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Bio-based solvents were investigated for the biocatalysed amidation reactions of various ester-amine combinations by Pseudomonas stutzeri lipase (PSL). Reactions were undertaken in a range of green and potentially bio-based solvents including terpinolene, p-cymene, limonene, 2-methyl THF, ɣ-valerolactone, propylene carbonate, dimethyl isosorbide, glycerol triacetate and water. Solvent screenings demonstrated the importance and potential of using non-polar bio-based solvents for favouring aminolysis over hydrolysis; whilst substrate screenings highlighted the unfavourable impact of reactants bearing bulky para- or 4-substituents. Renewable terpene-based solvents (terpinolene, p-cymene, D-limonene) were demonstrated to be suitable bio-based media for PSL amidation reactions. Such solvents could provide a greener and more sustainable alternative to traditional petrochemical derived non-polar solvents. Importantly, once the enzyme (either PSL or CALB) binds with a bulky para-substituted substrate, only small reagents are able to access the active site. This therefore limits the possibility for aminolysis to take place, thereby promoting the hydrolysis. This mechanism of binding supports the widely accepted ‘Ping Pong – Bi Bi’ mechanism used to describe enzyme kinetics. The work highlights the need to further investigate enzyme activity in relation to para- or 4-substituted substrates. A priority in PSL chemistry remains a methodology to tackle the competing hydrolysis reaction.

Research into the synthesis and application of green solvents is a developing field of interest relevant to a variety of chemical manufacturers. Tunable solvents utilising carbon dioxide lend themselves to a reduction in work-up solvent and can help avoid the need for energy intensive solvent recovery options such as distillation. This perspective examines the potential benefits of tunable solvents to the chemical manufacturing industry and the environment. Freed from the usual reliance on distillation, high boiling solvents with an inherently lessened potential to cause air emissions can be used as part of a tunable solvent system. The application of bio-based solvents as tunable reaction media is highlighted as an area for further development.

An improved multi-parameter approach to the correlation of enzymatic activity and solvent properties is proposed. The development of the correlation facilitated the introduction of renewable solvents in a CALB catalysed synthesis of fatty esters.