S.-S. Xue et al. / Journal of Molecular Catalysis A: Chemical 424 (2016) 297–303
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Fig. 1. Hydrolysis of single enantiomer of the substrates (2.5 M) a) Boc-Lys(Boc)-ONp, b) Boc-Ala-ONp promoted by CuL1 (Red circles), by CuL2 (blue circles), by L1 (red
triangles), by L2 (blue triangles), by Cu(ClO4)2 (black squares). Solid ones are for the l-isomer and hollow ones are for the d-isomer for all the cases. The reactions were
performed in a 10% MeCN solution in HEPES buffer (pH 7.2, 50 mM) promoted by 50 M different catalysts at 298 0.1 K. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
a higher catalytic efficiency and more pronounced enantios-
electivity than the secondary face linked “face-to-face” bisCD
ysis rate (kinL = 13.9 × 10−5 s−1) among all the tested substrates
(Table 1), which is twice of the largest value of our previous study
[36]. The value is 479-fold and 10-fold higher than the spontaneous
and d-isomer hydrolysis, respectively (Fig. 1a, Table 1). Boc-Ala-
ONp enantiomers resulted in moderate hydrolytic efficiency and
enantioselectivity compared with Boc-Lys(Boc)-ONp promoted by
ONp with a lower or comparative rate compared with L2 (Table 1).
We consider that the coordination of copper(II) ion shortened the
linker length of L2, and the spaced distance was adjusted too nar-
row, which is detrimental to the hydrolysis reactions, especially for
the long alkyl chain-possessing substrates [19,46].
amino acid esters using the same catalyst [36]. The decrease of
enantioselectivity can be related to the hydrophobic interaction
between alkyl chain and the CDs is weaker than that between
aromatic moiety and the CDs. The value is also moderate among
which can also be resulted from weaker interaction between
alkyl chain and the CD cavity. As for Boc-Ala-ONp, the short alkyl
chain analogs, the enantiomer selectivity value was decreased
to 3.6. The value is smaller than the imidazolyl--CD promoted
ones [30], but the preferred isomer is opposite. The Km values for
Boc-Ala-ONp are higher than that for Boc-Lys(Boc)-ONp, indicating
weaker binding capability of CuL1. These data suggested that the
alkyl chains of Boc-Lys(Boc)-ONp participate in governing the
enantioselective hydrolysis. On the other hand, similar Km values
for Boc-Lys(Boc)-ONp enantiomers were observed, with (6.5 1.4)
for l-isomer and (6.0 1.2) for d-isomer, indicating the similar
binding affinity for the two enantiomers with CuL1. The hydrolysis
enantioselectivity was proposed coming from the different geome-
tries of the substrates that were regulated by the closely linked
CD chiral cavities [36]. Electrospray ionization mass spectrometry
(ESI-MS) analysis of CuL1 in solution mixed with each enantiomer
of Boc-Lys(Boc)-ONp and Boc-Ala-ONp individually was performed
(Supplementary Figs. S4–S7). The catalyst-substrate (1:1) com-
plexes were detected by ESI-MS for all cases, which confirmed the
presence of Michalis intermediates along the reaction pathway.
However, for Boc-Ala-ONp enantiomers, the ESI-MS signals of
catalyst-substrate (1:1) complexes were much weaker than that
for Boc-Lys(Boc)-ONp enantiomers, indicating weaker interaction
with CuL1. The hydrolytic product NP and Boc-Lys(Boc)-OH or
Boc-Ala-OH were also detected, see Supplementary Figs. S4–S7.
The “back-to-back” bisCD complex CuL1 favoured the enantios-
elective hydrolysis of the amino acid-containing substrates. For
either CuL1 or CuL2, the chiral CD cavities in our models have
an overall preference for the l-isomers of the amino acid esters.
Additionally, changing the CD orientation of catalyst reduced the
enantioselectivity more than that induced by changing the sub-
strate, suggesting that the CD orientation plays a vital role in the
enantioselective hydrolysis. Thus, the initial kinetic study indicated
consistent results with the study using the same catalysts with
aromatic amino acid ester analogs as substrates [36].
Further kinetic studies of the hydrolysis of Boc-Lys(Boc)-ONp
the dependence of the rate constants on the CuL1 concentra-
tion [28,29,47–49]. By increasing the initial catalyst concentration
from 5.0 to 100.0 M, deviation from linear increase was observed
(Fig. 2), which implied that the catalyst-substrate complex formed
in prior to the catalytic reaction. The kinetic parameters were
gathered in Table 2.
moted by CuL1. The value was among the largest ones which were
gained by using CD-based [29–31] or other surfactants containing
complex [49,50] as artificial mimics, and are close to those cases
by using enzymes or modified enzymes [5,47,50,51], for the
hydrolysis of amino acid esters or peptide esters. The hydrolysis
of Boc-Lys(Boc)-ONp enantiomers promoted by CuL1 resulted in
more pronounced enantioselectivity than that of Boc-Ala-ONp,
with an enantiomer selectivity of 8.8. The enantiomer selectiv-
ity value is smaller than the largest value (15.7) for aromatic
2.3. Chiral HPLC analysis
chromatography (HPLC) analysis was performed with racemic
were taken and subjected to HPLC analysis with CHIRALPAK®
IC as the chiral stationary phase [36,47]. As shown in Supple-
mentary Fig. S9, when 38% and 47% of Boc-Lys(Boc)-ONp was
consumed, the remaining L:D ratios were 28:72 and 31:69, respec-
tively (Table 3). Based on these conversions and the remaining
Boc-l-Lys(Boc)-ONp and Boc-d-Lys(Boc)-ONp were calculated to
be 86:14 and 76:24, respectively, indicating that the correspond-
ing hydrolyzed product (Boc-Lys(Boc)-OH) were present at e.r. of
86:14 and 76:24 (L/D) (Table 3). The results demonstrated that CuL1
possessed high enantioselectivity for the hydrolysis of racemic