Chain-terminus triggered chiral memory in an optically inactive 3 10-helical peptide

Article abstract of DOI:10.1021/ja805647k

A new optically inactive 3₁₀-helical peptide with a side-chain cross-linking was found to exhibit chiral memory stored in the peptide backbone, whose chirality was induced by a noncovalent interaction of a chiral molecule at the N-terminal end of the peptide. Copyright

Full text of DOI:10.1021/ja805647k

Published on Web 08/23/2008
Chain-Terminus Triggered Chiral Memory in an Optically Inactive 310-Helical
Peptide
Naoki Ousaka,† Yoshihito Inai,‡ and Reiko Kuroda*,†,§
Japan Science and Technology Agency, ERATO-SORST Kuroda Chiromorphology Team, 4-7-6, Komaba,
Meguro-ku, Tokyo 153-0041, Japan, Department of Frontier Materials, Graduate School of Engineering, Nagoya
Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan, and Department of Life Sciences,
Graduate School of Arts and Sciences, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
Received July 20, 2008; E-mail: ckuroda@mail.ecc.u-tokyo.ac.jp
Control of screw sense of helical polymers and oligomers that
are composed of an achiral backbone by covalent or noncovalent
chiral interactions has been developed mostly in the past two
decades.1 In 1999, Yashima and his co-workers demonstrated that
external chiral stimuli induced helical sense in a dynamically
optically inactive helical polymer and the induced macromolecular
helicity was maintained in a solution for a long period of time after
the complete replacement of the chiral molecule with an achiral
one (“chiral memory”).2a Such a memory system in helical
molecules has so far been limited to artificial polymeric backbones
(polyacetylene2a-c and polyisocyanide2d), although there have been
many reports on supramolecular systems.3
Figure 1. Side and top views of the right-handed structure of 1 as energy-
minimized by the semiempirical MO calculation (AM1 method14). Hydrogen
atoms except for the amino and amide protons were omitted for clarity.
Dotted lines represent intramolecular hydrogen bonds.15
Herein, we report “chiral memory” of an optically inactive 310-
helical peptide4 which is composed of only achiral amino acids
and possesses a chiral recognition site at the N-terminal region with
a single intramolecular side-chain cross-linking.
temperature.11 One [Aib(3)] of the two resonances shows a significant
downfield shift with decreasing temperature, and the other [∆ZPhe(2)]
becomes sharp at lower temperatures but disappears at higher tem-
peratures.11 Chemical shift of the former NH is sensitive to DMSO-
d6 percentage in CDCl3 solution (Figure S3). These data suggest the
lack of intramolecular H-bonding for the two protons.13 The structure
of 1 optimized by semiempirical MO calculation (AM1 method14 in
MOPAC2007) afforded a typical 310-helical conformation; the averaged
|φ|/|Ψ| torsion angles for ∆ZPhe(2) to Aib(9) were |50|°/|33|° (Figure
1). Additionally, the side-chain constraint in peptide 1 promotes the
310-helical structure and destabilizes an R-helix due to severe strain.10c
All the proton signals of the piperidine rings of 1 are very sharp
and distributed over a wide chemical-shift range, because the motion
of the two piperidine rings is constrained (Figure S2).10c Both
methylene protons of -CH2-C6H4-CH2- (∼3.6-3.8 ppm) in the
bridging moiety show diastereotopic splitting. Moreover, N-terminal
R, ꢀ-methylene protons of the ꢀ-Ala residue (∼2.4 and 3.0 ppm)
are observed individually (Figure S2), as assigned by a 2D TOCSY
spectrum (see Figure S4), although they are placed distant from
the bridging region. Observation of these diastereotopic protons
indicates peptide 1 as a whole exhibits slow helix inversion.
A N-terminal free decapeptide, H-ꢀ-Ala-∆ZPhe-Aib-∆ZPhe-Api-
Aib2-Api-Aib2-OMe with the side-chain cross-linking between Api5
residues (1) [∆ZPhe ) (Z)-R,ꢀ-dehydrophenylalanine, Aib )
R-aminoisobutyric acid, Api ) 4-aminopiperidine-4-carboxylic acid,
OMe ) methoxy], was designed based on the following facts. (i)
CR-Tetrasubstituted R-amino acids such as Aib and Api residues
promote a helical structure strongly.4,5 (ii) Noncovalent multipoint
interaction between the N-terminal segment H-ꢀ-Ala-∆ZPhe-Aib-6
in 310-helix and an N-protected amino acid preferentially induces
a one-handed helix in the achiral peptide chain (termed as a
“noncovalent chiral domino effect”6a,b,7).8 (iii) Absorption band
of ∆ZPhe residues (λmax ∼ 280 nm) is located away from those of
the usual solvents and peptide bond, and the residues in every other
place enable us to identify induced helicity.6,7,9 (iv) The side-chain
cross-linking9 of Api residues at i and i + 3 in a 310-helix stabilizes
the entire structure and decelerates the helix inversion of the
optically inactive 310-helical peptide.10c,d
The optically inactive peptide 1 shows no CD signal in achiral
media; however, the copresence of Boc-L-Leu-OH (L-2, Boc ) tert-
butoxycarbonyl) in the chloroform solution induced a split CD
pattern around 286 nm as shown in Figure 2A.16,17 This is
assignable to the ∆ZPhe residues adopting a right-handed helix.6,7,9
The mirror image CD corresponding to a left-handed helix was
observed in the presence of D-2 (Figure 2A). A small amount (30
µL) of the chloroform solution containing 1 and L-2 as described
above was added to a solution (770 µL) of excess Boc-Aib-OH
(3), an achiral guest, at 20 °C, and the time dependence of the CD
intensity of 1 at 271 nm was measured (Figure 2B). Interestingly,
under a large excess of 3 ([3]/[L-2] ) ca. 445/1), the induced CD
signal slowly reduced its intensity and reached an undetectable level
only after ca. 20 min (Figure 2B). The apparent binding constant
The solution structure of 1 was investigated by a 1H NMR technique
and additionally by computational analysis. Variable temperature 1H
NMR spectra of 1 in CDCl3 are shown in Figure S2. The two amide
NH resonances (8.09 and ∼9.76 ppm at 20 °C) depend heavily on
† Japan Science and Technology Agency.
‡ Nagoya Institute of Technology.
§ The University of Tokyo.
9
12266 J. AM. CHEM. SOC. 2008, 130, 12266–12267
10.1021/ja805647k CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: Synthetic procedure, charac-
terization, and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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(4) The peptide 310-helix is characterized by three amino acids per turn and
the intramolecular hydrogen bonds between i (CdO) and i + 3 (NH): (a)
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Figure 2. (A) CD (top) and absorption (bottom) spectra of 1 in the presence
of L-2 (blue line) or D-2 (red line) in chloroform at room temperature; [1]
) 0.94 mM and [2] ) 6.0 mM. ∆ꢀ and ꢀ were expressed with respect to
the molar concentration of ∆ZPhe residues. Solutions were left to stand for
over 30 min at room temperature prior to measurements to reach an
equilibrium state. (B) Time dependence of the CD intensity at 271 nm of
1 in the presence of L-2 and large excess of 3 at 0, 10, 20, and 30 °C; [1]
) 3.5 × 10-2 mM, [L-2] ) 0.23 mM, [3] ) 100 mM, [3]/[L-2] ) ca.
445/1.
Table 1. Pseudo Rate Constant (krac, s-1) and Half-Life Time (t1/2
min) for the Racemization of Complex 1·3a
,
temp (°C)
krac (s-1
)
t1/2 (min)b
0
10
20
30
1.17 × 10-4
5.46 × 10-4
2.32 × 10-3
8.29 × 10-3
49.3
10.6
2.5
0.7
a Conditions: in chloroform, [1] ) 3.5 × 10-2 mM, [L-2] ) 0.23
b
(5) (a) Wysong, C. L.; Yokum, T. S.; Morales, G. A.; Gundry, R. L.;
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(6) (a) Inai, Y.; Ousaka, N.; Okabe, T. J. Am. Chem. Soc. 2003, 125, 8151–
8162. (b) Inai, Y.; Ousaka, N.; Ookouchi, Y. Biopolymers 2006, 82, 471–
481. (c) Inai, Y.; Komori, H. Biomacromolecules 2004, 5, 1231–1240. (d)
Komori, H.; Inai, Y. J. Org. Chem. 2007, 72, 4012–4022.
mM, [3] ) 100 mM, [3]/[L-2] ) ca. 445/1. t1/2 (min) ) ln 2/(2krac
·
60).
(Kapp, M-1) of 1 to 3 was estimated to be similar to that of 1 to L-2
by competition experiments.18 In this condition, contribution of L-2
to the decay of the CD intensity is almost negligible, and the
complexation between 1 and 3 (1·3) is judged to be complete based
on the Kapp value. Additionally, the rate of guest-guest exchange
in the complex is faster than the NMR time scale even at low
temperature (-10 °C).7c Thus, there is no attribute of the slow
exchange between 1·L-2 and 1·3 complexes to the decay of the
CD intensity. Consequently, the racemization of the 1·3 complex
obeys pseudofirst-order kinetics in this condition.
(7) (a) Inai, Y.; Tagawa, K.; Takasu, A.; Hirabayashi, T.; Oshikawa, T.;
Yamashita, M. J. Am. Chem. Soc. 2000, 122, 11731–11732. (b) Inai, Y.;
Hirano, T. ITE Lett. Batteries, New Technol. Med. 2003, 4, 485–488. (c)
Ousaka, N.; Inai, Y.; Okabe, T. Biopolymers 2006, 83, 337–351.
(8) ꢀ-Ala residue rather than Gly is chosen to enhance the binding affinity
towards chiral acid.6,7c
(9) Pieroni, O.; Fissi, A.; Pratesi, C.; Temussi, P. A.; Ciardelli, F. J. Am. Chem.
Soc. 1991, 113, 6338–6340.
The pseudofirst-order rate constants (krac, s-1) and half-life time
for the racemization of the 1·3 complex at several temperatures
(0, 10, 20, and 30 °C) were estimated by linear regression analysis
for the initial slope (r2 ) 0.999) (Table 1). It is surprising that krac
of the 1·3 complex is 105 times smaller than the related 310-helical
decapeptide Z-Aib10-OtBu (Z ) benzyloxycarbonyl, OtBu ) tert-
butoxy).19 Arrhenius and Eyring plots of the kinetic data from Table
1 provide the following thermodynamic parameters (Figure S7),
(10) The side-chain cross-linking in the 310-helix: (a) Boal, A. K.; Guryanov,
I.; Moretto, A.; Crisma, M.; Lanni, E. L.; Toniolo, C.; Grubbs, R. H.;
O’Leary, D. J. J. Am. Chem. Soc. 2007, 129, 6986–6987. (b) Schievano,
E.; Bisello, A.; Chorev, M.; Bisol, A.; Mammi, S.; Peggion, E. J. Am.
Chem. Soc. 2001, 123, 2743–2751. (c) Ousaka, N.; Sato, T.; Kuroda, R.
J. Am. Chem. Soc. 2008, 130, 463–465. (d) Ousaka, N.; Tani, N.; Sekiya,
R.; Kuroda, R. Chem. Commun. 2008, 2894–2896.
(11) Two out of nine amide NH protons could not be observed because of the
overlapping with aromatic resonances (see Supporting Information).
(12) Similar broadening was also observed in N-terminal free peptide possessing
∆ZPhe residue at the second position from the N-termimus.6,7
(13) For example: Polese, A.; Formaggio, F.; Crisma, M.; Valle, G.; Toniolo,
C.; Bonora, G. M.; Broxterman, Q. B.; Kamphuis, J. Chem.sEur. J. 1996,
2, 1104–1111.
Ea ) 97.7 kJ mol-1, ∆G‡20 ) 86.6 kJ mol-1, ∆H‡ ) 95.4 kJ mol-1
,
and ∆S‡ ) 30.0 J mol-1 K-1. A positive value for the entropy of
activation indicates the transition state of the helix reversal process
is more disordered than the folded state. The slow process and high
barrier of helix inversion originate from the severe strain in the
bridging region.20 The obtained parameters suggest that the induced
helicity is preserved for ∼1 week at -30 °C (half-life time).
In summary, we have demonstrated the first example of a “chiral
memory” system in a helical oligomer, which is composed of a
biological backbone with a single side-chain cross-linking. In this
system, noncovalent chiral interaction at the N-terminal region leads
to modulation of the original chain chirality, and the induced helix
sense preference is stored in the peptide backbone assisted by the
side-chain cross-linking. We are currently studying the effects of
peptide main-chain length, solvents, and an additional cross-linking
on the rate of helix inversion to achieve a complete memory of
induced helicity (asymmetric synthesis) in peptide helices.
(14) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902–3909.
(15) The molecular graphics were drawn with: Thompson, M. A. ArgusLab 4.0.1;
Planaria Software LLC: Seattle, WA, 2004 (http://www.arguslab.com).
(16) The solution stood for over 30 min at room temperature before CD
measurement to establish an equilibrium state.
(17) The apparent binding constant (Kapp, M-1) of 1 for L-2 was estimated to
be 2.3 × 103 M-1, which is obtained from nonlinear fitting of the
corresponding CD data (1:1 binding model6a) (Figure S5).
(18) Induced CD intensity of 1 with equimolar mixture of L-2 and 3 was about
one-half the value as compared with that of 1 with L-2 (Figure S6).
(19) Hummel, R.-P.; Toniolo, C.; Jung, G. Angew. Chem., Int. Ed. Engl. 1987,
26, 1150–1152.
(20) In methanol/chloroform (77/3; v/v) mixed solution which prevents interac-
tion between 1 and L-2, kinetic and thermodynamic parameters of 1 were
also obtained; Ea ) 93.4 kJ mol-1, ∆G‡ ) 85.1 kJ mol-1, ∆H‡ ) 91.1
20
kJ mol-1, and ∆S‡ ) 20.5 J mol-1 K-1 (Figures S8-S9 and Table S1).
These values are comparable to those of the complex 1 · 3. Thus, there is
little effect of guest binding on the rate of helix inversion of 1.
JA805647K
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J. AM. CHEM. SOC. VOL. 130, NO. 37, 2008 12267