Noncovalent domino effect on helical screw sense of chiral peptides possessing C-terminal chiral residue

Article abstract of DOI:10.1021/ja017126w

Recently, a novel chiral intermolecular interaction was found in an N-deprotected achiral nonapeptide that undergoes the predominance of one-handed screw sense through the addition of chiral small carboxylic acid (Inai, Y.; Tagawa, K.; Takasu, A.; Hirabay

Full text of DOI:10.1021/ja017126w

Published on Web 02/20/2002
Noncovalent Domino Effect on Helical Screw Sense of Chiral
Peptides Possessing C-Terminal Chiral Residue
Yoshihito Inai,* Yuya Ishida, Kenichi Tagawa, Akinori Takasu, and
Tadamichi Hirabayashi
Contribution from the Department of EnVironmental Technology and Urban Planning,
Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho,
Showa-ku, Nagoya 466-8555, Japan
Received September 21, 2001
Abstract: Recently, a novel chiral intermolecular interaction was found in an N-deprotected achiral
nonapeptide that undergoes the predominance of one-handed screw sense through the addition of chiral
small carboxylic acid (Inai, Y.; Tagawa, K.; Takasu, A.; Hirabayashi, T.; Oshikawa, T.; Yamashita, M. J.
Am. Chem. Soc. 2000, 122, 11731). We here clarify to what extent such noncovalent chiral domino effect
affects the helical screw sense of an N-deprotected chiral peptide. Two chiral peptides consisting of
C-terminal L-Leu (1) or L-Leu2 (2) and the preceding achiral helical octapeptide segment were employed.
NMR and IR spectroscopy, and energy calculation indicated that both peptides adopt a helical conformation
in chloroform. Peptide 1 showed a small excess of a left-handed screw sense for the achiral helical
octapeptide, but peptide 2 strongly preferred a right-handed screw sense. The addition of chiral Boc amino
acid to a chloroform solution of peptide 1, depending on its chirality, underwent a unique helix-to-helix
transition or led to remarkable stabilization of the original left-handed screw sense. Peptide 2 retained the
original right-handed screw sense on addition of chiral Boc-amino acid, but its helical stability changed to
some extent depending on its added chirality. Therefore, the importance of noncovalent domino effect for
controlling the helical screw sense or helical stability of a chiral peptide has been demonstrated here for
the first time. In addition, we here have presented a unique system that both N-terminal noncovalent and
C-terminal covalent domino effects operate simultaneously on the helical screw sense of a single achiral
segment and have compared both powers for inducing the screw sense bias.
Introduction
about the possibility that the site-specific action of external
stimulus upon a terminal moiety of a helical polymer can control
To control the helical screw sense of biological macromol-
ecules1 or synthetic polymers2 through external stimuli such as
pH, light, temperature, solvent, and chiral molecules is of
academic as well as of practical importance in a wide range of
chemical fields such as biochemistry, polymer chemistry,
supramolecular chemistry, analytical chemistry, and chiral
separation and chiral pharmaceutical technologies. In most of
previous studies on the control of a helical screw sense, such
external stimuli act on a whole polymer molecule, including
the main and/or side chains. On the other hand, little is known
the whole helical screw sense. Recently, we found that the
predominance of one-handed screw sense is induced for an
N-deprotected achiral nonapeptide by the addition of a chiral
small carboxylic acid, of which chiral stimulus acts on the
N-terminal amino group to generate the helical screw sense
bias.3 This noncovalent domino effect will provide new insight
into the nature of chiral interactions between a helical segment
and a chiral molecule in peptide and protein science. However,
one might ask at this point to what extent such noncovalent
domino effect, in fact, affects the helical screw sense or helical
stability of a chiral peptide.
* To whom correspondence should be addressed: inai@mse.nitech.ac.jp.
(1) For examples of polypeptidesa-e or DNAf,g, see: (a) Blout, E. R.; Carver,
J. P.; Gross, J. J. Am. Chem. Soc. 1963, 85, 644. (b) Overberger, C. G.;
David, K.-H. Macromolecules 1972, 5, 373. (c) Watanabe, J.; Okamoto,
S.; Satoh, K.; Sakajiri, K.; Furuya, H.; Abe, A. Macromolecules 1996,
29, 7084. (d) Ueno, A.; Takahashi, K.; Anzai, J.; Osa, T. J. Am. Chem.
Soc. 1981, 103, 6410. (e) Ciardelli, F.; Pieroni, O.; Fissi, A.; Carlini, C.;
Altomare, A. Br. Polym. J. 1989, 21, 97. (f) Saenger, W. Principles of
Nucleic Acid Structure; Springer-Verlag: New York, 1984; Chapter
12. (g) Mahadevan, S.; Palaniandavar, M. Chem. Commun. 1996, 114,
2547.
(2) (a) Yashima, E.; Maeda, Y.; Okamoto, Y. J. Am. Chem. Soc. 1998, 120,
8895. (b) Okamoto, Y.; Nakano, T.; Ono, E.; Hatada, K. Chem. Lett. 1991,
525. (c) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336. (d) Schlitzer, D.
S.; Novak, B. M. J. Am. Chem. Soc. 1998, 120, 2196. (e) Maxein, G.;
Zentel, R. Macromolecules 1995, 28, 8438.
To demonstrate the importance of the novel chiral interaction
in biopolymers, we here adopted the following N-deprotected
nonapeptide 1 and decapeptide 2 containing achiral heli-
cogenic residues4 of R-aminoisobutyric acid (Aib) and R,â-
(3) Inai, Y.; Tagawa, K.; Takasu, A.; Hirabayashi, T.; Oshikawa, T.; Yamashita,
M. J. Am. Chem. Soc. 2000, 122, 11731.
(4) (a) Prasad, B. V. V.; Balaram, P. CRC Crit. ReV. Biochem. 1984, 16, 307.
(b) Benedetti, E.; Bavoso, A.; Di Blasio, B.; Pavone, V.; Pedone, C.; Crisma,
M.; Bonora, G. M.; Toniolo, C. J. Am. Chem. Soc. 1982, 104, 2437. (c)
Jain, R.; Chauhan, V. S. Biopolymers 1996, 40, 105. (d) Pieroni, O.; Fissi,
A.; Pratesi, C.; Temussi, P. A.; Ciardelli, F. J. Am. Chem. Soc. 1991, 113,
6338.
9
2466 VOL. 124, NO. 11, 2002 J. AM. CHEM. SOC.
10.1021/ja017126w CCC: $22.00 © 2002 American Chemical Society

Noncovalent Domino Effect on Helical Screw Sense
A R T I C L E S
3 × CH3 Boc), 0.90 (6H, bs, 2 × CH3 Leu(7)). FT-IR (cm-1, in KBr):
3280, 1741, 1686, 1661, 1626, 1536.
didehydrophenylalanine (∆ZPhe).
Boc-(Aib-∆ZPhe)4-L-Leu-OMe (3). The nonapeptide was prepared
by ring-opening reaction of Boc-Aib-∆ZPhe azlactone5 with H-(Aib-
∆ZPhe)3-L-Leu-OMe prepared from Boc-(Aib-∆ZPhe)3-L-Leu-OMe, in
a manner similar to Boc-(Aib-∆ZPhe)3-L-Leu-OMe. Yield 42%. RfA
0.66; RfB 0.87; RfC 0.49; RfD 0.81. Anal. Calcd. for C64H79N9O12 H2O:
C 64.90, H 6.89, N 10.64. Found: C 65.09, H 6.80, N 10.48. 600 MHz
1H NMR (δ, in CDCl3): 9.02 (1H, s, NH ∆ZPhe(4)), 8.99 (1H, s, NH
∆ZPhe(6)), 8.70 (1H, bs, NH ∆ZPhe(8)), 8.10 (2H, s, 2 × NH Aib(3)
+ Aib(7)), 8.05 (1H, s, NH Aib(5)), 7.95 (1H, d, J ) 7.8 Hz, NH
Leu(9)), 7.73 (1H, s, NH ∆ZPhe(2)), 7.56-7.10 (24H, m, 4 × (CâH +
phenyl) ∆ZPhe), 5.17 (1H, s, NH Aib(1)), 4.60 (1H, m, CRH Leu(9)),
3.56 (3H, s, COOCH3), 1.93-1.6 (3H, m, CâH2 + CγH Leu(9)), 1.58
+ 1.56 + 1.49 + 1.29 + 1.21 (24H, s + s + bs + bs + bs, 8 × CH3
Aib), 1.36 (9H, s, 3 × CH3 Boc), 0.83 (6H, bs, 2 × CH3 Leu(9)).
FT-IR (cm-1, in KBr): 3277, 1737, 1660, 1625, 1536.
H-(Aib-∆ZPhe)4-L-Leu-OMe (1). Nonapeptide 3 (60 mg, 52 µmol)
was dissolved in trifluoroacetic acid (0.6 mL)/dichloromethane (0.6
mL) at 0 °C, and then the solution was allowed to stand at 0 °C for 5
h, and concentrated in vacuo. After addition of 5% NaHCO3 solution,
the residue was extracted with chloroform, and the organic solution
was dried over MgSO4. The product was purified by precipitation from
chloroform/n-hexane to give 1 in 45 mg yield (82%). RfA 0-0.21; RfB
0.51-0.81; RfC 0.42; RfD 0.61. Anal. Calcd. for C59H71N9O10 H2O: C
65.36, H 6.79, N 11.63. Found: C 65.51, H 6.54, N 11.51. 600 MHz
1H NMR (δ, in CDCl3 containing 8.9 vol % (CD3)2SO): 9.40 (1H, s,
NH ∆ZPhe(8)), 9.13 (1H, s, NH ∆ZPhe(6)), 8.80 (1H, s, NH ∆ZPhe-
(4)), 8.24 (1H, s, NH Aib(7)), 8.21 (1H, s, NH Aib(5)), 8.06 (1H, s,
NH Aib(3)), 8.04 (1H, d, J ) 7.7 Hz, NH Leu(9)), 7.58-7.21 + 6.75
(24H, m + s, 4 × (CâH + phenyl) ∆ZPhe), 4.63 (1H, m, CRH Leu(9)),
3.64 (3H, s, COOCH3), 1.96 + 1.85 (2H, m + bs, CâH2 Leu(9)), 1.68-
1.6 (1H, m, CγH Leu(9)), 1.65 + 1.63 + 1.53 + 1.31 + 1.21 (24H, s
+ s + bs + bs + s, 8 × CH3 Aib), 0.90 (6H, bs, 2 × CH3 Leu(9)).
FT-IR (cm-1, in chloroform containing 9 vol %-(CH3)2SO; [1] ) 1.0
mM): 1730, 1659, 1627, 1537; (cm-1, in KBr): 3269, 1738, 1658,
1622, 1538. In the NOESY spectrum, the relative intensity (%) of NiH-
These peptides possess a C-terminal chiral residue (L-Leu)
or segment (L-Leu2) that will induce the excess of one-handed
screw sense for the preceding achiral helical segment -(Aib-
∆ZPhe)4- through the C-terminal covalent domino effect. Here,
addition of chiral carboxylic acid will give rise to the N-terminal
noncovalent domino effect to lead to the terminal control of
the original helical screw sense. This is also a very unique
system for understanding novel factors governing the helical
screw sense of polymer molecules, i.e., the covalent chiral effect
from one-side terminal of a helical segment, and noncovalent
chiral effect generating from the other terminal operate simul-
taneously on the helical screw sense of an achiral segment, as
if it were a tug-of-war game on the achiral helical chain.
Experimental Section
Materials. All amino acids and coupling reagents were purchased
from Tokyo Kasei Co. (Tokyo, Japan) or Kokusan Chemical Works
Ltd. (Tokyo, Japan). Boc-amino acid (Boc ) t-butoxycarbonyl) was
prepared by a standard procedure with (Boc)2O, or was purchased from
Kokusan Chemical Works Ltd. Size exclusion column (TOYOPEARL
HW-40) for purification of products was commercially available from
TOSOH Co. (Tokyo, Japan). Chloroform dried over CaSO4 was distilled
onto CaSO4 before use. N,N-Dimethylformamide (DMF) was purified
by distillation with ninhydrin under a reduced pressure. Thin-layer
chromatography (TLC) was done on precoated silica plates in the
following solvent systems: (A) ethyl acetate, (B) methanol, (C)
chloroform-methanol (9:1), and (D) 1-butanol-acetic acid-water (7:
2:1): single spot in the TLC was obtained for each of the final products
and their intermediates, as shown below.
N
i+1H (i - i + 1) cross-peaks on setting the diagonal volume of the
∆ZPhe(4) NH to 100% was as follows: 1.2 (3-4), 1.3 (4-5), 1.2 (5-6),
1.0 (6-7), 1.2 (7-8), and 1.4 (8-9).
Boc-Aib-∆ZPhe-OMe. The dipeptide was prepared by ring-opening
reaction of Boc-Aib-∆ZPhe azlactone5 with methanol.6 Yield 98%. RfA
0.78; RfB 0.83; RfC 0.73; RfD 0.73. 600 MHz H NMR (δ, in CDCl3):
1
8.19 (1H, s, NH ∆ZPhe), 7.55-7.29 (6H, m, CâH + phenyl ∆ZPhe),
4.94 (1H, s, NH Aib), 3.83 (3H, s, COOCH3), 1.56 (6H, s, 2 × CH3
Aib), 1.45 (9H, s, 3 × CH3 Boc). FT-IR (cm-1, in KBr): 3401, 3378,
1714, 1699, 1685, 1641, 1504.
Boc-(Aib-∆ZPhe)3-L-Leu-OMe. Boc-(Aib-∆ZPhe)2-L-Leu-OMe5 (2.6
g, 3.3 mmol) was dissolved in trifluoroacetic acid (5 mL)/dichlo-
romethane (15 mL) at 0 °C, and then the solution was allowed to stand
for 3 h at 0 °C, and concentrated in vacuo. After addition of 5%
NaHCO3 solution, the residue was extracted with chloroform, and the
organic solution was dried over MgSO4. After removing the solvent,
the residue was dissolved in DMF (10 mL), and to the solution was
added Boc-Aib-∆ZPhe azlactone5 (1.1 g, 3.3 mmol) at 0 °C. The mixture
was stirred for 2 h at 0 °C, then for 24 h at room temperature,
concentrated in vacuo, and the residue was dissolved in chloroform.
The solution was washed with 10% NaCl, 5% KHSO4, 10% NaCl, 5%
NaHCO3, and 10% NaCl solutions, and dried over MgSO4. The product
was purified by recrystallization from chloroform/n-hexane to give the
heptapeptide in 1.5 g yield (50%). RfA 0.70; RfB 0.87; RfC 0.53; RfD
Boc-(Aib-∆ZPhe)m-OMe (m ) 2-4). Peptides (m ) 2-4) were
prepared by ring-opening reaction of Boc-Aib-∆ZPhe azlactone5 with
H-(Aib-∆ZPhe)n-OMe (n ) 1-3) prepared from Boc-(Aib-∆ZPhe)n-
OMe, in a manner similar to the preparation of Boc-(Aib-∆ZPhe)3-L-
Leu-OMe.
Boc-(Aib-∆ZPhe)2-OMe. Yield 78%. RfA 0.73; RfB 0.81; RfC 0.58;
1
RfD 0.73. 600 MHz H NMR (δ, in CDCl3): 8.71 (1H, s, NH ∆ZPhe-
(4)), 7.75 (1H, s, NH Aib(3)), 7.52 (1H, s, NH ∆ZPhe(2)), 7.71-7.19
(12H, m, 2 × (CâH + phenyl) ∆ZPhe), 4.87 (1H, s, NH Aib(1)), 3.82
(1H, s, COOCH3), 1.67 + 1.46 (12H, s + s, 4 × CH3 Aib), 1.44 (9H,
s, 3 × CH3 Boc). FT-IR (cm-1, in KBr): 3313, 3286, 1718, 1670,
1636, 1528, 1503.
1
0.82. 600 MHz H NMR (δ, in CDCl3): 8.95 (1H, s, NH ∆ZPhe(4)),
8.74 (1H, s, NH ∆ZPhe(6)), 8.12 (1H, s, NH Aib(3)), 8.02 (1H, s, NH
Aib(5)), 7.97 (1H, d, J ) 7.9 Hz, NH Leu(7)), 7.74 (1H, s, NH ∆Z-
Phe(2)), 7.63-7.18 (18H, m, 3 × (CâH + phenyl) ∆ZPhe), 5.14 (1H,
s, NH Aib(1)), 4.68 (1H, m, CRH Leu(7)), 3.64 (3H, s, COOCH3), 1.99-
1.6 (3H, m, CâH2 + CγH Leu(7)), 1.64 + 1.62 + 1.50 + 1.48 + 1.30
+ 1.28 (18H, s + s + s + s + bs + bs, 6 × CH3 Aib), 1.44 (9H, s,
Boc-(Aib-∆ZPhe)3-OMe. Yield 69%. RfA 0.65; RfB 0.85; RfC 0.53;
1
RfD 0.69. 600 MHz H NMR (δ, in CDCl3): 8.77 (2H, s, 2 × NH
∆ZPhe(4) + ∆ZPhe(6)), 8.03 (1H, s, NH Aib(3)), 7.76 (1H, s, NH Aib-
(5)), 7.63 (1H, s, NH ∆ZPhe(2)), 7.74-7.18 (18H, m, 3 × (CâH +
(6) Chauhan, V. S.; Kaur, P.; Sen, N.; Uma, K.; Jacob, J.; Balaram, P.
(5) Inai, Y.; Kurokawa, Y.; Hirabayashi, T. Biopolymers 1999, 49, 551.
Tetrahedron 1988, 44, 2359.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2467

A R T I C L E S
Inai et al.
phenyl) ∆ZPhe), 4.96 (1H, s, NH Aib(1)), 3.75 (1H, s, COOCH3), 1.69
+ 1.55 + 1.24 (18H, s + s + s, 6 × CH3 Aib), 1.43 (9H, s, 3 × CH3
Boc). FT-IR (cm-1, in KBr): 3296, 1715, 1686 (sh), 1666, 1632, 1531.
(CâH2 + CγH) Leu(9) + Leu(10)), 1.7 + 1.61 + 1.57 + 1.39 + 1.32
+ 1.22 (24H, s + s + s + s + s + s, 8 × CH3 Aib), 0.96-0.76 (12H,
m, 4 × CH3 Leu(9) + Leu(10)). FT-IR (cm-1, in chloroform; [2] )
1.0 mM): 3285 (br), 1732, 1659, 1626, 1537; (cm-1, in KBr): 3275,
1741, 1659, 1625, 1536. In the NOESY spectrum, the relative intensity
(%) of NiH-Ni+1H (i - i + 1) cross-peaks on setting the diagonal
volume of the ∆ZPhe(6) NH to 100% was as follows: 6.9 (3-4), 6.7
(4-5), 11.8 (5-6), 10.6 (6-7), 12.2 (7-8), 6.9 (8-9), and 12.2 (9-10); in
Boc-(Aib-∆ZPhe)4-OMe (4). Yield 76%. RfA 0.65; RfB 0.85; RfC 0.53;
1
RfD 0.75. 600 MHz H NMR (δ, in CDCl3): 9.06 (1H, s, NH ∆ZPhe-
(4)), 8.88 (1H, s, NH ∆ZPhe(6)), 8.77 (1H, s, NH ∆ZPhe(8)), 8.08 (1H,
s, NH Aib(3)), 8.06 (1H, s, NH Aib(5)), 7.90 (1H, s, NH Aib(7)), 7.65
(1H, s, NH ∆ZPhe(2)), 7.75-7.17 (24H, m, 4 × (CâH + phenyl) ∆Z-
Phe), 5.05 (1H, s, NH Aib(1)), 3.69 (3H, s, COOCH3), 1.70 + 1.60 +
1.30 + 1.25 (24H, s + s + bs + bs, 8 × CH3 Aib), 1.43 (9H, s, 3 ×
CH3 Boc). FT-IR (cm-1, in KBr): 3295, 1729, 1688, 1660, 1628, 1532.
other cross-peaks, 10.8 for N9H-CR H, 8.7 for N10H-CR10H, and 6.0
9
for CR H-N10H.
9
1
Spectroscopic Measurements. H NMR spectra were recorded on
Bruker DRX-600 (600 MHz) or DPX-200 (200 MHz) spectrometers
at 299 K for peptide 1 (5 mM) in 8.9 vol %-(CD3)2SO/CDCl3 and for
peptide 2 (9 mM) in CDCl3. All chemical shifts in parts per million
(ppm) were determined using tetramethylsilane as an internal standard.
NOESY spectra were measured on Bruker DRX-600 (600 MHz) using
a Bruker standard pulse program (noesytp)7 with a mixing time of 700
ms, 64 transients per t1, 2 K data points in the t2 domain, and 256
points in the t1 domain. The data processing and analysis were
performed with the XWINNMR software (ver 2.5). FT-IR spectra were
recorded on a JASCO FT/IR-430 spectrometer in KBr, in 9 vol
%-(CH3)2SO/chloroform for peptide 1, and in chloroform for peptide
2. CD and UV spectra were recorded at ambient temperature (17-20
°C) for a chloroform solution of peptide (0.14 mM) containing various
amounts of carboxylic acid (0-200 mM) on JASCO J-500 and JASCO
V-550 spectrometers, respectively.
Boc-(Aib-∆ZPhe)4-OH (5). To a solution of peptide 4 (650 mg, 0.62
mmol) in methanol (75 mL) and dioxane (75 mL) was added 1 M
NaOH solution (3.1 mL, 3.1 mmol) at 0 °C. Then the reaction mixture
was stirred at room temperature for 2 days until TLC (ethyl acetate)
indicated that the saponification process was complete. After concentra-
tion in vacuo, the mixture was inserted into a KHSO4 solution (pH )
2-3) to obtain a white precipitate. The precipitate was washed with
distilled water until the washed water became neutral, and was dried
in vacuo to give 5 in 480 mg yield (75%). RfA 0-0.24; RfB 0.90; RfC
0.37-0.54; RfD 0.80. 600 MHz H NMR (δ, in CDCl3 containing 20
1
vol %-(CD3)2SO): 9.69 (1H, s, NH ∆ZPhe(2)), 9.21 (1H, s, NH ∆Z-
Phe(4)), 9.09 (1H, s, NH ∆ZPhe(6)), 8.83 (1H, s, NH ∆ZPhe(8)), 8.49
(1H, s, NH Aib(3)), 8.30 (1H, s, NH Aib(7)), 8.23 (1H, s, NH Aib(5)),
7.70-7.17 (24H, m, 4 × (CâH + phenyl) ∆ZPhe), 6.68 (1H, s, NH
Aib(1)), 1.66 + 1.57 + 1.34 + 1.25 (24H, s + s + bs + bs, 8 × CH3
Aib), 1.45 (9H, s, 3 × CH3 Boc). FT-IR (cm-1, in KBr): 3273, 1721,
1663, 1627, 1534.
Conformational Energy Calculation. Energy-minimized conforma-
tions of peptides 1 and 2 were obtained using the semiempirical
molecular orbital (MO) calculation (AM1 method)8 in MOPAC97.8 The
minimization with a MOPAC97 keyword of MMOK was carried out
for the variables of all bond lengths, bond angles, and torsion angles.
An initial conformation of L-Leu and L-Leu2 moieties for the AM1
calculation was obtained using the modified PEPCON9 for conforma-
tional energy calculation on ∆ZPhe-containing peptides, and that of
-(Aib-∆ZPhe)4- was set to a standard left-handed 310-helix for 1 (φ )
60.0°, ψ ) 30.0°, and ω ) 180.0°) and to a standard right-handed one
for 2 (φ ) -60.0°, ψ ) -30.0°, and ω ) 180.0°)10 on the basis of the
experimental data.
Boc-(Aib-∆ZPhe)4-L-Leu2-OMe (6). Peptide 5 (400 mg, 0.39 mmol)
and HCl‚H-L-Leu2-OMe (125 mg, 0.42 mmol) were dissolved in DMF
(5 mL), cooled to 0 °C. To the solution were added 1-hydroxybenzo-
triazole monohydrate (65 mg, 0.42 mmol), dicyclohexylcarbodiimde
(88 mg, 0.42 mmol), and N-methylmorphorine (47 µL, 0.42 mmol).
The reaction mixture was stirred at 0 °C for 3 h, and at room
temperature for 4 days. The mixture was concentrated in vacuo, and
the residue was redissolved in chloroform. After dicyclohexylurea was
removed by filtration, the solution was washed with 10% NaCl, 5%
KHSO4, 10% NaCl, 5% NaHCO3, and 10% NaCl solutions, and then
dried over MgSO4. The product was purified using a silica gel column
eluted with ethyl acetate, and then a size-exclusion column (TOYO-
PEARL HW-40) eluted with DMF. The resulting residue was subjected
to precipitation from chloroform/diethyl ether to give 6 in 230 mg yield
(47%). RfA 0.52; RfB 0.85; RfC 0.44; RfD 0.80. Anal. Calcd. for
C70H90N10O13 H2O: C 64.80, H 7.15, N 10.80. Found: C 65.01, H
Results and Discussion
Conformation of Peptides 1 and 2. The achiral segment
-(Aib-∆ZPhe)4- in both peptides can be expected to generate
two “enantiomeric” (left- and right-handed) helices, since
oligopeptides bearing -(Aib-∆ZPhe)m- (m ) 2 or 4) were found
to form a 310-helical structure in solution and in the solid
states.3,5,11 Actually, a helical conformation of peptides 1 and 2
was evidenced by 1H NMR and FT-IR spectroscopy in solution.
NOESY spectra of both peptides gave marked cross-peaks of
1
7.07, N 10.78. 600 MHz H NMR (δ, in CDCl3): 9.15 (1H, s, NH
∆ZPhe(6)), 9.06 (1H, s, NH ∆ZPhe(8)), 9.05 (1H, s, NH ∆ZPhe(4)),
8.34 (1H, s, NH Aib(7)), 8.19 (1H, s, NH Aib(3)), 8.14 (1H, s, NH
Aib(5)), 7.94 (1H, bs, NH Leu(9)), 7.85 (1H, bs, NH ∆ZPhe(2)), 7.63
(1H, bs, NH Leu(10)), 7.60-7.21 (24H, m, 4 × (CâH + phenyl) ∆Z-
Phe), 5.30 (1H, s, NH Aib(1)), 4.56 (2H, m, 2 × CRH Leu(9) + Leu-
(10)), 3.67 (3H, s, COOCH3), 2.0-1.7 (6H, m, 2 × (CâH2 + CγH)
Leu(9) + Leu(10)), 1.71 + 1.64 + 1.60 + 1.46 + 1.38 + 1.26 + 1.19
(24H, s + s + s + bs + bs + s + bs, 8 × CH3 Aib), 1.43 (9H, s, 3 ×
CH3 Boc), 0.92 + 0.84 (12H, m, 4 × CH3 Leu(9) + Leu(10)). FT-IR
(cm-1, in KBr): 3277, 1740, 1659, 1624, 1534.
(7) Bodenhausen, G.; Kogler, H.; Ernst, R. R. J. Magn. Res. 1984, 58, 370.
(8) The AM1 method in MOPAC97 was employed: Dewar, M. J. S.; Zoebisch,
E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902.
For MOPAC97, see: Stewart, J. J. P. MOPAC97, Fujitsu Ltd, Tokyo, Japan,
1998.
(9) For PEPCON, see: (a) Momany, F. A.; McGuire, R. F.; Burgess, A. W.;
Scheraga, H. A. J. Phys. Chem. 1975, 79, 2361. (b) Beppu, Y. Comput.
Chem. 1989, 13, 101. (c) Sisido, M. Peptide Chem. 1991; Suzuki, A., Ed.;
1992; pp 105-110. For the modified one, see: (d) Inai, Y.; Kurashima,
S.; Hirabayashi, T.; Yokota, K. Biopolymers 2000, 53, 484. (e) Inai, Y.;
Hirabayashi, T. Biopolymers 2001, 59, 356. (f) Inai, Y.; Oshikawa, T.;
Yamashita, M.; Hirabayashi, T.; Kurokawa, Y. Bull. Chem. Soc. Jpn. 2001,
74, 959.
(10) (a) Paterson, Y.; Rumsey, S. M.; Benedetti, E.; Nemethy, G.; Scheraga, H.
A. J. Am. Chem. Soc. 1981, 103, 2947. (b) Ramachandran, G. N.;
Sasisekharan, V. AdV. Protein Chem. 1968, 23, 283.
(11) (a) Inai, Y.; Kurokawa, Y.; Hirabayashi, T. Macromolecules 1999, 32, 4575.
(b) Inai, Y.; Oshikawa, T.; Yamashita, M.; Hirabayashi, T.; Ashitaka, S. J.
Chem. Soc., Perkin Trans. 2 2001, 892. (c) Inai, Y.; Ashitaka, S.;
Hirabayashi, T. Polym. J. 1999, 31, 246. (d) Inai, Y.; Kurokawa, Y.; Ida,
A.; Hirabayashi, T. Bull. Chem. Soc. Jpn. 1999, 72, 55.
H-(Aib-∆ZPhe)4-L-Leu2-OMe (2). Boc group of decapeptide 6 was
removed in a manner similar to that of peptide 1. Yield 83%. RfA
0-0.12; RfB 0.59-0.80; RfC 0.37; RfD 0.53. Anal. Calcd. for C65H82N10O11
H2O: C 65.20, H 7.07, N 11.70. Found: C 65.09, H 7.01, N 11.56.
1
600 MHz H NMR (δ, in CDCl3): 9.36 (1H, s, NH ∆ZPhe(4)), 9.12
(1H, s, NH ∆ZPhe(6)), 9.09 (1H, s, NH ∆ZPhe(8)), 8.40 (1H, s, NH
Aib(7)), 8.17 (1H, s, NH Aib(5)), 8.06 (1H, s, NH Leu(9)), 7.68 (1H,
s, NH Leu(10)), 7.47 (1H, s, NH Aib(3)), 7.57-7.20 + 6.93 (24H, m
+ s, 4 × (CâH + phenyl) ∆ZPhe), 4.51 (1H, m, CRH Leu(10)), 4.42
(1H, m, CRH Leu(9)), 3.66 (3H, s, COOCH3), 2.0-1.6 (6H, m, 2 ×
9
2468 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Noncovalent Domino Effect on Helical Screw Sense
A R T I C L E S
On the basis of the exciton chirality method17 and theoretical
CD calculation,3,11a,18 the split CD sign corresponds to a left-
handed screw sense for a 310- or R-helix. Thus, the C-terminal
L-Leu residue induces a left-handed screw sense for the achiral
helical segment -(Aib-∆ZPhe)4- through the covalent domino
effect. The left-handedness induced by a C-terminal L-residue
has also been observed in Aib peptides containing one L-residue
in the C-terminal position,19 or in an analogous peptide Boc-
(Aib-∆ZPhe)2-L-Leu-OMe in solution.5 Schellman also noted
that many right-handed helical segments in proteins ended with
a residue in a left-handed conformation.20 According to the
rational mechanism proposed in ref 19a and 19b, the left-handed
screw sense induced by a C-terminal L-residue ester should be
interpreted on the basis of an unfavorable O(i-2)‚‚‚ Oi interaction
taking place between the carbonyl oxygen atom of (i-2)th residue
from the C-terminus and either oxygen atom of the ester group
of the C-terminal ith residue if the sign of the φi angle of the
ith residue is the same as that of the preceding 310-helical
segment. This unfavorable interaction can be removed by the
φi angle with the opposite sign of the φi-2 angle. Thus, because
an L-amino acid strongly tends to adopt negative φi values, a
left-handed screw sense (positive φi-2) should be induced for
the preceding achiral segment. A similar mechanism should be
applied to the left-handed screw sense of peptide 1 because the
energy-minimized conformation (Figure 2A) gives φ9 ) -124°
for the C-terminal L-Leu(9) residue and φ7 ) +43° for the
preceding Aib(7) residue, leading to the disappearance of an
unfavorable O7‚‚‚O9 interaction. This negatively large φ9 value
should be supported by a large JNH-CH value of L-Leu(9) (7.7
Hz), which corresponds to φ9 ) -88°.21
Figure 1. Solvent dependence on NH chemical shifts of (A) peptides 1
and (B) 2 in CDCl3/(CD3)2SO mixtures. The ∆ZPhe(2) NH resonance could
not be observed due to its broadening. The titration experiment for peptide
1 started at 8.9 vol % of (CD3)2SO, because of its less solubility in pure
CDCl3.
NiH-Ni+1H resonances in the segment of Aib(3) to ∆ZPhe(8),
thus indicating the presence of 310- or R-helix.12 Figure 1 shows
the variation in NH chemical shifts of peptides 1 and 2 with
concentration of (CD3)2SO13 in CDCl3.
Six NH resonances of ∆ZPhe(4) to Leu(9) residues for peptide
1 and seven NHs of ∆ZPhe (4) to Leu(10) residues for 2 are
shielded from solvent due to intramolecular hydrogen bonding,
of which the pattern corresponds to a 310-helix14 supported by
consecutive (i + 3) f i hydrogen bonds starting from NH ∆Z-
Phe(4) f CO Aib(1). The helical conformation was also
supported by the positions of amide I absorption bands of their
FT-IR spectra in solution: 1659 and 1627 cm-1 for 1, and 1659
and 1625 cm-1 for 2, which can be assigned to saturated amino
acid and ∆ZPhe residues,15 respectively. Furthermore, as shown
in Figure 2,16 energy minimization of both peptides gave a 310-
helical conformation for the achiral heptapeptide segment ∆Z-
Phe(2)-∆ZPhe(8): average values for ∆ZPhe(2)-∆ZPhe(8)
residues are φ ) 38.3°, ψ ) 41.3°, and ω ) 178.9° for
peptide 1, and φ ) -39.1°, ψ ) -41.2°, and ω ) 179.7°
for peptide 2. In both energy-minimized peptides, six NHs of
∆ZPhe(4)-Leu(9) residues participated in consecutive (i + 3)
f i hydrogen bonds starting from NH ∆ZPhe(4) f CO Aib(1),
appropriate for a 310-helix.
On the other hand, peptide 2 showed a strong exciton splitting
assignable to a right-handed screw sense, thus indicating that
the chiral dipeptide segment induces predominantly a right-
handed screw sense for the preceding achiral segment. Interest-
ingly, the covalent chiral domino effect induced by the
C-terminal L-Leu2 segment is dramatically different from that
by the C-terminal L-Leu residue in the determination of the
whole screw sense. This finding might also clarify a role of
C-terminal chiral L-L doublet in the choice of a right-handed
screw sense for the preceding segment, or more generally in
one of protein folding mechanisms. In the energy-minimized
conformation (Figure 2B), the L-Leu2 segment adopts a non-
helical conformation characterized by φ9 ) -89°, ψ9 ) 50°,
φ10 ) -119°, and ψ10 ) -49°. Here, the proton pair of CRH
Leu(9)-NH Leu(10) is close to each other (2.8 Å), which is
supported by the observation of a marked NOE signal for the
corresponding proton pair (6.0%). In addition, the negatively
large φ9 value should be supported by a large JNH-CH value of
Covalent Domino Effect on Helical Screw Sense. The
preferred screw sense of peptides 1 and 2 prior to addition of
chiral carboxylic acid was investigated. Peptide 1 in chloroform
showed an exciton splitting centered at around 280 nm assign-
able to ∆ZPhe residue (Figure 3).
(12) Wu¨thrich, K.; Billeter, M.; Braun, W. J. Mol. Biol. 1984, 180, 715.
(13) Pitner, T. P.; Urry, D. W. J. Am. Chem. Soc. 1972, 94, 1399.
(14) For 310-helical structures, see: Toniolo, C.; Benedetti, E. Trends Biochem.
Sci. 1991, 16, 350, and references therein.
1
L-Leu(9) (7.9 Hz in 200 MHz H NMR spectrum in CDCl3),
which corresponds to φ9 ) -90°.21 Further systematic inves-
(15) FT-IR absorption data of ∆ZPhe-containing peptides taking a 310-helical
conformation in solution were presented.15a Unlike helical oligopeptides
consisting of only saturated amino acids,15b two characteristic peaks in an
amide I band region were observed: first peak at ca. 1665-1655 cm-1
and second peak at ca. 1628-1625 cm-1, of which are assigned to saturated
amino acid and ∆ZPhe residues in helical segments, respectively. A shift
to lower wavenumbers in the second peak should be ascribed to partial
contribution of resonance between carbonyl and styryl groups in a ∆ZPhe
residue. See: (a) Inai, Y.; Sakakura, Y.; Hirabayashi, T. Polym. J. 1998,
30, 828. (b) Kennedy, D. F.; Crisma, M.; Toniolo, C.; Chapman, D.
Biochemistry 1991, 30, 6541. Furthermore, two dehydropeptides adopting
a typical 310-helix in the crystalline state11b showed similar absorption bands
in the solid state: i.e., for Boc-(Aib-∆ZPhe)2-Aib-OMe, 1662 and 1628
tigations on covalent chiral domino effects on helical screw
(17) (a) Harada, N.; Chen, S. L.; Nakanishi, K. J. Am. Chem. Soc. 1975, 97,
5345. For CD analysis of ∆ZPhe-containing peptides, see: (b) Pieroni, O.;
Fissi, A.; Jain, R. M.; Chauhan, V. S. Biopolymers 1996, 38, 87.
(18) Inai, Y.; Ito, T.; Hirabayashi, T.; Yokota, K. Biopolymers 1993, 33, 1173.
(19) (a) Benedetti, E.; Saviano, M.; Iacovino, R.; Pedone, C.; Santini, A.; Crisma,
M.; Formaggio, F.; Toniolo, C.; Broxterman, Q. B.; Kamphuis, J.
Biopolymers 1998, 46, 433. (b) Pengo, B.; Formaggio, F.; Crisma, M.;
Toniolo, C.; Bonora, G. M.; Broxterman, Q. B.; Kamphuis, J.; Saviano,
M.; Iacovino, R.; Rossi, F.; Benedetti, E. J. Chem. Soc., Perkin Trans. 2,
1998, 1651.
cm-1; for Boc-L-Pro-(Aib-∆ZPhe)2-Aib-OMe, 1660 and 1629 cm-1
.
(16) The molecular graphics were illustrated using the molecular modeling
software: Butch Software Studio FREE WHEEL for Windows: 0.60E for
Molecular Modeling Software, Japan, 2001.
(20) Schellman, C. In Protein Folding; Jaenicke, R., Ed.; Elsevier: Amsterdam,
1980; pp 53-64.
(21) Pardi, A.; Billeter, M.; Wu¨thrich, K. J. Mol. Biol. 1984, 180, 741.
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2469

A R T I C L E S
Inai et al.
Figure 2. Stereoviews of energy-minimized conformations of (A) peptides 1 and (B) 2 by the semiempirical MO calculation (AM1 method).8 Peptide 1
gives a left-handed 310-helical conformation for the achiral heptapeptide segment ∆ZPhe(2)-∆ZPhe(8), and peptide 2 gives a right-handed 310-helical conformation
for the achiral segment.
Figure 4. CD (top) and UV absorption (bottom) spectra of peptide 1 in
chloroform containing (A) Boc-D-Pro-OH or (B) Boc-L-Pro-OH: [1] ) 0.14
mM and [Boc-Pro-OH] ) 0-150 mM. ∆ꢀ and ꢀ are expressed with respect
to the molar concentration of ∆ZPhe residues.
Figure 3. CD (top) and UV absorption (bottom) spectra of peptides 1
(dashed line) and 2 (solid line) in chloroform.
signals with a positive peak at longer wavelengths decreased
sense are in progress. However, we can conclude that the single
C-terminal L-Leu residue gives a small excess of a left-handed
screw sense for the preceding achiral segment, and that the
C-terminal L-Leu doublet induces strongly a right-handed screw
sense.
Noncovalent Domino Effect in Peptide 1. As described
above, peptide 1 alone prefers a left-handed screw sense in
chloroform through the C-terminal covalent domino effect. Here,
the addition of chiral carboxylic acid (Boc-Pro-OH) leads to a
dramatic change in the original CD spectrum, as shown in Figure
4.
On the addition of Boc-D-Pro-OH (Figure 4A), the split CD
sign corresponding to a left-handed screw sense was retained,
but the split amplitude markedly increased: ca. 5-fold at [Boc-
D-Pro-OH] ) 150 mM. Obviously, the original left-handed helix
induced by the C-terminal covalent domino effect can be
markedly stabilized by the chiral stimulus of Boc-D-Pro-OH.
Conversely, the addition of Boc-L-Pro-OH gives rise to a
remarkable helix-to-helix transition from left- to right-handed
screw sense, as shown in Figure 4B. The amplitude of split CD
with an increase in [Boc-L-Pro-OH] of 0-12 mM. Then an
opposite split CD pattern with a negative peak at longer
wavelengths began to appear at [Boc-L-Pro-OH] ) 18 mM, and
the split amplitude increased with further addition of Boc-L-
Pro-OH (24-150 mM). Therefore, the chiral stimulus of Boc-
L-Pro-OH gives rise to destabilization of the original left-handed
helix, and subsequently leads to a right-handed helix. The
directions of the helical screw senses induced by Boc-D-Pro-
OH and Boc-L-Pro-OH agreed with those observed for achiral
peptide H-(Aib-∆ZPhe)4-Aib-OMe.3 Similar tendency was ob-
served for addition of other chiral Boc-amino acids (Ala, Leu,
Val, and Phe), as shown in Figure 5 and Table 1. Namely, Boc-
L-amino acid tends to destabilize the original left-handed helix
or lead to a helix-to-helix transition, whereas the corresponding
Boc-D-amino acid stabilizes the left-handed helix.
N-Boc-protected peptide 1, Boc-(Aib-∆ZPhe)4-L-Leu-OMe,
also showed a split CD spectrum similar to that of peptide 1
preferring a left-handed screw sense. However, the CD pattern
and intensity were unaffected by the addition of a large excess
(480-fold) of chiral Boc-Pro-OH. Obviously, the N-terminal
9
2470 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Noncovalent Domino Effect on Helical Screw Sense
A R T I C L E S
Figure 5. CD spectra of peptide 1 in chloroform containing (A) Boc-D-
amino acid ((a) Ala, (b) Leu, (c) Val, (d) Phe, and (e) Pro) or (B) the
corresponding Boc-L-amino acid: [1] ) 0.14 mM; [Boc-amino acid] ) 65
mM. The CD spectra of the mixtures containing Boc-L (or D)-Phe-OH are
not shown below 272 nm due to overlap of the peptide and Boc-L (or D)-
Phe-OH signals.
Table 1. Signs of Splitting Cotton Effects and ∆ꢀ Values for
Induced CD of Peptide 1 with Chiral Carboxylic Acida
first Cotton effect
sign ∆ꢀ (λ/nm)
second Cotton effect
acid
sign
∆ꢀ (λ/nm)
none
+
+
+
+
+
+
-
2.5/300
8.3/301
3.4/301
4.1/301
4.5/301
3.5/301
-
-
-
-
-
-
+
+
+
+
+
2.8/264
12.4/270
5.3/269
6.6/268
Boc-D-Pro-OH
Boc-D-Ala-OH
Boc-D-Val-OH
Boc-D-Leu-OH
Boc-D-Phe-OH
Boc-L-Pro-OH
Boc-L-Ala-OH
Boc-L-Val-OH
Boc-L-Leu-OH
Boc-L-Phe-OH
7.5/268
b
Figure 6. Titration curves of the values of induced CD signals at 270 nm
(∆ꢀ270) in the complexation of peptide 1 (0.14 mM) with (A) Boc-D-Pro-
OH (open square), or (B) Boc-L-Pro-OH (open circle) in chloroform.
Titration curves of ∆ꢀ270 in the mixture of peptide 1 with Boc-Gly-OH
(closed triangle) and with Boc-DL-Pro-OH (D/L ) 50/50; open triangle) are
plotted. The fraction of peptide 1 complexed (solid line) is also superim-
3.0/302
6.4/270
2.8/279
3.9/275
4.7/273
2.6/279
c
-
-
0.1/309
0.7/304
c
posed; the fraction was calculated using the binding constant (K ) 28 M-1 3
)
a [1] ) 0.14 mM and [Boc-amino acid] ) 65 mM in chloroform.
between Boc-L-Pro-OH and H-Aib-OMe for the model compound of an
b Overlapped with CD signal of Boc-D-Phe-OH. c Not observed.
N-terminal moiety of peptide 1, according to ref 3.
amino group for interacting with a chiral carboxylic acid is
required for controlling the original helical screw sense of chiral
peptide 1. Figure 6 shows the CD titration curves upon the
complexation of peptide 1 with Boc-D-Pro-OH (Figure 6A) or
Boc-L-Pro-OH (Figure 6B) in chloroform.
the N-terminal noncovalent effects give a tug at the original
screw sense arising from the C-terminal covalent effect toward
their preferred ones.
Noncovalent Domino Effect in Peptide 2. Figure 7 shows
the effect of chiral carboxylic acid on CD spectra of peptide 2
in chloroform.
The CD intensity increased remarkably with an increase in
the concentration of chiral Boc-Pro-OH and reached a saturation
value over 100 mM essentially. Also, the CD spectrum of
peptide 1 was almost unaffected by adding a large excess of
achiral Boc-Gly-OH (ca. 40-1100-fold) instead of chiral Boc-
amino acid (Figure 6), thus indicating that achiral acid-base
interaction does not influence the helical screw sense or helical
stability of chiral peptide 1 essentially. Racemic Boc-DL-Pro-
OH (D/L ) 50/50) also gave no marked changes in the original
CD spectrum (Figure 6), implying that the binding affinity of
peptide 1 to Boc-D-Pro-OH is almost the same as that to Boc-
L-Pro-OH. Moreover, the CD amplitudes (∆ꢀ270) induced by
chiral Boc-Pro-OH could be essentially attributed to the fraction
of peptide 1 complexed (Figure 6); the fraction was calculated
using the binding constant (K ) 28 M-1)3 between Boc-L-Pro-
OH and H-Aib-OMe for the model compound of an N-terminal
moiety of peptide 1, according to ref 3. Therefore, the chiral
acid-base interaction at the N-terminal position, i.e., nonco-
valent domino effect, controls the original left-handed screw
sense of chiral peptide 1, as shown in Chart 1A. In peptide 1,
In contrast to peptide 1, peptide 2 retains the original right-
handed screw sense on the addition of Boc-D-Pro-OH or Boc-
L-Pro-OH. This should be ascribed to a strong propensity of
peptide 2 to adopt a right-handed helical conformation. Thus,
noncovalent chiral interaction seems not to be strong enough
to induce a dramatic helix-to-helix transition in a chiral peptide
adopting a stable one-handed helix. However, the split amplitude
of the original CD spectrum increased markedly with an increase
in the Boc-L-Pro-OH concentration, whereas the addition of Boc-
D-Pro-OH did not change the amplitude essentially. This strongly
implies that the original right-handed helix is more stabilized
by the chiral stimulus of Boc-L-Pro-OH. Similar tendency was
observed for addition of other chiral Boc-amino acids (Ala, Leu,
Val, and Phe), as shown in Figure 8 and Table 2. Namely, Boc-
L-amino acids tend to more stabilize the original right-handed
helix, compared with the corresponding Boc-D-amino acids.
To eliminate effect of achiral acid-base interaction on CD
spectra, the split CD amplitude was plotted against concentra-
9
J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002 2471

A R T I C L E S
Inai et al.
Table 2. Signs of Splitting Cotton Effects and ∆ꢀ Values for
Induced CD of Peptide 2 with Chiral Carboxylic Acida
first Cotton effect second Cotton effect
acid
sign
∆ꢀ (λ/nm)
sign
∆ꢀ (λ/nm)
none
-
-
-
-
-
-
-
-
-
-
-
17.7/302
18.5/302
19.5/301
19.1/301
19.4/301
19.1/302
21.3/302
20.0/302
20.5/302
21.0/303
20.0/302
+
+
+
+
+
+
+
+
+
+
+
24.8/270
24.6/270
25.6/270
25.1/270
Boc-D-Pro-OH
Boc-D-Ala-OH
Boc-D-Val-OH
Boc-D-Leu-OH
Boc-D-Phe-OH
Boc-L-Pro-OH
Boc-L-Ala-OH
Boc-L-Val-OH
Boc-L-Leu-OH
Boc-L-Phe-OH
25.9/270
b
31.6/270
30.1/270
31.0/270
31.9/271
b
Figure 7. CD spectra of peptide 2 in chloroform containing (A) Boc-D-
Pro-OH and (B) Boc-L-Pro-OH: [2] ) 0.14 mM and [Boc-Pro-OH] )
0-100 mM.
a [2] ) 0.14 mM and [Boc-amino acid] ) 100 mM in chloroform.
b Overlapped with CD signal of Boc-L (or D)-Phe-OH.
Figure 9. (A) Titration curves of the amplitudes of split CD signals in the
complexation of peptide 2 (0.14 mM) with Boc-D-Pro-OH (open square),
Boc-L-Pro-OH (open circle), Boc-DL-Pro-OH (D/L ) 50/50; open triangle),
or Boc-Gly-OH (closed triangle) in chloroform. The dashed line represents
the arithmetic mean of the two amplitudes induced by Boc-D-Pro-OH (open
square) and Boc-L-Pro-OH (open circle). (B) Titration curves of the
amplitudes of split CD signals in the complexation of N-Boc-protected
peptide 2 (0.14 mM) with Boc-D-Pro-OH (open square), Boc-L-Pro-OH
(open circle), or Boc-Gly-OH (closed triangle) in chloroform.
Figure 8. CD spectra of peptide 2 in chloroform containing (A) Boc-D-
amino acid (Ala, Leu, Val, Phe, and Pro) or (B) the corresponding Boc-
L-amino acid: [1] ) 0.14 mM; [Boc-amino acid] ) 65 mM. The CD spectra
of the mixtures containing Boc-L (or D)-Phe-OH are not shown below 272
nm due to overlap of the peptide and Boc-L (or D)-Phe-OH signals.
Chart 1. Preferred Helical Screw Sense of Chiral Peptides 1 (A)
and 2 (B) through Both Noncovalent and Covalent Domino Effects;
the Directions of Arrows Represent the Induction of Left- (LH) or
Right-handed (RH) Screw Sense.
whereas Boc-L-Pro-OH stabilizes it markedly. The directions
of the helical screw senses induced by Boc-L-Pro-OH and Boc-
D-Pro-OH agreed with those observed for achiral peptide
H-(Aib-∆ZPhe)4-Aib-OMe.3 In Figure 9A, the CD amplitude
induced by Boc-DL-Pro-OH (D/L ) 50/50) was slightly larger
over the concentration range of Boc-Pro-OH (0-200 mM) than
the arithmetic mean (dashed line) of the two amplitudes induced
by Boc-D-Pro-OH and Boc-L-Pro-OH. Because Boc-L-Pro-OH
induces a right-handed screw sense in peptide 1 and achiral
peptide H-(Aib-∆ZPhe)4-Aib-OMe,3 peptide 2 exhibiting a
strong bias in favor of a right-handed screw sense might slightly
prefer the binding to Boc-L-Pro-OH rather than to Boc-D-Pro-
OH. N-Boc-protected peptide 2, Boc-(Aib-∆ZPhe)4-L-Leu2-
OMe, also showed a split CD spectrum corresponding to a right-
handed screw sense. Although addition of Boc-L-Pro-OH, Boc-
D-Pro-OH, or Boc-Gly-OH slightly increased its original CD
amplitude, the three titration curves (Figure 9B) did not differ
from one another essentially. Obviously, the N-terminal amino
group is required for controlling the stabilization of the original
right-handed helix. Therefore, noncovalent chiral domino effect
in peptide 2 can contribute even to the helical stability of a
chiral peptide prevailing one-handed helix strongly, as shown
in Chart 1B.
tions of Boc-D-Pro-OH, Boc-L-Pro-OH, and achiral Boc-Gly-
OH, as shown in Figure 9A.
Compared with the titration curve of Boc-Gly-OH, Boc-D-
Pro-OH destabilizes somewhat the original right-handed helix,
9
2472 J. AM. CHEM. SOC. VOL. 124, NO. 11, 2002

Noncovalent Domino Effect on Helical Screw Sense
A R T I C L E S
Conclusions
both noncovalent and covalent domino effects operate simul-
taneously on the helical screw sense of a single achiral segment
and have compared both powers for inducing the screw sense
bias. As a result of the tug-of-war game on the achiral helical
chain, the chiral domino effect governing the whole screw sense
is the N-terminal noncovalent type for peptide 1, and the
C-terminal covalent type for peptide 2, as shown in Chart 1.
In the present work, we have demonstrated, for the first time,
the importance of noncovalent chiral domino effect operating
on the N-terminal position of chiral peptides. This effect is
capable of controlling the original helical screw sense or helical
stability of a chiral peptide, thus providing novel insights into
the nature of chiral intermolecular interactions of a helical
peptide with a chiral molecule in peptide and protein science.
In particular, the effect is sensitive to a chiral peptide exhibiting
a small excess of one-handed helix; i.e., peptide 1 possessing
only one chiral residue undergoes a dramatic helix-to-helix
transition or remarkable stabilization of one-handed helix upon
complexation with chiral carboxylic acid, depending on its
chirality. Second, we here have presented a unique system that
Acknowledgment. This work was supported by the Ministry
of Education, Culture, Sports, Science, and Technology of Japan
under Grant to Y. I. We are extremely grateful to the editor
and the referees for very fruitful discussion and helpful
suggestions.
JA017126W
9
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