Optimization of solvent-free enzymatic esterification in eutectic substrate reaction mixture

Area of Science:

• Biocatalysis
• Enzyme Technology
• Green Chemistry

Background:

• Enzymatic esterification offers a sustainable alternative to traditional chemical synthesis.
• Deep eutectic solvents (DES) are emerging as novel reaction media, potentially reducing environmental impact.
• Solvent-free enzymatic reactions minimize waste and simplify product purification.

Purpose of the Study:

• To investigate the esterification of (-)-menthol and lauric acid (LA) catalyzed by Candida rugosa lipase (CRL).
• To explore the use of a (-)-menthol:LA deep eutectic solvent (DES) as a reaction medium and substrate pool under solvent-free conditions.
• To optimize the esterification process by evaluating key parameters like water activity (a_w), enzyme loading, and temperature.

Main Methods:

• Utilized a deep eutectic solvent formed by (-)-menthol and lauric acid (3:1 mol/mol) as a solvent-free reaction system.
• Employed response surface methodology (RSM) to optimize reaction conditions, focusing on thermodynamic water activity (a_w), CRL amount (m_CRL), and temperature (T).
• Quantified product formation and substrate conversion using analytical methods to validate model predictions.

Main Results:

• Achieved a high conversion rate of 95 ± 1% for lauric acid within one day under optimized conditions (a_w = 0.55, m_CRL = 60 mg, T = 45 °C).
• Reached a final product concentration of 1.36 ± 0.04 M (-)-menthyl lauric acid ester after 2.25 days.
• Demonstrated excellent agreement between experimental product formation rates and RSM model predictions.

Conclusions:

• The developed solvent-free esterification process using CRL in a (-)-menthol:LA DES is highly efficient and sustainable.
• Optimized reaction parameters, particularly water activity, are crucial for maximizing enzymatic esterification performance.
• This approach offers a promising green alternative for the synthesis of (-)-menthyl lauric acid ester.