Biocatalysis in microaqueous systems (solvent-based and solvent-free conditions): Historical perspectives and recent...

A microaqueous system is generally defined as a system containing a small amount of water in the bulk of a medium, yet playing a pivotal role that exists between two extreme systems: aqueous systems and nonaqueous (or anhydrous, or non-aqueous, or dry) systems, which contain no water in the bulk. The technical adjective microaqueous was first introduced in 1987 and has since gained widespread acceptance. This review summarizes recent advances in research concerning biocatalysis in microaqueous systems, with special attention paid to the historical development of the field and with notable contributions from Japan. Microaqueous biosystems are categorized into two types, those utilizing organic solvents and those that are organic solvent-free. Eight advantages of microaqueous biosystems are highlighted, along with four specific benefits of organic solvent-free systems. Mathematical equations are proposed to describe two types of relationships between water bound to proteins and free water dissolved in the bulk organic solvents, one being applicable to water-miscible solvents and the other to water-immiscible solvents. Subsequently, an overview of various biochemical reactions in microaqueous systems is provided, followed by a discussion of the types of enzymes studied, techniques for establishing microaqueous biosystems, characteristic changes in enzymes in organic solvents, and criteria for selecting suitable solvents, including three biobased green solvents. Lastly, microaqueous bioreactors are briefly introduced. Two novel contributions are presented: classification of organic solvents into three categories with respect to water solubility (water-miscible, water-immiscible, and water-insoluble (hydrocarbons)); and a revised decision flowchart for selecting the most suitable system for biocatalytic reactions.