6470
Z. Zhang / Tetrahedron Letters 49 (2008) 6468–6470
Figure 2. Selected chromatograms of the ES processes corresponding to entries 1,3,4,5,6, and 9 are shown. The black, red, and blue lines are of the samples at the beginning,
the middle, and the end of the reaction, respectively. In the case of 2-Br–Phe and 2-Cl–Phe, since the reactions were completed before the second sampling (red), the blue line
is absent. The retention times (min) varied from one substrate to another. The
the experiments.
L-enantiomer which eluted slower was scavenged. The elution conditions were the same for all
Structurally, the substrates used in this study are all derivatives of
Phe with diverse substitutions except for Trp and homo-Phe, which
is a 2-amino 4-phenylbutyric acid. It is possible for them to share a
common pathway and enzymes. However, different substrates
might be transformed by different sets of enzymes. The isolation
of IAA from the experiment with rac-Trp clearly indicates the
involvement of Trp’s IAA pathway. This implies the decarboxyl-
ation by either Trp 2-monooxygenase (EC 1.13.12.3) or 20-dioxy-
genase (EC 1.13.99.3) or both. At this point, it is not clear if these
two enzymes were solely responsible for the scavenging process.
Nevertheless, it seems that the ES recognizes the stereogenic cen-
ter regardless of the structural diversity of the substrates. Even
though the substitutions on the aromatic ring of the substrates
might affect the reaction rate of the enzyme through changes in
their substrates’ solubility and velocity of penetration/binding to
the enzyme’s active site, they did not dramatically alter its
enantioselectivity.
of ES could be used in many scientific fields, due to its simplicity
and usefulness. Ongoing work is being done in identifying the
enzymes responsible for the reaction in this ES system.
Acknowledgments
The author thanks Professor Lihe Zhang for his encouragement,
and also extends acknowledgment to Yebin Zhang and Hongying
Guo for technical assistance. This work was supported by The
National Natural Science Foundation of China (Project No.
20542006).
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It remains to be seen which enzymes are responsible for the
L-ES system. Nevertheless, this work uncovers a rich realm of
discoveries. The practical usefulness of this work provides an
invaluable target for further studies in sequencing, cloning, expres-
sion, genetic engineering, enzymology, et cetera so as to identify
and investigate the potential of these biocatalysts in synthetic
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In summary, we demonstrated an
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D
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D
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