Chiral recognition of amino acid esters by zinc porphyrin derivatives

Article abstract of DOI:10.1039/b100942g

Three novel chiral zinc porphyrins (4a-4c) with protected chiral amino acid substituents as chiral sources were synthesized. Their chiral recognition of amino acid methyl esters was investigated using UV-vis spectrophotometric titration. Some enantioselec

Full text of DOI:10.1039/b100942g

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Chiral recognition of amino acid esters by zinc porphyrin derivatives
Chuan Zhong Wang,a Zhi Ang Zhu,*a Ying Li,a Yun Ti Chen,a Xin Wen,b Fang Ming Miao,b
Wing Lai Chanc and Albert S. C. Chanc
a Department of Chemistry, Nankai University, T ianjin, 300071, P. R. China.
E-mail: zazhu=nankai.edu.cn
b Department of Chemistry, T ianjin Normal University, T ianjin, 300074, P. R. China
c Opening L aboratory of Asymmetry Synthesis, Hong Kong Polytechnic University, Hong Kong
Received (in Montpellier, France) 26th January 2001, Accepted 13th March 2001
First published as an Advance Article on the web 15th May 2001
Three novel chiral zinc porphyrins (4aÈ4c) with protected chiral amino acid substituents as chiral sources were
synthesized. Their chiral recognition of amino acid methyl esters was investigated using UV-vis spectrophotometric
titration. Some enantioselectivities obtained are higher than that of the known system, and the highest achieved in
our study is 21.54 using host 4a with PheOCH as guest. We also show that higher enantioselectivity can be
3
obtained at lower temperature. Circular dichroism spectra of chiral porphyrin derivatives binding to enantiomers of
guests show great shape and intensity di†erences. Molecular modeling was performed to understand chiral
recognition on a molecular level, and the results are in good agreement with the experimental data.
Chiral recognition is an attractive subject in the area of hostÈ
guest chemistry, since it is a fundamental process for a range
of chemical and biological phenomena.1h3 Several types of
host molecules for chiral recognition studies have been
reported4h10 because of their biological and chemical impor-
tance. Among them, porphyrins are relatively accessible mol-
ecules, and this can be seen in the zinc and aluminium
porphyrin studies reported over the past 15 years.11h16
Methyl esters of amino acids are usually employed as guest
molecules in chiral recognition studies. Certain chiral porphy-
rin receptors exhibit an enantioselectivity for amino acid
esters as high as 7.5, the highest value reported so far.11
In current studies, the focus on chiral recognition is to
enhance the enantioselectivity. It has been proposed that the
enantioselectivity of the host for amino acid esters could be
e†ectively enhanced by constructing a speciÐc chiral environ-
ment derived from protected amino acids on host molecules.
Therefore a series of chiral porphyrin-amino acid derivatives
have been prepared from 5a,10b,15a,20b-tetrakis(o-amino-
phenyl)porphyrin (a,b,a,b-TAPP) and protected amino acids
for an amino acids enantioselectivity study. The compound
a,b,a,b-TAPP17 was selected because it is one of the most
commonly used for biochemical models. Protected amino
acids were chosen because they can be easily obtained and are
available in abundance.18 As anticipated, they proved to be
e†ective hosts for chiral recognition and their zinc complexes
achieved surprising enantioselectivities, as high as 21.54, the
highest value to date that we know of. The causes of their high
selectivities will be investigated in this paper.
TAPP20 were all prepared following previously documented
methods.
1H NMR spectra were recorded on either a Varian INOVA
500Nb or Bruker AC-P 200 NMR spectrometer, tetra-
methylsilane (TMS) was used as internal reference for all
spectra. UV-vis spectra were measured on a Beckman DU-8B
spectrophotometer with a thermostatted cell compartment.
Circular dichroism (CD) spectra were taken on a JASCO-715
spectropolarimeter with a thermostatted cell compartment.
Infrared spectra (KBr disc) were recorded on a Bio-Rad FTS
135 FT-IR spectrometer. Carbon, hydrogen and nitrogen con-
tents were determined with a PerkinÈElemer 240 elemental
analyzer. Molecular modeling was carried out with the Sybyl
6.3 program (Tripos Inc. St. Louis, MO, 1998) using the
Tripos force Ðeld on an SGI Indigo II workstation.
Syntheses
Synthesis of chiral porphyrins (3a–c). To a solution of 67.52
mg (0.1 mmol) of a,b,a,b-TAPP in 40 mL of dichloromethane
was added 0.5 mmol of proctected amino acid. The reaction
mixture was stirred at [10 ¡C for 10 min. After addition of
103 mg (0.5 mmol) of dicyclohexylcarbodiimide (DCC), the
solution was stirred at 0 ¡C for 12 h. After Ðltration through a
glass frit under vacuum, the solution was washed successively
with 5% aqueous citric acid solution, water, 5% aqueous
sodium carbonate solution, and water. The solution was dried
and evaporated. The residue obtained was dissolved in 5 mL
of dichloromethane and column chromatographed on silica
gel (dichloromethaneÈether 5 : 1).
Experimental
3a. 108 mg (80% yield); 1H NMR (CDCl ): d [2.60 (s, 2H,
3
pyrrole-NH), 0.44 (d, 12H, CH ), 0.73È2.05 [m, 36H, (CH ) ],
3
3 3
4.06 (m, 4H, CH), 4.30 (s, 4H, CONH), 7.11 (s, 4H, ArNH),
Materials and physical measurements
7.24È7.82 (m, 16H, ArH), 8.63È8.77 (m, 8H, b-pyrrole-H).
All reagents and solvents were of commercial reagent grade
and were used without further puriÐcation. Dry dichloro-
Anal. calc. for C H N O : C, 67.14; H, 6.38; N, 12.36;
76 86 12 12
found: C, 67.01; H, 6.03; N, 12.53%. UV-vis (CH Cl ):
2 2
methane was obtained by distilling over CaCl . Amino acid
j
/nm 419.6 (Soret), 514.6, 548.8, 590.4, 647.2; IR (KBr,
2
max
methyl esters were prepared according to the reported
cm~1): 3380.93 (CONÈH), 3323.03 (pyrrole NÈH), 2931.04
(ÈCH ), 2854.96 (ÈCH È), 1700.72 (C2O), 1522.28 (pyrrole).
methods.11 BocAla and BocPro, benzoyl-Gln19 and a,b,a,b-
3
2
DOI: 10.1039/b100942g
New J. Chem., 2001, 25, 801È806
801
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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3b. 112 mg (77% yield); 1H NMR (CDCl ): d [2.64 (s, 2H,
annealing to ensure the last result is a minimum energy con-
3
pyrrole-NH), 0.83È0.89 (m, 24H, CH ), 1.15È1.41 (m, 36H,
formation at a high level. Consequently D,L-AlaOCH were
2
CH ), 3.43È3.51 (m, 4H, CH), 5.27 (s, 4H, CONH), 7.10 (s, 4H,
3
3
added on the porphyrin plane and systematic conformational
ArNH), 7.33È7.84 (m, 16H, ArH), 8.61È8.74 (m, 8H, b-pyrrole-
searches were carried out. With the conformations of the
H). Anal. calc. for C
84 94 12 12
11.48; found: C, 68.52; H, 6.21; N, 11.53%. UV-vis (CH Cl ):
H
N
O
: C, 68.93; H, 6.47; N,
4a É L-AlaOCH and 4a É D-AlaOCH complexes obtained
3
3
above energetic analyses were performed.
2 2
j
/nm 419.8 (Soret), 514.2, 547.8, 590.8, 647.0; IR (KBr,
cm~1): 3380.18 (CONÈH), 3328.43 (pyrrole NÈH), 2931.43
max
Results and discussion
(ÈCH ), 2854.72 (ÈCH È), 1698.18 (C2O), 1528.97 (pyrrole).
3
2
3c. 132 mg (82% yield); 1H NMR (CDCl ): d [2.66 (s, 2H),
The chiral framework of these porphyrin derivatives (4aÈc)
was constructed by condensing a,b,a,b-TAPP 120 with pro-
tected amino acids 2 to give the chiral porphyrins 3aÈc. Their
zinc complexes 4aÈc were prepared based on a previously
reported method (Scheme 1).21 Complexation experiments
were performed in dichloromethane solution. Visible spectros-
copy, circular dichroism spectra (CD), and molecular model-
ing were employed to study the enantioselectivity of L and D
amino acid esters by chiral zinc porphyrins.
3
0.89 (t, 4H), 1.21 (s, 8H), 1.51 (m, 20H), 3.44 (s, 4H), 6.81 (s,
4H), 7.05È8.76 (m, 36H), 8.79 (s, 8H). Anal. calc. for
C
H N O : C, 68.90; H, 5.15; N, 13.97; found: C, 68.88;
92 82 16 12
H, 5.02; N, 13.71%. UV-vis (CH Cl ): j /nm 422.1 (Soret),
516.4, 550.4, 589.6, 546.6; IR (KBr, cm~1): 3379.18 (CONÈH),
3312.7 (pyrrole NÈH), 1665.46 (C2O), 1525.7 (pyrrole).
2
2
max
Synthesis of zinc(II) porphyrins (4a–c). To a solution of 0.05
mmol of porphyrin (3aÈc) in 50 mL of dichloromethane, was
UV-vis spectrophotometric titration
added l0 mL of a saturated solution of Zn(OAc) in methanol.
2
Absorption spectra of zinc porphyrins binding with amino
acid esters of varying concentrations show an isosbestic point
in the Soret band, which indicates 1 : 1 complexation between
the host and guest molecules. The absorption spectra of 4a are
shown in Fig. 1. The association constant K is given by eqn.
(1):
The reaction mixture was stirred at room temperature for 2 h,
and the solvent evaporated. The solid obtained was dissolved
in 50 mL of dichloromethane. The organic layer was washed
twice with water, then dried over anhydrous Na SO over-
2
4
night and concentrated. The crude product was column chro-
matographed on silica gel (dichloromethaneÈether 5 : 1).
4a. 64 mg (90% yield); 1H NMR (CDCl ): d 0.84 (d, 12H,
1
1
1
1
3
\
É
]
(1)
CH ), 1.18È1.42 [m, 36H, (CH ) ], 3.40 (m, 4H, CH), 4.31 (s,
3
3 3
A [ A Ka
c
a
4H, CONH), 7.07 (s, 4H, ArNH), 7.30È7.98 (m, 16H, ArH),
8.71 (s, 8H, b-pyrrole-H). Anal. calc. for C Zn: C,
0
L
H
N O
where A is the absorbance of zinc porphyrin solution, A is
the absorbance in the presence of a guest at concentration c ,
76 84 12 12
0
64.15; H, 5.95; N, 11.81; found: C, 63.85; H, 5.71; N, 11.65%.
L
UV-vis (CH Cl ): j /nm 426.3 (Soret), 557.0, 594.8; IR
and a is a constant. The quantity 1/(A [ A) has a linear rela-
2
2
max
0
tion to 1/c ; the association constant, K, can be obtained from
L
(KBr, cm~1): 3398.62 (CONÈH), 2932.44 (ÈCH ), 1688.96
3
(C2O), 1522.78 (pyrrole).
the ratio of the intercept to the slope. The results are present-
4b. 66 mg (87% yield); 1H NMR (CDCl ): d 0.82È0.90 (m,
ed in Table 1. The binding between 4b and amino acid esters
was too weak to be accurately determined and therefore the
results are not included.
Association constants between hosts and amino acid esters
are in the range 102È104, which are much smaller than that of
ZnTPP.22,23 The relatively weak binding abilities are ascribed
to the steric repulsion between the side chain of the amino
acid esters and the branch group of the zinc porphyrins. For
host 4a, the association constants of D-amino acid esters are
larger than that of their optical antipodes. It is worthy of note
3
24H, CH ), 1.15È1.31 [m, 36H, (CH ) ], 3.44È3.47 (m, 4H,
2
3 3
CH), 5.10 (m, 4H, CONH), 7.31 (m, 4H, ArNH), 7.44È7.80 (m,
16H, ArH), 8.67 (s, 8H, b-pyrrole-H). Anal. calc. for
C
H N O Zn: C, 66.07; H, 6.07; N, 11.01; found: C,
84 92 12 12
65.85; H, 5.82; N, 11.36%. UV-vis (CH Cl ): j /nm 405.7
2
2
max
(sh), 427.1 (Soret), 557.8, 595.5; IR (KBr, cm~1): 3381.62
(CONÈH), 2975.68, 2928.1 (ÈCH ), 2854.46 (ÈCH È), 1699.97
(C2O), 1520.66 (pyrrole).
3
2
4c. 71 mg (85% yield); 1H NMR (CDC1 ): d 0.91 (t, 4H),
3
1.23 (s, 8H) 1.52 (m, 20H), 3.45 (s, 4H), 6.86 (s, 4H), 7.03È8.73
that the enantioselectivity for D,L-PheOCH is very high at
21.54, which is the highest value that we are aware of. This is
believed to arise from the bulkier aromatic group in
PheOCH . The structure of the D-enantiomer of the guest
3
(m, 36H), 8.75 (s, 8H). Anal. calc. for C
66.28; H, 4.84; N, 13.44; found: C, 66.12; H, 4.45; N, 13.19%.
H N O Zn: C,
92 80 16 12
UV-vis (CH Cl ): j /nm 428.5 (Soret), 558.0, 595.8; IR
2
2
max
3
(KBr, cm~1): 3387.52 (CONÈH), 929.37 (ÈCH ), 2856.03
molecules can be better aligned with the host 4a, the simpliÐed
3
(ÈCH È), 1665.71 (C2O), 1523.63 (pyrrole).
recognition process is shown in Scheme 2. The steric repulsion
between the larger groups in 4a and the L-enantiomer is unfa-
vorable for ““close bindingÏÏ of the host and guest. Accord-
ingly, the association of 4a with the L-guest becomes weaker
and the association constant smaller than with the D-guest.
2
UV-vis spectrophotometric titrations
To a solution of about 5.0 ] 10~6 M of 4 in dichloromethane
was added a solution of a-amino acid esters in dichloro-
methane at room temperature. Changes in the absorbance of
the Soret band were monitored at ten di†erent concentrations
of the guest compounds in the range 10~3È10~2 M.
Table 1 Enantioselectivity of 4a and 4c binding to amino acid esters
in dichloromethane at 25 ¡C
Molecular modeling of the chiral recognition of AlaOCH
using 4a
3
Guest
K(4a)
K /K (4a)
K(4c)
K /K (4c)
D
L
L
D
Molecular modeling was performed with the Tripos force Ðeld
as implemented in Sybyl 6.3 software (Tripos Inc.) on an SGI
Indigo II workstation. The 3D structure of 4a was constructed
by using the SKETCH module. The macrocyclic ring of the
porphyrin was constrained and energy minimization was
carried out with a gradient of 0.05 kcal mol~1. Taking the
optimized geometry as the starting conformation, conformer
searching of host 4a involved random search and simulated
L-AlaOCH
D-AlaOCH
L-LeuOCH
D-LeuOCH
L-PheOCH
1967
8230
195
2351
136
2930
288
1802
464
336
876
786
752
176
1025
900
4.18
12.06
21.54
6.26
1.38
1.11
4.27
1.14
3
3
3
3
3
D-PheOCH
3
L-IleOCH
D-IleOCH
3
3
802
New J. Chem., 2001, 25, 801È806

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Scheme 1
On the other hand, host 4c prefers to bind with the L-
enantiomer rather than the D. As a consequence, the ratio of
to K is less than 1; the values of K /K are presented in
K
D
L
L
D
Table 1 to characterize the enantioselectivity. The selectivity
of host 4c for PheOCH (K /K \ 4.27) is also much higher
3
L
D
than that for other amino acid esters. The data in Table 1 also
Fig. 1 UV-vis titration of 4a with L-LeuOCH in dichloromethane
3
at 298 K: [4a] \ 8.23 ] 10~6 M; [L-LeuOCH ] \ 0, 0.8, 1.0, 1.2, 2.0,
3
4.0, 8.0 and 12.0 ] 10~3 M.
Scheme 2
New J. Chem., 2001, 25, 801È806
803

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indicate that the chiral recognition ability of 4a is much
stronger than that of 4c.
The enantioselectivity of hosts is temperature dependent;
this can be see in the LeuOCH binding study (Fig. 2). The
3
enantioselectivity at 15 ¡C is nearly halved at 30 ¡C. This phe-
nomenon can be explained by the vanÏt Ho† equation:
*H
RT
*S
R
D
D
ln K \ [
]
]
(2)
(3)
D
*H
RT
*S
R
L
L
ln K \ [
L
By combining eqns. (2) and (3), one Ðnds:
K
K
*H [ *H
*S [ *S
D
L
D
L
]
D
L
ln
\ [
\ [
RT
R
**H **S
]
(4)
RT
R
In Fig. 2, ln(K /K ) shows a good linear relationship with
D
L
1/T . From the slope, the value of **H is negative, therefore,
the value of K /K will decrease exponentially as the tem-
D
L
perature increases.
Enthalpy–entropy compensation relationship
Thermodynamic parameters are determined from the vanÏt
Ho† equation and an enthalpyÈentropy compensation e†ect
was found in our system. The relationship can be expressed
as:5
T *S \ a*H ] T *S
(5)
Fig. 3 EnthalpyÈentropy compensation relationship of complexes of
(a) 4a and (b) 4c.
0
According to the classiÐcation of Rekarsky and Inoue5 the
slope a and intercept T *S provide good indexes as to confor-
0
mational changes during complex formation and the extent of
L-amino acid esters, CD spectra exhibit a negative Cotton
e†ect induced in the Soret band, which is very similar to that
seen with 4a. With D-amino acid esters, the spectra show posi-
tive Cotton e†ects. These results suggest that D-guest mol-
ecules are highly localized on hosts and bind more tightly
than their optical antipodes.
The mechanism leading to the induced CD spectra of these
systems is still ambiguous. The experimental results support
the explanation given by Ogoshi and co-workers,13,24 where
the coupling between the electric and magnetic transition
moments of the carbonyl group and the electric transition
desolvation, respectively. A slope of a \ 0.797 and an inter-
cept of 10.56 kJ mol~1 were obtained for 4a [Fig. 3(a)] and a
slope a \ 0.828 and intercept of 10.75 kJ mol~1 for 4c [Fig.
3(b)]. The slopes are less than unity, which indicates that rela-
tively small conformational changes may accompany the
process complex formation in both cases. The slope using 4c is
smaller than with 4a, indicating that the conformational
change is comparatively easier. The intercepts of the two
systems have almost the same value, which shows that the
extent of desolvation is almost the same.
Circular dichroism studies
Fig. 4 shows the CD spectra of 4a and 4a with L-AlaOCH or
3
D-AlaOCH in dichloromethane. In all complexes with
3
Fig. 4 Circular dichroism spectra of host 4a (5 ] 10~6 M) and its
Fig. 2 The inÑuence of temperature on the enantioselectivity of 4a
complexes with D,L-AlaOCH (5 ] 10~3 M) in dichloromethane at
3
with LeuOCH as guest.
298 K.
3
804
New J. Chem., 2001, 25, 801È806

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moments of porphyrin chromophores lead to the observed
Cotton e†ects. This mechanism suggests that the D-
enantiomer is more stable on the porphyrin 4a plane. The
orientations of the carbonyl groups of the two guest enantio-
mers are opposite but not symmetric.
out-of-plane bending) and total external energy (1È4 van der
Waals energy, van der Waals energy). The remaining terms
are molecular energy components minus the energy of the
porphyrin ring. The 1È4 van der Waals refers to interactions
of non-neighbouring atoms, and van der Waals to the inter-
actions of neighbouring ones. These energies depend on the
potential functions and their associated parameters that make
up the force Ðeld and, accordingly, are force Ðeld dependent.
They have little physical signiÐcance. Nonetheless, we provide
these energies to examine how the individual terms compris-
ing particular force Ðelds di†er in D vs. L complexes. In Table
2 the largest di†erence in energy appears in the van der Waals
energy. The D-enantiomer is thus more tightly bound in part
because the system has less steric repulsion and the non-
bonded interactions are more favorable.
Molecular modeling
The minimum energy conformation of 4a [Fig. 5(a)] was
obtained by the method of random search and simulated
annealing. The conformation has C symmetry. The branch
2
groups of the host provide a chiral cavity for guest molecules.
On the basis of this conformation, the minimum energy con-
formations of host 4a binding with L- and D-AlaOCH were
3
obtained by rotating the ZnÈN bond in 10¡ increments and
then performing a geometry optimization. The resulting opti-
mized geometries are shown in Fig. 5(b) and (c). The calcu-
These component energies are also partitioned into internal
and external energies. The internal energy is simply the sum of
the Ðrst four component energies, and external energy is the
sum of the van der Waals and the 1È4 van der Waals contri-
butions. Table 2 shows that the great di†erences between the
two recognition processes originate from the external energy.
lated energy of the L-AlaOCH complex is 80.104 kcal mol~1
3
and that of D-AlaOCH is 78.801 kcal mol~1, indicating that
3
the latter is more stable than the former. Consistent with this
experiment, the D-enantiomer is more tightly bound to the
host than its optical antipode and, as anticipated, the com-
puted binding energies are overestimated because the e†ect of
solvent is neglected.25
The energy components are included in Table 2. Energy in
aggregates means total energy of the porphyrin ring
(excluding substituents) deÐned as a plane, it includes total
internal energy (bond stretching, angle bending, torsional,
Conclusion
This paper reports the enantioselective binding of D,L-amino
acid esters to novel chiral porphyrins 4aÈ4c. Experimental
results reveal that the host 4a prefers to bind with the D-
enantiomers of guests, while 4c prefers the optical antipodes,
the L-enantiomers. The strongest enantioselective binding of
D,L-amino acid esters is provided by 4a with PheOCH as
3
guest, which is as high as 21.54 at 25 ¡C. The enantioselectivity
K /K is temperature dependent. The induced CD spectra of
D
L
the complexes verify the aforementioned results at a qualit-
ative level. Molecular modeling has been performed to
examine the enantioselectivity of host 4a, which is found to
arise mainly from the di†erent steric repulsion with the two
guest enantiomers.
Acknowledgements
This work is supported by a grant from the National Natural
Science Foundation (no. 29871018), State Key Laboratory of
Coordination Chemistry, Nanjing University and The Hong
Kong Polytechnic University ASD Fund.
References and notes
1
2
3
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7
8
9
K. Yannakopoulou, D. Mentzafos, I. M. Marridis and K.
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Table 2 Average energies (kcal mol~1) of L- and D-AlaOCH with
3
host molecule 4a
Energy term
E
E
E [ E
L
D
L
D
10 B. Chankvetadze, C. Yamamoto and Y. Okamoto, Chem. L ett.,
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Energy in aggregates
Bond stretching
Angle bending
61.05
61.05
0
3.58
22.23
44.92
0.57
71.30
10.10
[62.35
[52.25
80.10
3.69
22.90
44.90
1.08
72.57
9.84
[64.66
[54.82
78.80
[0.11
[0.67
0.02
[0.51
[1.27
0.26
2.31
2.57
1.30
Torsional
Out-of-plane bending
Total internal energy
1È4 van der Waals
van der Waals
Total external energy
Total energy
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805

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