Enantioselective Molecular Recognition between β-Sheets

Article abstract of DOI:10.1021/ja031632z

This communication asks whether homochiral or heterochiral interaction is preferred between enantiomeric β-sheets and finds that homochiral pairing is strongly preferred. Interactions between β-sheets occur widely among proteins through pairing of the hyd

Full text of DOI:10.1021/ja031632z

Published on Web 02/21/2004
Enantioselective Molecular Recognition between â-Sheets
De Michael Chung and James S. Nowick*
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
Received December 11, 2003; E-mail: jsnowick@uci.edu
The coming together of two hands in prayer and the coming
together of two hands in a handshake represent fundamentally
different interactions among chiral objects. Heterochiral and ho-
mochiral interactions of this sort can also occur among biomolecular
structures. This communication asks which type of interaction is
preferred between â-sheets and finds that homochiral pairing is
strongly preferred to heterochiral pairing.
Interactions between â-sheets occur widely among proteins in
biologically important processes as diverse as the dimerization of
HIV-1 protease, the interaction between Ras oncoproteins and ki-
nase enzymes, and the aggregation of â-amyloid.1 These interactions
are of course homochiral, because nature produces proteins of only
one chirality. Whether homochiral interactions between â-sheets
are preferred, however, is not clear from various reports scattered
throughout the literature: Furhop and co-workers reported the pre-
cipitation of heterochiral â-sheets upon mixing aqueous solution
of poly(D-lysine) and poly(L-lysine).2 Maggio and co-workers have
demonstrated that the aggregation of â-amyloid occurs with homo-
chiral selectivity, while Gervais and co-workers have found that
the inhibition of â-amyloid aggregation by small peptides derived
from â-amyloid occurs with heterochiral selectivity.3,4 Although
enantiomeric HIV-1 protease has been synthesized, the formation
of heterochiral HIV-1 protease dimers has not been reported.5,6
Although the edges of both L- and D-â-sheets put forth the same
pattern of hydrogen-bond donor and acceptor groups, the side chains
point in opposite directions. Homochiral pairing of â-sheets
generates structures in which the pleats and side chains of adjacent
â-strands are parallel to each other, while heterochiral pairing of
â-sheets generates structures in which the pleats and side chains
are antiparallel (Chart 1).
Previous studies in our laboratory have established that â-sheets
such as L-Leu-Leu (1a) and L-Val-Val (1b) exist as well-defined
dimers in organic solvents (Chart 2).7 â-Sheets 1a and 1b form
Chart 2. Homochiral â-Sheet Dimer
Chart 1
homodimers (1a‚1a and 1b‚1b) in CDCl3 solution. When mixed,
these compounds equilibrate to form a heterodimer (1a‚1b). The
anilide and hydrazide NH resonances of these species are well-
To test which pairing is preferred, we have prepared and studied
â-sheets 1, which comprise all L-amino acids, and â-sheets 2, which
comprise all D-amino acids.7 Variants of each of these compounds
were prepared that display different amino acids at two of the non-
hydrogen-bonded â-sheet interaction sites (R1 and R2). These var-
iants are referred to by their chirality and the residues they display
as follows: L-Leu-Leu (1a), L-Val-Val (1b), L-Val-Ala (1c), L-Ala-
Val (1d), D-Leu-Leu (2a), D-Val-Val (2b), and D-Val-Ala (2c).
1
resolved in the H NMR spectrum, permitting the identification
and quantification of the dimers (Figure 1). â-Sheets 1c and 1d
also form homo- and heterodimers that exhibit well-resolved anilide
and hydrazide NH resonances.
When the L-â-sheets (1) are mixed with the enantiomeric D-â-
sheets (2), homochiral â-sheet dimers predominate, and only small
quantities of heterochiral â-sheet dimers form. Thus, mixing of
9
3062
J. AM. CHEM. SOC. 2004, 126, 3062-3063
10.1021/ja031632z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Formation of Homo- and Heterochiral â-Sheet Dimersa
1a and 2a
1b and 2b
1c and 2c
1d and 2c
dimer ratio
K
95.8:4.2b
0.0079
3.1
97.9:2.1b
0.0018
3.9
98.5:1.5b
0.0009
4.2
42.5:53.6:3.9c
0.0068
3.2
∆G (kcal/mol)d
a
b
c
d
CDCl3, 253 K. [1‚1+2‚2]:[1‚2]. [1d‚1d]:[2c‚2c]:[1d‚2c]. Sta-
tistically corrected free-energy difference (∆G ) -RT ln(K/4)).
acid residues are involved in the interactions of 1 and 2, the free-
energy differences observed correspond to a thermodynamic
preference of 0.6-0.8 kcal/mol per interacting residue.9
A number of explanations may be envisioned for the high
enantioselectivity of molecular recognition between â-sheets.
Favorable nonbonded contacts between the adjacent â-strands may
occur when the pleats and side chains point in the same direction.
This model might also explain the preferential formation of
heterochiral â-sheets in poly(D-lysine) and poly(L-lysine), as
heterochiral â-sheet formation should minimize repulsion between
the cationic lysine side chains.2 Alternatively, the well-known twist
of â-sheets might dictate that homochiral â-strands, which should
twist in the same direction, fit together better than heterochiral
â-strands, which should twist in opposite directions.
Figure 1. 1H NMR spectra of the hydrazide and anilide NH groups of
L-Leu-Leu peptide 1a (lower), L-Val-Val peptide 1b (middle), and a mixture
of the two peptides (upper). Spectra were recorded at 500 MHz in CDCl3
at 253 K at 2.0 mM of each peptide.
The enantioselective recognition between â-sheets described
herein differs from the widely studied enantioselective binding of
ligands by chiral receptors, because it involves interactions between
partners of comparable size and achieves selectivity through the
type of shape complementarity that occurs in a handshake, rather
than the sort of lock-and-key complementarity that typically
characterizes molecular recognition between partners of largely
unequal sizes.10
Acknowledgment. This paper is dedicated to Professor Julius
Rebek, Jr. on the occasion of his 60th birthday. We thank the NSF
for grant support (CHE-0213533) and Dr. Bao Nguyen for
collecting and processing the 800 MHz EXSY data.
Figure 2. 1H NMR spectra of the hydrazide and anilide NH groups of
L-Leu-Leu peptide 1a (lower), D-Leu-Leu peptide 2a (middle), and a mixture
of the two peptides (upper). Spectra were recorded at 500 MHz in CDCl3
at 253 K at 2.0 mM of each peptide. The peak at 10.52 ppm is an impurity
present in 2a.
Supporting Information Available: Synthetic procedures and
extensive 1D and 2D 1H NMR spectral data for â-sheets 1 and 2 (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Maitra, S.; Nowick, J. S. In The Amide Linkage: Structural Significance
in Chemistry, Biochemistry, and Materials Science; Greenberg, A.,
Breneman C. M., Liebman, J. F., Eds.; Wiley: New York, 2000; Chapter
15.
(2) Fuhrhop, J.-H.; Krull, M.; Bu¨ldt, G. Angew. Chem., Int. Ed. Engl. 1987,
26, 699-700.
(3) Esler, W. P.; Stimson, E. R.; Fishman, J. B.; Ghilaridi, J. R.; Vinters, H.
V.; Mantyh, P. W.; Maggio, J. E. Biopolymers 1999, 49, 505-514.
(4) Chalifour, R. J.; McLaughlin, R. W.; Lavoie, L.; Morissette, C.; Tremblay,
N.; Boule´, M.; Sarazin, P.; Ste´a, D.; Lacombe, D.; Tremblay, P.; Gervais,
F. J. Biol. Chem. 2003, 278, 34874-34881.
(5) Milton, R. C. L.; Milton, S. C. F.; Kent, S. B. H. Science 1992, 256,
1445-1448.
(6) (a) Petsko, G. A. Science 1992, 256, 1403-1404. (b) Jung, G. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1457-1459. (c) Haack, T.; Gonza´lez, M.
J.; Sa´nchez, Y.; Giralt, E. Lett. Pept. Sci. 1997, 4, 377-386.
(7) (a) Nowick, J. S.; Lam, K. S.; Khasanova, T. V.; Kemnitzer, W. E.; Maitra,
S.; Mee, H. T.; Liu, R. J. Am. Chem. Soc. 2002, 124, 4972-4973. (b)
Nowick, J. S.; Chung, D. M. Angew. Chem., Int. Ed. 2003, 42, 1765-
1768.
Figure 3. 2D EXSY spectrum of a mixture of L-Leu-Leu peptide 1a and
D-Leu-Leu peptide 2a. The spectrum was recorded at 800 MHz in CDCl3
at 308 K at 2.0 mM of each peptide using a 500-ms mixing time. EXSY
cross-peaks are marked “EX”.
equimolar quantities of the L-Leu-Leu homochiral dimer (1a‚1a)
and D-Leu-Leu homochiral dimer (2a‚2a) in CDCl3 solution results
in the formation of new anilide and hydrazide NH resonances
(Figure 2). A 2D EXSY experiment demonstrates that the new
species exchanges with the homochiral species and corroborates
that the new species is the heterochiral dimer 1a‚2a (Figure 3).8
Quantification of these species by integration or deconvolution of
the anilide NH resonances reveals a 95.8:4.2 mixture of homochiral
and heterochiral â-sheet dimers at 253 K. This ratio corresponds
to a homochiral dimer-heterochiral dimer equilibrium constant of
0.0079 (K ) [1a‚2a]2/[1a‚1a][2a‚2a]) and a statistically corrected
free-energy difference of 3.1 kcal/mol (∆G ) -RT ln(K/4)). Small
quantities of heterochiral dimer also form upon mixing of the other
L- and D-â-sheets (Table 1).
(8) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935-967.
(9) Although the low concentration of heterochiral dimer precludes rigorously
establishing the precise mode of heterochiral interaction (e.g., through
NOE studies), it is difficult to envision a mode of dimerization other than
antiparallel â-sheet formation that would be sufficiently strong to provide
slow exchange on the NMR time scale under the conditions of these
experiments. Because alternative modes of heterochiral interaction cannot
be precluded rigorously, the value of 0.6-0.8 kcal/mol per interacting
residue should be taken as a lower limit for the preference of antiparallel
homochiral â-sheet formation over antiparallel heterochiral â-sheet
formation.
These studies establish that homochiral pairing of â-sheets is
preferred to heterochiral pairing, at least within the context of
nonpolar side chains and a low-polarity solvent. Since five amino
(10) Webb, T. H.; Wilcox, C. S. Chem. Soc. ReV. 1993, 22, 383-395.
JA031632Z
9
J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3063