Cavity effects on the enantioselectivity of chiral amido[4]resorcinarene stereoisomers

Article abstract of DOI:10.1002/anie.200460663

The dramatic effects of the size and hydrophilic/hydrophobic properties of cavities on the intrinsic reaction kinetics and dynamics is shown by the gas-phase reaction of (R)-(-)-2-butylamine and complexes of stereoisomeric amido[4]resorcinarene hosts with aromatic amino acids. The graph shows the kinetic plots of the base-induced loss of L-Phe (green), L-Tyr (red), and L-dopa (blue).

Full text of DOI:10.1002/anie.200460663

Angewandte
Chemie
Host–Guest Systems
Cavity Effects on the Enantioselectivity of Chiral
Amido[4]resorcinarene Stereoisomers**
Bruno Botta, Deborah Subissati, Andrea Tafi,
Giuliano Delle Monache, Antonello Filippi, and
Maurizio Speranza*
Enzyme catalysis plays a crucial role in all biochemical
processes. Natural and artificial enzymes normally exhibit a
high enantioselectivity toward chiral molecules as a conse-
quence of shape-specific noncovalent attractive and repulsive
intermolecular interactions.[1–5] An important step toward the
elucidation of enzyme mechanisms requires a comprehensive
kinetic study using simplified models under conditions, such
as the gas phase, where the molecule/receptor interactions are
not perturbed by medium effects.[6]
Calixarenes lend themselves to this task since they are
synthetic macrocycles endowed with a cavity-shaped archi-
tecture, which can be made asymmetric by the presence of
chiral pendant groups. A peculiar feature of calixarene
macrocycles is their nonplanar cyclophane structure, which
may give rise to relatively stable stereoisomeric and con-
formational forms. A pertinent case is that of 2,8,14,20-
tetrakis(l-valinamido)[4]resorcinarene (1), which can display
at least four stable stereoisomeric structures, that is, flattened
cone (1aL), 1,2-alternate (1bL), chair 1 (1cL) (Figure 1), and
chair 2.[7] Its stereoisomerism allows the tailoring of variously
shaped cavities, which can be probed to evaluate their effect
on ion or molecular recognition.[8,9]
Herein, we report the first comparative gas-phase study
along these lines. The intrinsic enantioselectivity of the 1aL,
1bL, and 1cL diastereomeric structures was checked by
introducing their proton-bonded complexes with the pure d
and l enantiomers of some aromatic amino acids (A), such as
phenylalanine (Phe), tyrosine (Tyr), and 3,4-dihydroxy-
phenylalanine (dopa), into the cell of a Fourier-transform
ion cyclotron resonance mass spectrometer (FT-ICR-MS) and
measuring the exchange rate of the amino acid A with
[*] Prof. B. Botta, Dr. D. Subissati, Dr. A. Filippi, Prof. M. Speranza
Dipartimento di Studi di Chimica e Tecnologia
delle Sostanze Biologicamente Attive
Università “La Sapienza”, 00185 Roma (Italy)
Fax: (+39)06-4991-3602
E-mail: maurizio.speranza@uniroma1.it
Prof. A. Tafi
Dipartimento Farmaco Chimico Tecnologico
Università di Siena, 53100 Siena (Italy)
Dr. G. Delle Monache
Istituto Chimica del Riconoscimento Molecolare
Università Cattolica del Sacro Cuore, 00168 Roma (Italy)
[**] This work was supported by the Ministero dell’Istruzione dell’Uni-
versità e della Ricerca (MIUR) and the Consiglio Nazionale delle
Ricerche (CNR). The authors express their gratitude to F. Angelelli
for technical assistance.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 4767 –4770
DOI: 10.1002/anie.200460663
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Figure 2. Kinetic plots for the gas-phase reaction between (R)-(ꢀ)-2-
butylamine (BR) and [(1cL)·H·A]+ (A=l-Phe (open circles;
P(BR)=4.1”10ꢀ8 mbar); l-Tyr (solid circles; P(BR)=6.2”10ꢀ8 mbar);
l-dopa (diamonds; P(BR)=2.6”10ꢀ7 mbar)).
the linear [1·H·A]þfast decay curve, provides an estimate of the
relative distribution of [1·H·A]þfast and [1·H·A]sþlow, respectively
(Table 1).
Table 1: Percent distribution of isomeric [1·H·A]+ structures.
Host (1)
Guest (A)
[1·H·A]þfast
[1·H·A]þslow
1aL
d-dopa
l-dopa
d-Tyr
l-Tyr
d-dopa
l-dopa
20ꢁ4
19ꢁ3
42ꢁ3
44ꢁ2
48ꢁ2
39ꢁ3
80ꢁ4
81ꢁ3
58ꢁ3
56ꢁ2
52ꢁ2
61ꢁ3
Figure 1. Formulas and side views of local minimum geometry of
2,8,14,20-tetrakis(l-valinamido)[4]resorcinarene (R=l-CH2CONH-
CH(iPr)COOEt) in the flattened-cone 1aL, 1,2-alternate 1bL, and
chair 1 1cL structures.
1cL
1cL
(S)-(+)- and (R)-(ꢀ)-2-butylamine (BS and BR, respectively)
[Eq. (1)].
The second-order rate constants k, calculated from the
corresponding k’/[B] ratios, are listed in Table 2. Their values,
compared with the relevant collision rate constants (kcoll),[10]
provide a measure of the efficiency of the reaction (eff =
k/kcoll, figures in parentheses in Table 2). The enantioselec-
tivity factor 1 is defined as the ratio of the k value for the
reaction of a given base B with the [1·H·AD]+ complex and
that for the same reaction with the [1·H·AL]+ complex. The
enantioselectivity factor x is defined as the ratio of the k value
for the reaction of a given [1·H·A]+ complex with (R)-(ꢀ)-2-
butylamine and that for the same reaction with (S)-(+)-2-
butylamine. The 1 < 1 terms of Table 2 indicate that the
[1·H·AL]+ complex is more reactive than its [1·H·AD)]+
diastereomer toward the same base B. The reverse is true
when 1 > 1. The x > 1 terms of Table 2 indicate that (R)-(ꢀ)-
2-butylamine is more reactive than (S)-(+)-2-butylamine
toward the same [1·H·A]+ complex. The reverse is true
when x < 1.
Comparative analysis of the kinetic data reported in
Tables 1 and 2 shows the strict relationship between the
structural features of the selected stereoisomeric hosts and
their kinetic enantioselectivity toward a given chiral guest. In
a previous study,[9] a structure (denoted as up) was assigned to
the [(1aL)·H·(dopa)]þfast isomer in which the guest is located on
the hydrophobic aromatic rim of the host. Conversely, in the
[(1aL)·H·(dopa)]þslow structure (denoted as down), the guest is
½1 Á H Á AŠþ þ B ! ½1 Á H Á BŠþ þ A
ð1Þ
The rate constants k’ for the reaction defined in Equa-
tion (1) were obtained from the slopes of the pseudo-first-
order rate plots (ln(I/I0) versus t), where I is the abundance of
complex [1·H·A]+ at the delay time t and I0 is the sum of the
abundances of [1·H·A]+ and [1·H·B]+. Irrespective of the
configurations of A and B, linear rate plots are invariably
observed with all [1·H·A]+ complexes except those with 1cL,
Tyr ; 1aL, dopa; and 1cL, dopa (see Figure 2 and Supporting
Information).
The bimodal kinetics shown by these latter systems reveal
the presence of several stable isomeric [1·H·A]+ structures.
Above a certain reaction time, for example 6 s for [(1cL)·H·(l-
Tyr)]+ and 12 s for [(1cL)·H·(l-dopa)]+ (Figure 2), the bimo-
dal curves become linear (correlation coefficient r2 = 0.990
and 0.976, respectively) which indicates that, at that delay
time, only the less-reactive isomer [1·H·A]þslow is present. The
time dependence of the more-reactive fraction [1·H·A]þfast can
be inferred from the overall [1·H·A]+ decay after subtracting
the first-order decay of the [1·H·A]þslow fraction. The good
linearity of the resulting curve (r2 = 0.998 and 0.991, respec-
tively) suggests that only a single isomer is present in the more
reactive [1·H·A]þfast fraction. Its y intercept, as well as that of
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Angewandte
Chemie
Table 2: Exchange rate constants (k”10ꢀ10 cm3 moleculeꢀ1 sꢀ1).
chiral
hydrophilic
R pendant
groups, not only along the down-
like cavity of the host but also near
its up-like region. This effect is also
mirrored by the reduced difference
Host
(1)
Guest
(A)
(R)-(ꢀ)-C4H9NH2
(S)-(+)-C4H9NH2
k
eff
k
eff
x
1aL
d-Phe
l-Phe
3.60ꢁ0.03
2.20ꢁ0.03
(0.32)
(0.20)
3.56ꢁ0.04
2.28ꢁ0.04
1.56ꢁ0.05
1.42ꢁ0.02
1.36ꢁ0.02
1.04ꢁ0.03
1.26ꢁ0.05
1.82ꢁ0.20
0.69ꢁ0.11
0.064 ꢁ0.008
0.079 ꢁ0.009
0.81ꢁ0.19
(0.32)
(0.20)
1.01ꢁ0.02
0.96ꢁ0.03
in
reaction
rates
of
and
1=1.64ꢁ0.03
0.86ꢁ0.02
[(1cL)·H·(dopa)]fþast
d-Tyr
l-Tyr
(0.08)
(0.10)
(0.13)
(0.13)
0.61ꢁ0.02
0.79ꢁ0.02
[(1cL)·H·(dopa)]þslow relative to
1.07ꢁ0.02
that of the [(1aL)·H·(dopa)]fþast
/
1=0.80ꢁ0.04
2.28ꢁ0.06
d-dopafast
l-dopafast
(0.20)
(0.27)
(0.11)
(0.16)
1.81ꢁ0.12
1.65ꢁ0.24
[(1aL)·H·(dopa)]þslow pair (Table 2).
The size and physical properties of
the up-like and down-like regions of
the chair stereoisomer 1cL are
markedly different from those of
the up and down regions of the
flattened cone stereoisomer 1aL.
This may account for their opposite
enantioselectivity toward dopa
(1 factors in Table 2). The diverse
orientation of the incoming base B
toward [1·H·(dopa)]+ (1 = 1aL, 1cL)
may play a role as well, as suggested
by the appreciable differences in
3.00ꢁ0.09
1=0.76ꢁ0.04
0.073 ꢁ0.008
0.100 ꢁ0.008
1=0.73ꢁ0.13
d-dopaslow
l-dopaslow
(0.006)
(0.009)
(0.006)
(0.007)
1.14ꢁ0.28
1.27ꢁ0.25
1bL
d-Phe
l-Phe
4.06ꢁ0.09
3.78ꢁ0.07
(0.36)
(0.34)
5.15ꢁ0.06
4.93ꢁ0.06
1.04ꢁ0.03
2.93ꢁ0.03
3.81ꢁ0.06
0.77ꢁ0.02
1.91ꢁ0.06
1.55ꢁ0.05
1.23ꢁ0.08
(0.46)
(0.44)
0.79ꢁ0.02
0.77ꢁ0.02
1=1.07ꢁ0.05
2.29ꢁ0.04
d-Tyr
l-Tyr
(0.21)
(0.27)
(0.26)
(0.34)
0.78ꢁ0.02
0.80ꢁ0.03
3.06ꢁ0.06
1=0.75ꢁ0.03
1.57ꢁ0.06
d-dopa
l-dopa
(0.14)
(0.13)
(0.17)
(0.14)
0.82ꢁ0.02
0.95ꢁ0.03
1.47ꢁ0.05
1=1.07ꢁ0.07
the
(Table 2).
corresponding
x terms
1cL
d-Phe
l-Phe
3.67ꢁ0.04
3.20ꢁ0.05
(0.33)
(0.29)
3.61ꢁ0.04
3.25ꢁ0.04
1.11ꢁ0.03
3.95ꢁ0.11
3.00ꢁ0.13
1.32ꢁ0.09
0.97ꢁ0.04
0.61ꢁ0.03
1.59ꢁ0.13
0.87ꢁ0.10
1.12ꢁ0.10
0.78ꢁ0.09
0.095 ꢁ0.034
0.046 ꢁ0.008
2.08ꢁ1.33
(0.32)
(0.29)
1.02ꢁ0.02
0.98ꢁ0.04
The apparently linear rate plots
1=1.15ꢁ0.03
3.39ꢁ0.08
associated with the attack by base B
on
d-Tyr
(0.30)
(0.29)
(0.35)
(0.27)
0.86ꢁ0.04
1.08ꢁ0.07
fast
l-Tyr
3.24ꢁ0.06
the
diastereomeric
fast
[(1bL)·H·(dopa)]+ complexes can
be correlated not only with the
occurrence of a single complex
structure, but also with the occur-
rence of different structures having
1=1.05ꢁ0.04
0.90ꢁ0.03
d-Tyrslow
l-Tyrslow
(0.08)
(0.05)
(0.09)
(0.06)
0.93ꢁ0.06
0.95ꢁ0.09
0.58ꢁ0.03
1=1.55ꢁ0.14
1.27ꢁ0.03
d-dopafast
l-dopafast
(0.11)
(0.04)
(0.08)
(0.10)
1.45ꢁ0.23
0.42ꢁ0.08
0.47ꢁ0.04
1=2.68ꢁ0.30
0.094 ꢁ0.006
0.041 ꢁ0.005
1=2.30ꢁ0.44
comparable
reactivity
toward
d-dopaslow
l-dopaslow
(0.008)
(0.004)
(0.009)
(0.004)
0.99ꢁ0.64
0.86ꢁ0.35
base B. Although no clear-cut evi-
dence is presently available,[11] the
reaction patterns shown by the
([1·H·(dopa)]+ (W= 1aL; 1cL) ster-
eoisomers lend support to the latter
hypothesis. This view conforms well
located at the hydrophilic rim of the host among the four
chiral R pendant groups (Figure 1). The relative abundance
of these isomeric structures (20 ꢁ 4 versus 80 ꢁ 4%; Table 1)
qualitatively conforms to their computed stability difference
(20 kJmolꢀ1).[9] The loss of dopa from the up structure of
[(1aL)·H·(dopa)]þfast is promoted by the uptake of base B into
the chiral hydrophilic rim of the host. This accounts for the
high x values (1.81 ꢁ 0.12 (d-dopa); 1.65 ꢁ 0.24 (l-dopa)) in
Table 2. A similar uptake is prevented in the down structure
of [(1aL)·H·(dopa)]þslow by the presence of the guest in the
hydrophilic cavity of the host. In this case, the amine B can
only interact from outside the chiral cavity of the host, which
can then exert only in part its asymmetry toward it (x = 1.14 ꢁ
0.28 (d-dopa); 1.27 ꢁ 0.25( l-dopa); Table 2).
with the
1
and x enantioselectivity values of the
[(1bL)·H·(dopa)]+ complexes, which are in the middle of
those of their [1·H·(dopa)]+ (1 = 1aL, 1cL) analogues
(Table 2).
The general enantioselectivity picture shown by the
[1·H·(dopa)]+ complexes finds some confirmation in their
[1·H·(Tyr)]+ analogues. Although here the unequivocal
assignment of two stable isomeric structures is limited only
to the [(1cL)·H·(Tyr)]+ complex (Table 1), nevertheless the
same 1 value trend is observed for both the [1·H·(Tyr)]+ and
[1·H·(dopa)]+ series, which increases in the order: 1aL ꢂ
1bL < 1cL (Table 2). However, the complete absence of
phenolic OH groups in Phe has a remarkable effect on the
enantioselectivity of the reaction between base B and the
corresponding [1·H·(Phe)]+ complexes. In contrast to their
[1·H·(Tyr)]+ analogues, the [1·H·(Phe)]+ complexes invariably
exhibit 1 > 1 values which increase in the reverse order: 1cL ~
1bL < 1aL. This selectivity picture finds some analogies with
that observed in the gas-phase reaction of base B with
Similarly, the comparatively more level distribution of
the isomeric [(1cL)·H·(dopa)]þfast and [(1cL)·H·(dopa)]sþlow
structures (48 ꢁ 2 versus 52 ꢁ 2% (d-dopa); 39 ꢁ 3 versus
61 ꢁ 3% (l-dopa); Table 1) indicates that their stability
difference is somewhat reduced by the orientation of the
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diastereomeric [(1aL)·H·A]+ (A = alanine (Ala) and serine
necessitated the use of an ion gauge whose sensitivity was dependent
on the nature of the chemical species. The correction of the ionization
gauge reading was achieved by first determining the rate constant of
the reaction between the CH4C+ radical cation and CH4 in the FT-ICR
instrument at a given nominal methane pressure, and then comparing
the result obtained with the average value of the rate constants
reported for this process (1.13 ” 10ꢀ9 cm3 moleculeꢀ1 sꢀ1).[13] Subse-
quently, the correction factor needed for amine B was estimated by
using the method based on an indicated linear dependence of the
response of the ionization gauge with the polarizability of the base in
question.[14]
(Ser)) complexes.[8,9] Indeed, 1 > 1 values have been mea-
sured for [(1aL)·H·(Ala)]+, whereas the [(1aL)·H·(Ser)]+
analogues invariably display 1 < 1 values. This opposite
enantioselectivity is attributed to the different structures of
[(1aL)·H·(Ala)]+ and [(1aL)·H·(Ser)]+, the first having the Ala
guest located outside the host cavity in proximity to two
adjacent pendant groups, and the latter having the Ser guest
preferentially located inside the host cavity among the four
pendant groups (the down structure).[9]
In conclusion, chiral recognition of the selected aromatic
amino acids markedly depends on the structural features of
the tetrakis(l-valinamido)[4]resorcinarene receptor, and
mainly on the spatial orientation of its chiral R pendant
groups. The flattened cone structure 1aL is characterized by a
hydrophobic achiral (up) and a hydrophilic chiral (down)
region. Dopa and Tyr, with OH functionalities on their
aromatic ring, are preferentially hosted in the down region.
Base-induced removal of their l enantiomers is faster than
that of the d forms. The reverse is true for Phe where
OH phenolic groups are absent. Two adjacent pendant groups
lean outward from both the up-like and down-like cavities of
1cL, and the other two lean over its down-like region. Both
1cL cavities seem capable of hosting the dopa and Tyr guests.
In this case, base-induced removal of their l enantiomers is
slower than that of the d enantiomers. Only one pendant
group leans over the up-like structure of the 1,2-alternate
stereoisomer 1bL. The others form a sort of chiral down niche,
which may accommodate better the selected amino acid
guests. This may explain why the [(1bL)·H·A]+ complexes
apparently exhibit only a single structural regioisomer, and
why the base-induced removal of their amino acid guest
displays the lowest measured enantioselectivity as regards
both the configuration of the leaving guest and that of the
incoming base.
Received: May 14, 2004
Keywords: enantioselectivity · enzyme models · host–guest
.
systems · kinetics · macrocycles
[1] F. Diederich, Angew. Chem. 1988, 100, 372; Angew. Chem. Int.
Ed. Engl. 1988, 27, 362.
[2] J.-M. Lehn, Angew. Chem. 1988, 100, 91; Angew. Chem. Int. Ed.
Engl. 1988, 27, 89.
[3] D. Cram, Angew. Chem. 1988, 100, 1041; Angew. Chem. Int. Ed.
Engl. 1988, 27, 1009.
[4] K. S. Jeong, J. Rebek, Jr., J. Am. Chem. Soc. 1988, 110, 3327, and
references therein.
[5] N. Pant, A. D. Hamilton, J. Am. Chem. Soc. 1988, 110, 2002.
[6] For recent reviews on gas-phase enantioselectivity, see: a) M.
Speranza, Adv. Phys. Org. Chem. 2004, 39, 147; b) M. Speranza,
Int. J. Mass Spectrom. 2004, 232, 277.
[7] B. Botta, G. Delle Monache, P. Salvatore, F. Gasparrini, C.
Villani, M. Botta, F. Corelli, A. Tafi, E. Gacs-Baitz, A. Santini, C.
Carvalho, D. Misiti, J. Org. Chem. 1997, 62, 932.
[8] B. Botta, M. Botta, A. Filippi, A. Tafi, G. Delle Monache, M.
Speranza, J. Am. Chem. Soc. 2002, 124, 7658.
[9] B. Botta, A. Tafi, M. Botta, G. Delle Monache, A. Filippi, M.
Speranza, Chem. Eur. J. 2004, 10, 4126.
[10] T. Su, J. Chem. Phys. 1988, 88, 4102 – 4103, 5355.
[11] Careful inspection of Figures in the Supporting Information does
not exclude the possibility of bimodal reaction kinetics between
base B and the diastereomeric [(1bL)·H·(dopa)]+ complexes.
[12] Observation of the same time dependence of [1·H·A]+ when
unquenched or collisionally quenched by methane in the FT-
ICR cell indicates that the [1·H·A]+ complexes are translation-
ally and vibrationally thermal when reacting with base B.
[13] Y. Ikezoe, S. Matsuoka, M. Takebe, A. A. Viggiano, Gas-Phase
Ion–Molecule Reaction Rate Constants Through 1986, Maruzen
Company, Tokyo, 1987.
Experimental Section
Optically pure 1aL, 1bL, and 1cL were coproduced from the BF3·Et2O-
catalyzed cyclization of the l enantiomer of (E)-N-[1-carboxyethyl)-
2-methylpropyl]-2,4-dimethoxycinnamamide and isolated according
to established procedures.[7] The d and l enantiomers of the amino
acids (A) were obtained from Aldrich Co. and used without further
purification. The same source provided the (R)-(ꢀ) (BR) and (S)-(+)
(BS) enantiomers of 2-butylamine, which were purified in the vacuum
manifold with several freeze–thaw cycles.
[14] J. E. Bartmess, R. M. Georgiadis, Vacuum 1983, 33, 149.
The experiments were performed at room temperature in an
APEX 47e FT-ICR mass spectrometer equipped with an ESI source
(Bruker Spectrospin) and a resonance cell (“infinity cell”) situated
between the poles of a superconducting magnet (4.7 T). Stock
solutions of 1 (1 ” 10ꢀ5 m) in 1:3 H2O/CH3OH, which contained a
fivefold excess of the appropriate amino acid A, were electrosprayed
through a heated capillary (1308C) into the external source of the FT-
ICR mass spectrometer. The resulting ions were transferred into the
resonance cell by a system of potentials and lenses and thermalized by
collisions with methane pulsed into the cell through a magnetic
valve.[12] Abundant signals, which corresponded to the natural
isotopomers of the proton-bound complex [1·H·A]+, were monitored
and isolated by broad-band ejection of the accompanying ions. The
[1·H·A]+ family was then allowed to react with the chiral amine B
present in the cell at a fixed pressure of 4 ” 10ꢀ8 to 3 ” 10ꢀ7 mbar.
Accurate measurement of the amine B pressure in the resonance cell
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