From the Gibb’s equation given above it follows that only the entropy term ΔΔS
6
¼
(but not the enthalpy ΔΔH
6 ¼ ) is influenced by the temperature. Thus, the selectivity
of an enzymatic reaction depends on the temperature as follows:
• At temperatures below T rac the contribution of entropy is minimal and the
stereochemical outcome of the reaction is mainly dominated by the activation
enthalpy difference (ΔΔH
6 ¼
). The optical purity of product(s) will thus decrease
with increasing temperature.
• On the other hand, at temperatures greater than T rac , the reaction is controlled
mainly by the activation entropy difference (ΔΔS
6 ¼
) and enthalpy plays a minor
role. Therefore, the optical purity of product(s) will increase with increasing
temperature.
However, the major product obtained at a temperature above T rac will be the
antipode to that below T rac , thus a temperature-dependent reversal of stereochemistry is predicted. The validity of this rationale has been proven with the asymmetric reduction of ketones using a dehydrogenase from Thermoanaerobium
brockii [311] (Sect. 2.2.2). In contrast to the above-mentioned dehydrogenases
from thermophilic organisms, the majority of hydrolases used for biotransformations (except Candida antarctica lipase B) possess more restricted thermal operational limits, which narrows the possibility of a significant selectivity
enhancement by variation of the reaction temperature. From the data available,
it can be seen that upon lowering the temperature both an increase [312] or a
decrease in the selectivity of hydrolase reactions may be observed [313],
depending on whether the reaction was performed above or below the racemic
temperature (T rac ) of the enzyme used. The modest upper temperature of about
50
C for the majority of enzymes represents a serious limitation, while impressive effects have been observed upon cooling (À20 to À60
C) [314–316]. In
order to enhance reaction rates of organic-chemical reactions, microwave
(MW) irradiation has become fashionable [317].
8
While conventional heating is due to polychromatic infrared radiation, microwaves are generated in a monochromatic manner. The benefit of MW heating has
been proven in numerous types of organic reactions, but the existence of special
microwave-effects (the so-called hot-spot theory) is still heavily debated [318–
321]. For enzyme-catalyzed reactions, MW heating has been shown to be superior
to conventional heating by leading to reduced enzyme deactivation and enhanced
selectivities [322–324].
Enzyme Engineering Molecular biology has enabled the redesign of enzymes
possessing improved performance in terms of enhanced stability at extreme
8 By definition, the range of microwave irradiation extends from 1 to 300 GHz; however, due to the
resonance frequency of water (19.5 GHz), most of the applications are close to the latter range,
i.e., 0.9 and 2.45 GHz.
76
2 Biocatalytic Applications
6
¼
(but not the enthalpy ΔΔH
6 ¼ ) is influenced by the temperature. Thus, the selectivity
of an enzymatic reaction depends on the temperature as follows:
• At temperatures below T rac the contribution of entropy is minimal and the
stereochemical outcome of the reaction is mainly dominated by the activation
enthalpy difference (ΔΔH
6 ¼
). The optical purity of product(s) will thus decrease
with increasing temperature.
• On the other hand, at temperatures greater than T rac , the reaction is controlled
mainly by the activation entropy difference (ΔΔS
6 ¼
) and enthalpy plays a minor
role. Therefore, the optical purity of product(s) will increase with increasing
temperature.
However, the major product obtained at a temperature above T rac will be the
antipode to that below T rac , thus a temperature-dependent reversal of stereochemistry is predicted. The validity of this rationale has been proven with the asymmetric reduction of ketones using a dehydrogenase from Thermoanaerobium
brockii [311] (Sect. 2.2.2). In contrast to the above-mentioned dehydrogenases
from thermophilic organisms, the majority of hydrolases used for biotransformations (except Candida antarctica lipase B) possess more restricted thermal operational limits, which narrows the possibility of a significant selectivity
enhancement by variation of the reaction temperature. From the data available,
it can be seen that upon lowering the temperature both an increase [312] or a
decrease in the selectivity of hydrolase reactions may be observed [313],
depending on whether the reaction was performed above or below the racemic
temperature (T rac ) of the enzyme used. The modest upper temperature of about
50
C for the majority of enzymes represents a serious limitation, while impressive effects have been observed upon cooling (À20 to À60
C) [314–316]. In
order to enhance reaction rates of organic-chemical reactions, microwave
(MW) irradiation has become fashionable [317].
8
While conventional heating is due to polychromatic infrared radiation, microwaves are generated in a monochromatic manner. The benefit of MW heating has
been proven in numerous types of organic reactions, but the existence of special
microwave-effects (the so-called hot-spot theory) is still heavily debated [318–
321]. For enzyme-catalyzed reactions, MW heating has been shown to be superior
to conventional heating by leading to reduced enzyme deactivation and enhanced
selectivities [322–324].
Enzyme Engineering Molecular biology has enabled the redesign of enzymes
possessing improved performance in terms of enhanced stability at extreme
8 By definition, the range of microwave irradiation extends from 1 to 300 GHz; however, due to the
resonance frequency of water (19.5 GHz), most of the applications are close to the latter range,
i.e., 0.9 and 2.45 GHz.
76
2 Biocatalytic Applications