Buffer and Polymer Molecular Weight Affect Zinc Myoglobin-Mediated PET-RAFT Polymerizations

Area of Science:

• Biocatalysis
• Polymer Chemistry
• Photochemistry

Background:

• Protein photocatalysts offer versatile synthesis routes for small organic molecules.
• Their application in photocatalytic polymer synthesis, particularly in photoinduced electron/energy transfer reversible-activation fragmentation chain transfer (PET-RAFT) polymerizations, is not well understood.
• The influence of polymer molecular weight and protein structure on PET-RAFT remains largely unexplored.

Purpose of the Study:

• To investigate the impact of buffer conditions and macro chain-transfer agent (macro-CTA) molecular weight on zinc myoglobin-mediated PET-RAFT chain-extension polymerizations.
• To elucidate the relationship between macro-CTA size and the accessibility of the RAFT end group to the protein photocatalyst.
• To provide insights into optimizing protein-based catalysts for PET-RAFT polymerization.

Main Methods:

• Zinc myoglobin-mediated PET-RAFT chain-extension polymerizations were conducted under varying buffer conditions (Tris vs. PBS).
• The effect of different macro-CTA molecular weights (22 kg/mol and 75 kg/mol) on polymerization rates was evaluated.
• Solvent-accessible surface area (SASA) calculations were performed on model polymers to assess RAFT end group accessibility.

Main Results:

• Chain extensions in Tris buffer exhibited a 17% higher apparent rate constant compared to those in PBS buffer.
• A 22 kg/mol macro-CTA demonstrated a 40% faster apparent rate constant for chain extension compared to a 75 kg/mol macro-CTA.
• Increased macro-CTA molar mass correlated with decreased solvent-accessible surface area of the RAFT end group, explaining the reduced polymerization rate.

Conclusions:

• Buffer composition significantly influences the efficiency of protein-catalyzed PET-RAFT polymerizations.
• Polymer molecular weight, specifically the size of the macro-CTA, plays a crucial role in determining PET-RAFT polymerization rates.
• Protein photocatalyst performance in PET-RAFT is dependent on the accessibility of the reactive chain end, which is affected by polymer chain length.