Compensation effect between differential activation enthalpy and entropy in subtilisin-catalyzed kinetic resolutions of secondary alcohols

Article abstract of DOI:10.1246/cl.2000.782

Thermodynamic parameters for subtilisin-catalyzed kinetic resolutions of secondary alcohols were determined, and a compensation effect between ΔΔH? and ΔΔS? was found. These data are explained in terms of the transition-state model.

Full text of DOI:10.1246/cl.2000.782

782
Chemistry Letters 2000
Compensation Effect between Differential Activation Enthalpy and Entropy
in Subtilisin-Catalyzed Kinetic Resolutions of Secondary Alcohols
Tadashi Ema,* Kunihiro Yamaguchi, Yuji Wakasa, Nobuaki Tanaka, Masanori Utaka, and Takashi Sakai*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima, Okayama 700-8530
(Received April 19, 2000; CL-000371)
Thermodynamic parameters for subtilisin-catalyzed kinetic
It has been reported that the thermodynamic parameters,
resolutions of secondary alcohols were determined, and a com-
pensation effect between ∆∆H‡ and ∆∆S‡ was found. These
data are explained in terms of the transition-state model.
∆∆H‡ and ∆∆S‡, can be estimated according to equation 1,7–9
where ∆∆H‡ = ∆H‡fast – ∆H‡slow and ∆∆S‡ = ∆S‡fast – ∆S‡
.
slow
Lipases are abnormal enzymes in that they show high
enantioselectivity and broad substrate specificity simultaneous-
ly unlike other enzymes.1 In order to rationalize this abnormal
feature observed in the lipase-catalyzed kinetic resolutions of
secondary alcohols, we have proposed a stereo-sensing mecha-
nism (Figure 1b).2 In this transition-state model the enantiose-
lectivity is explained by the conformational requirements and
repulsive interactions, and any attractive binding interaction
between enzyme's pockets and substrate's substituents is not
involved. In this sense lipases are considered to be “chemical
reagent-like”.3 Further study has revealed that such a concept
can be applied to subtilisin Carlsberg (Figure 1c).4 These tran-
sition-state models can rationalize the opposite empirical
rules4,5 as shown in Figure 1a. These transition-state models
have been derived on the basis of MO calculations2 and kinetic
study,2,4 and have been supported by the successful kinetic res-
olutions of a very large secondary alcohol having tetraphenyl-
porphyrin as a substituent.4,6 In this paper, we present the
results of the thermodynamic study to gain a deeper insight into
the origin of the enantioselectivity.
In the case of the lipase-catalyzed kinetic resolution of second-
ary alcohols, the E values10 are too high in many cases, which is
inappropriate for this purpose. By contrast, subtilisin shows
low to moderate enantioselectivities,4 which allows us to deter-
mine the reliable E values at a range of temperatures.
Therefore, we employed subtilisin Carlsberg (ChiroCLEC-BL,
Altus Biologics Inc.) as a biocatalyst, and carried out the kinetic
resolutions of 1–10 (Chart 1) with vinyl acetate in dry i-Pr2O at
10–40 °C. In all cases except for 10, the (S)-enantiomers react-
ed faster. The thermodynamic parameters calculated according to
equation 1 are listed in Table 1. The accuracy of the data is rela-
tively high despite the heterogeneous reactions because the E val-
ues were determined under the competitive reaction conditions.
In most cases, the ∆∆H‡ value contributes predominantly
to the E value. In other words, enantiomer discrimination is
enthalpy-driven. There is also a tendency that the ∆∆H‡ value
decreases as the two substituents of the secondary alcohols are
more unbalanced in bulkiness (Table 1). In terms of the transi-
tion-state models, the ∆∆H‡ value can be regarded as the differ-
ence in degree of repulsive interactions with the enzyme
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
783
is going on in the transition state.
We are grateful to the SC-NMR Laboratory of Okayama
University for the measurement of the NMR spectra. This work
was financially supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,
Sports and Culture of Japan.
References and Notes
1
For example: a) “Enzymatic Reactions in Organic Media,”
ed. by A. M. P. Koskinen and A. M. Klibanov, Blackie
Academic, Glasgow (1996). b) U. T. Bornscheuer and R. J.
Kazlauskas, “Hydrolases in Organic Synthesis,” Wiley-
VCH, Weinheim (1999).
2
3
T. Ema, J. Kobayashi, S. Maeno, T. Sakai, and M. Utaka,
Bull. Chem. Soc. Jpn., 71, 443 (1998).
between the enantiomers. Therefore, that tendency observed
for the ∆∆H‡ values is reasonable because the difference in
degree of repulsive interactions between the enantiomers will
increase as the two substituents of the secondary alcohols are
more unbalanced in bulkiness.
In the field of synthetic organic chemistry, many transition-
state models have been proposed for various stereoselec-
tive chemical reactions. R. E. Gawley and J. Aubé
“Principles of Asymmetric Synthesis,” Elsevier, Oxford
(1996).
Interestingly, we found a linear relationship between the
∆∆H‡ and ∆∆S‡ values for 1–10 (Figure 2). As the ∆∆H‡ value
becomes negatively large, the ∆∆S‡ value also becomes nega-
tively large.11 Equation 1 indicates that a negative ∆∆H‡ value
contributes to an increase in E value whereas a negative ∆∆S‡
value contributes to a decrease in E value. This partial compen-
sation effect can be understood by considering repulsive inter-
actions between the slower–reacting enantiomer and the
enzyme. In general, although repulsive interactions are unfa-
vorable in terms of enthalpy, they are favorable in terms of
entropy because the degree of disorder (or freedom) increases.
Because the degree of disorder of the amino acid residues and
the substrate moiety will increase with an increase in degree of
repulsive interactions, the entropy gain will increase propor-
tionally with an increase in enthalpy loss. This opposite but pro-
portional relationship between enthalpy and entropy can remain
even after the ∆H‡ and ∆S‡ values for the slower-reacting enan-
tiomer are subtracted from the corresponding values for the
faster-reacting enantiomer. Accordingly, the observed compen-
sation effect can be explained by the transition-state model, and
the thermodynamic data reported here help us understand what
4
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