Handbook of Solvents - George Wypych - ChemTech - Ventech!

872 Tsuneo Yamane
14.4.3 CHOICE OF SOLVENT FOR ENZYMATIC REACTION IN ORGANIC
SOLVENT
Tsuneo Yamane
Graduate School of Bio- and Agro-Sciences, Nagoya University, Nagoya,
Japan
14.4.3.1 Introduction
The ability of enzymes to catalyze useful synthetic biotransformations in organic media is
now beyond doubt. There are some advantages in using enzymes in organic media as opposed
to aqueous medium, including
1) shifting thermodynamic equilibrium to favor synthesis over hydrolysis,
2) reduction in water-dependent side reaction,
3) immobilization of the enzyme is often unnecessary (even if it is desired,
merely physical deposition onto solid surfaces is enough),
4) elimination of microbial contamination,
5) suitable for reaction of substrates insoluble and/or unstable in water, etc.
Here organic media as the reaction system are classified into two categories: substrates
dissolved in neat organic solvents and solvent-free liquid substrates. Although the
latter seems preferable to the former, if it works, there are a number of cases where the former
is the system of choice: for example, when the substrate is solid at the temperature of
the reaction, when high concentration of the substrate is inhibitory for the reaction, when the
solvent used gives better environment (accelerating effect) for the enzyme, and so forth.
Prior to carrying out an enzymatic reaction in an organic solvent, one faces choice of a
suitable solvent in the vast kinds of organic solvents. From active basic researches having
been carried out in the past two decades, there has been a remarkable progress in our understanding
of properties of enzymes in organic media, and in how organic solvents affect
them. Some researchers call the achievement ‘medium engineering’. A comprehensive
monograph was published in 1996 reviewing the progress. 1
In this article organic solvents often used for enzymatic reactions are roughly classified,
followed by influence of solvent properties on enzymatic reactions and then properties
of the enzymes affected by the nature of the organic solvents are briefly summarized.
14.4.3.2 Classification of organic solvents
Among numerous kinds of organic solvents, the ones often used for enzymatic reactions are
not so many, and may be classified into three categories (Table 14.4.3.1), 2 in view of the importance
of water content of the organic solvent concerned (see Section 14.4.3.3).
1) water-miscible organic solvents
These organic solvents are miscible with water at the temperature of the reaction. Any
cosolvent system having 0 - 100% ratio of the solvent/water can be prepared from this kind
of solvent. Note that some organic solvents having limited water solubility at ambient temperature,
and hence are not regarded as water-miscible, become miscible at elevated temperature.
2) Water-immiscible organic solvents