• Water molecule distribution on the IgG1 surface is heterogeneous and not accounted for by BET theory.
• True monolayer coverage occurs above 30% hydration, challenging the traditional BET monolayer definition.
• The developed mechanistic model aligns with experimental Type-II isotherms and redefines the BET monolayer as the onset of water cluster formation.

Conclusions:

• The BET monolayer parameter (Wm) signifies the onset of water cluster formation, not just monolayer adsorption.
• A new model based on MD simulations and hydrogen bonding provides a more accurate description of protein-water sorption isotherms.
• Understanding heterogeneous water distribution is crucial for predicting protein stability in lyophilized products.