Transfer of noncovalent chiral information along an optically inactive helical peptide chain: Allosteric control of asymmetry of the C-terminal site by external molecule that binds to the N-terminal Site

Article abstract of DOI:10.1021/jo801686m

This study aims at demonstrating end-to-end transfer of noncovalent chiral information along a peptide chain. The domino-type induction of helical sense is proven by using achiral peptides 1-m of bis-chromophoric sequence with different chain lengths: H-(

Full text of DOI:10.1021/jo801686m

VOLUME 74, NUMBER 4
February 20, 2009
Copyright 2009 by the American Chemical Society
Transfer of Noncovalent Chiral Information along an Optically
Inactive Helical Peptide Chain: Allosteric Control of Asymmetry of
the C-Terminal Site by External Molecule that Binds to the
N-Terminal Site
Naoki Ousaka†,§ and Yoshihito Inai*,‡
Department of EnVironmental Technology and Urban Planning and Department of Frontier Materials,
Shikumi College, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku,
Nagoya 466-8555, Japan
inai.yoshihito@nitech.ac.jp
ReceiVed August 1, 2008
This study aims at demonstrating end-to-end transfer of noncovalent chiral information along a peptide
chain. The domino-type induction of helical sense is proven by using achiral peptides 1-m of
bis-chromophoric sequence with different chain lengths: H-(Aib-∆ZPhe)m-(Aib-∆ZBip)2-Aib-OCH3 [m
) 2, 4, and 6; Aib ) R-aminoisobutyric acid; ∆ZPhe ) (Z)-R,ꢀ-didehydrophenylalanine; ∆ZBip ) (Z)-
ꢀ-(4,4′-biphenyl)-R,ꢀ-didehydroalanine]. They all showed the tendency to adopt a 310-helix. Whereas
peptide 1-m originally shows no circular dichroism (CD) signals, marked CD signals were induced at
around 270-320 nm based on both the ꢀ-aryl didehydroresidues by chiral Boc-proline (Boc ) tert-
butoxycarbonyl). The observed CD spectra were interpreted on the basis of the exciton chirality method
and theoretical CD simulation of several helical conformations that were energy-minimized. The
experimental and theoretical CD analysis reveals that Boc-L-proline induces the preference for a right-
handed helicity in the whole chain of 1-m. Such noncovalent chiral induction was not observed in the
corresponding N-terminally protected 1-m. Obviously, helicity induction in 1-m originates from the binding
of Boc-proline to the N-terminal site. In the 17-mer (1-6), the information of helix sense reaches the
16th residue from the N-terminus. We have monitored precise transfer of noncovalent chiral stimulus
along a helical peptide chain. The present study also proposes a primitive allosteric model of a single
protein-mimicking backbone. Here chiral molecule binding the N-terminal site of 1-6 controls the
chiroptical signals and helical sense of the C-terminal site about 30 Å away.
Introduction
structure cooperatively integrates chiral information of each
constituent into a linear chain.1 The transfer of chiral information
Tertiary structures of biopolymers are expressed as asymmetric
information originating from their backbone torsion. A helical
‡ Department of Frontier Materials.
† Department of Environmental Technology and Urban Planning.
§ Present address: JST ERATO-SORST Kuroda Chiromorphology Team.
10.1021/jo801686m CCC: $40.75
Published on Web 11/19/2008
2009 American Chemical Society
J. Org. Chem. 2009, 74, 1429–1439 1429

Ousaka and Inai
along a helical chain plays a key role for nucleation, propagation,
and termination of a one-handed helicity ubiquitous in asymmetric
living organisms.2-4
sense triggered by chiral stimulus. The N-terminal-free segment
binds chiral molecules such as N-terminally blocked amino acid
and peptide acid. A terminal twist originating in the formation
of the chiral complex gives rise to energy imbalance between
the two enantiomeric helices.12 Dynamic chiral information at
the N-terminal site induces the preference for a helical sense.
We have figuratively termed this phenomenon the noncovalent
chiral domino effect (NCDE).10b,12 Helix-sense induction of an
achiral linear segment also occurs through covalent incorporation
Control of helical sense has been extensively investigated
using artificial helical models based on achiral monomer
units.1,5-8 Here the helical sense of such an optically inactive
segment is induced by covalent or noncovalent chiral stimuli.1,5-8
These leading studies provide significant guidance as to how
chiral information transfers along a molecular chain to generate
a one-handed helix. Such an optically inactive helix usually
involves chromophoric groups in the main chain or side chain.
Electronic circular dichroism (CD) spectroscopy that focuses
on these chromophores can clarify a helical sense induced in
the backbone itself. Furthermore, elegant studies on artificial
helical backbones have been reported in which helix propagation
or reversal is visually detected.9 However, chiral stimulus is
hardly traceable on biopolymer frameworks, especially on
protein backbones, because their chemically chiral sequences
prevent us from monitoring transfer of site-specific chiral
information.
(7) For transfer of chiral information in supramolecular assemblies, see: (a)
Smulders, M. M. J.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc.
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We have proposed noncovalent chiral induction in optically
inactive helical peptides as a protein-mimicking backbone.10-12
Such sequences are composed primarily of unusual achiral
R-amino acids, whereby we can extract information of a helix
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1430 J. Org. Chem. Vol. 74, No. 4, 2009

Transfer of Chiral Information in a Helical Peptide
SCHEME 1. Achiral Peptides 1-m [H-(Aib-∆ZPhe)m-(Aib-∆ZBip)2-Aib-OMe, m ) 2, 4, and 6], 2 [H-(Aib-∆ZBip)4-Aib-OMe],
and 3 [H-(Aib-∆ZPhe)4-Aib-OMe] for the NCDE Studya
a Induction of helical sense in peptide 3 through noncovalent chiral interaction was reported in ref 10a.
of chiral moieties into the terminal site5d-g,8b,f-h,j,13 and can
be termed the covalent chiral domino effect (CCDE).10b,12
to the C-terminal region. The position-specific incorporation of
some chromophores offers conformational information about a
local chain.20 Hence we have designed alternating sequences
1-m of Aib with two types of ꢀ-aryl didehydroalanine (∆AA)
residues, illustrated in Scheme 1. Peptides 1-m are characterized
by the ∆ZPhe-based region with different chain lengths, with
their C-terminal connected to a pair of (Z)-4,4′-biphenyl-R,ꢀ-
didehydroalanine (∆ZBip) residues. The ∆ZBip residue extends
the π-conjugation system of ∆ZPhe moiety to a biphenyl moiety,
thus shifting the electronic transition to a longer wavelength.
In addition, the TDM direction of the ∆ZBip should roughly
resemble that in the ∆ZPhe case17a,c due to similar symmetry
in rotation about the Cꢀ-Cγ bond. A spatially twisted
∆ZBip-∆ZBip pair will function as an efficient CD probe at
wavelengths different from those of the electronic transition of
∆ZPhe residues. As a result, if the helicity induction occurs at
the whole molecular level, CD signals will be detected
simultaneously on both the ∆ZPhe and ∆ZBip residues. Notably,
CD signals for the latter chromophore provide direct evidence
for transfer of N-terminal chiral information to the C-terminal
region.
We also propose a primitive model for allosteric proteins in
which molecular binding at one site leads to control of structure
or activity of another remote region.21 Such remote control not
only is found in biological functions but also is elegantly
demonstrated in synthetic receptors or catalysts.22 In our present
model, an external chiral molecule should stimulate the N-
terminal site of 1-m to induce chiroptical signals and helical
sense of the C-terminal region. If this allosteric control
successfully occurs on 1-6 in a 310-helix,23 the Aib(1)-
∆ZBip(16) distance would be beyond 30 Å. This controllable
distance is comparable to the most efficient case in a synthetic
unimolecule.19a
Our NCDE experiments have not offered the precise route,
because the electronic CD spectroscopy focuses on the same
chromophores incorporated in the sequence. Here, achiral
alternating sequences of (Z)-R,ꢀ-didehydrophenylalanine
(∆ZPhe) and R-aminoisobutyric acid (Aib) have been used for
design of the optically inactive helical segment.10-16 The ∆ZPhe
chromophore is characterized by a strong absorption band at
around 280 nm, in which the transition dipole moment (TDM)
places roughly at the phenyl-carbonyl line.17 According to the
exciton chirality method,18 split-type CD patterns at around 280
nm are predicted for a -∆ZPhe-X-∆ZPhe- segment in a helical
conformation. This was true in chiral stimulus-induced CD
spectra of nonapeptides based on (∆ZPhe-X)n.10,11,13,17d We have
interpreted the CD spectra as helical sense induced in the whole
molecule. However, such split CD patterns do not always mean
a preference for a one-handed helix in the entire chain. They
can be derived merely from a local ∆ZPhe-∆ZPhe twist. In
this regard, we have not directly proven the NCDE that chain-
terminal chiral information spreads over the entire chain. Helix
propagation and termination are detected in elegant artificial
molecules.8f,9,19 However, it is still unclear on a single protein-
mimicking chain composed of R-amino acids that dynamic chiral
information transfers from one chain terminus to the opposite
end.
The present study aims at demonstrating that chiral stimulus
received at the N-terminus of achiral peptide is truly transferred
(15) (a) Inai, Y.; Oshikawa, T.; Yamashita, M.; Tagawa, K.; Hirabayashi,
T. Biopolymers 2003, 70, 310–322. (b) Inai, Y.; Oshikawa, T.; Yamashita, M.;
Hirabayashi, T.; Ashitaka, S. J. Chem. Soc., Perkin. Trans. 2 2001, 892–897.
(16) Nandel, F. S.; Khare, B. Biopolymers 2005, 77, 63–73.
(17) For CD and absorption studies for ∆ZPhe-containing peptides, see: (a)
Pieroni, O.; Fissi, A.; Jain, R. M.; Chauhan, V. S. Biopolymers 1996, 38, 97–
108. (b) Pieroni, O.; Montagnoli, G.; Fissi, A.; Merlino, S.; Ciardelli, F. J. Am.
Chem. Soc. 1975, 97, 6820–6826. (c) Inai, Y.; Ito, T.; Hirabayashi, T.; Yokota,
K. Biopolymers 1993, 33, 1173–1184. For CD spectra simulated for helical
sequence based on -(X-∆ZPhe)n-, see: (d) Komori, H.; Inai, Y. J. Phys. Chem.
A 2006, 110, 9099–9107.
(20) (a) Johnson, N. P.; Baase, W. A.; von Hippel, P. H. Proc. Natl. Acad.
Sci. U.S.A. 2004, 101, 3426–3431. (b) Johnson, N. P.; Baase, W. A.; von Hippel,
P. H. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 7169–7173. (c) Inai, Y.; Sisido,
M.; Imanishi, Y. J. Phys. Chem. 1990, 94, 6237–6243. (d) Inai, Y.; Sisido, M.;
Imanishi, Y. J. Phys. Chem. 1990, 94, 8365–8370.
(18) For the exciton chirality method, see: (a) Harada, N.; Chen, S. L.;
Nakanishi, K. J. Am. Chem. Soc. 1975, 97, 5345–5352. (b) Harada, N.; Nakanishi,
K. Circular Dichroic Spectroscopy. Exciton Coupling in Organic Stereochemistry;
University Science Books: Mill Valley, CA, 1983.
(19) Elegant models have been proposed for the remote control of confor-
mational information along a molecular frame: (a) Clayden, J.; Lund, A.;
Vallverdu´, L.; Helliwell, M. Nature 2004, 431, 966–971. (b) Mizutani, T.; Sakai,
N.; Yagi, S.; Takagishi, T.; Kitagawa, S.; Ogoshi, H. J. Am. Chem. Soc. 2000,
122, 748–749.
(21) For allosteric proteins, see: Monod, J.; Changeux, J. P.; Jacob, F. J.
Mol. Biol. 1963, 6, 306–329.
(22) (a) Shinkai, S.; Ikeda, M.; Sugasaki, A.; Takeuchi, M. Acc. Chem. Res.
2001, 34, 494–503. (b) Takeuchi, M.; Ikeda, M.; Sugasaki, A.; Shinkai, S. Acc.
Chem. Res. 2001, 34, 865–873. (c) Kovbasyuk, L.; Kra¨mer, R. Chem. ReV. 2004,
104, 3161–3187.
(23) The simulated 310-helical structure is shown later (Figure 3). For
structural features of the 310-helix, see ref 14g. See also: Barlow, D. J.; Thornton,
J. M. J. Mol. Biol. 1988, 201, 601–619.
J. Org. Chem. Vol. 74, No. 4, 2009 1431

Ousaka and Inai
TABLE 1. Peak Positionsa Selected in Amide I and II Absorption
Bands of Peptides 1-m, 2, and 3 in Chloroform27
peptide
amide I
amide II
1-6
1-4
1-2
2
1655
1656
1660
1661
1660
1624
1625
1628
1629
1629
1538
1537
1535
1535
1535
3
a In cm-1
.
1.27 Similar patterns are observed for analogous helical se-
quences repeating -(X-∆ZPhe)-10a-d,11a-c,28. In particular, such
an amide pattern is seen in analogous nonapeptide [Boc-(Aib-
∆ZPhe)4-Aib-OMe], which adopts a typical 310-helix in the
crystal state.15a These peaks have been assigned to carbonyl
stretching modes in a helical conformation,28 being observed
at higher wavenumbers for saturated residues and at lower
wavenumbers for ∆ZPhe residues. Peptides 2 and 3 also showed
a similar amide I profile.28 For amide II region, all peptides
gave a strong absorption band at 1535-1538 cm-1, of which
the position is also observed in Aib-containing oligopeptides.28b
Therefore peptides 1-m and 2 favor a helical conformation in
chloroform.
FIGURE 1. FT-IR absorption spectra of 1-6 in chloroform/methanol
(95/5, v/v %) and 1-4 and 1-2 in chloroform at ambient temperature:
[peptide] ) 3 mM.
A quick overview of the work is as follows. In peptides 1-m,
chiral Boc-proline induced CD signals at both absorption bands
of the ∆ZPhe and ∆ZBip residues.24 These CD results have been
compared with the helicity induction of nonapeptide 2 or 310a
based on either the ∆ZBip or ∆ZPhe chromophores, respectively.
In addition, the spectral patterns observed have been interpreted
on the basis of simulations of electronic transition states and
theoretical CD spectra. In conclusion, the NCDE can control
the helical sense of the entire chain substantially. In the 17-
mer (m ) 6), chiral information reaches the 16th residue, thus
demonstrating long-range allosteric control in a protein-mimick-
ing helical backbone.
Peptides containing Aib or ∆AA residues tend to take a 310-
helix or an R-helix.14 To identify the helix type, temperature
changes in amide NH resonances are shown in Figure 2.30 When
peptide 1-m adopts a 310-helix, two amide NH groups of the
N-terminal sequence lack hydrogen-bonding partners. In the case
of an R-helix, three N-terminal amide NHs are free from
1
intramolecular hydrogen bonding. In H NMR spectra of 1-m
Results and Discussion
(Figure S2 Supporting Information), NH protons of ∆AA
appeared at a considerably lower magnetic field.29 Thus, the
singlets above 8.9 ppm should be assigned to the ∆AA NH
protons, of which the number was estimated from relative
intensity to be 3 (1-2), 5 (1-4), and 7 (1-6). In each case, one
NH proton of ∆AA was not clearly found. This trend is often
observed in analogous helical peptides having an N-terminal
moiety of H-X-∆ZPhe(2)-, where the ∆ZPhe NH resonance does
not appear clearly in CDCl3.10b,c,11b-d,31 Thus, the lacking NH
should be assigned to the ∆ZPhe(2)’s.
In contrast, the singlets at around 7.9-8.4 ppm should be
assigned to the Aib NH groups, of which the numbers were
similarly estimated to be 3 (1-2), 5 (1-4), and 7 (1-6). In each
case, NOESY cross peaks allow us to identify one remaining
Aib NH in a higher magnetic field (Figures S3-S5, Supporting
Information). The Aib NH group showed the largest temperature
coefficient among the amide groups observed (Figure 2). The
identification of a helical conformation from the FT-IR absorp-
Helical Conformations of Peptides 1-m. It has been widely
revealed that incorporation of Aib or ∆ZPhe residues promotes
helical propensity.14 Sequences based on -Aib-∆ZPhe- or
-∆ZPhe-Aib- are shown to adopt a 310-helix in solution and
crystalline form, and on theoretical basis.10-16 Particularly, a
310-helix is found in peptide 3 or its N-Boc-protected form as
analogs of 1-m.10a,12,15a In addition, conformational space of
other ∆AA residues resembles that of ∆ZPhe.25 For instance,
oligopeptides containing two or more (Z)-R,ꢀ-didehydronaph-
thylalanine residues tend to take a 310-helical conformation.25b
Furthermore, a biphenyl side chain can be placed on a helical
peptide backbone.26
The preliminary information suggests the propensity for a
helical structure in 1-m. For the experimental evidence, FT-IR
absorption spectra of 1-m in chloroform are displayed in Figure
1, focusing on the amide I and II regions.
Two prominent absorption bands were commonly seen in the
amide I region. These peak positions are summarized in Table
(27) Although FT-IR absorption data of peptide 3 were reported in ref 10a,
the data have been updated here. FT-IR absorption spectra of 2 and 3 are shown
in Figure S1, Supporting Information.
(24) Our present study mainly aims at clarifying whether noncovalent chiral
information received at the N-terminal site of a chiral peptide is indeed transferred
to the C-terminal site. Effects of the N-terminal amino acid or of another Boc-
amino acid are not the current issue. These effects on induction of helicity were
already reported.10 Peptide 3 and analogous peptides H-X-(∆ZPhe-Aib)4-OMe
(X ) Aib, ꢀ-Ala, Gly, or N-methylglycine) commonly underwent induction of
a right-handed helix through addition of Boc-L-Y-OH (Y ) Pro, Leu, Ala, or
Val).10a-d Thus the choice of the N-terminal amino acid or Boc-amino acid is
not an essential factor in the current theme.
(25) (a) Inai, Y.; Oshikawa, T.; Yamashita, M.; Hirabayashi, T.; Hirako, T.
Biopolymers 2001, 58, 9–19. (b) Inai, Y.; Hirabayashi, T. Biopolymers 2001,
59, 356–369. (c) Inai, Y.; Oshikawa, T.; Yamashita, M.; Hirabayashi, T.;
Kurokawa, Y. Bull. Chem. Soc. Jpn. 2001, 74, 959–966.
(28) (a) Inai, Y.; Sakakura, Y.; Hirabayashi, T. Polym. J. 1998, 30, 828–
832. (b) Kennedy, D. F.; Crisma, M.; Toniolo, C.; Chapman, D. Biochemistry
1991, 30, 6541–6548. For IR absorption studies on other peptides possessing
∆ZPhe residues, see: (c) Gupta, A.; Mehrotra, R.; Tewari, J.; Jain, R. M.;
Chauhan, V. S. Biopolymers 1999, 50, 595–601.
(29) For example, see refs 10, 11, 13, 14o, 14p, and 15.
(30) For intramolecular hydrogen-bonding NH and temperature coefficient,
see: (a) Andersen, N. H.; Neidigh, J. W.; Harris, S. M.; Lee, G. M.; Liu, Z.;
Tong, H. J. Am. Chem. Soc. 1997, 119, 8547–8561. (b) Baxter, N. J.; Williamson,
M. P. J. Biomol. NMR 1997, 9, 359–369. (c) Stevens, E. S.; Sugawara, N.;
Bonora, G. M.; Toniolo, C. J. Am. Chem. Soc. 1980, 102, 7048–7050.
(31) This might be partly due to conformational fluctuation of the N-terminal
site that does not participate in intramolecular hydrogen bonds. In fact, the
∆ZPhe(2) NH resonance appears at a lower magnetic field through addition of
Boc-amino acid that is hydrogen-bonded to the N-terminal site.10b,c
(26) (a) Kuragaki, M.; Sisido, M. J. Phys. Chem. 1996, 100, 16019–16025.
(b) Butterfield, S. M.; Patel, P. R.; Waters, M. L. J. Am. Chem. Soc. 2002, 124,
9751–9755.
1432 J. Org. Chem. Vol. 74, No. 4, 2009

Transfer of Chiral Information in a Helical Peptide
FIGURE 2. Temperature dependence of NH chemical shifts of (A) 1-6, (B) 1-4, (C) 1-2, and (D) 2: CDCl3/(CD3)2SO (100/3.7, v/v %) for (A);
CDCl3 for (B)-(D). [peptide] ) 3 mM. Green number near a line indicates its temperature coefficient (-∆δ/∆T in ppb/deg).
tion pattern indicates that the N-terminal sequence involves
amide NH groups free from intramolecular hydrogen bonding.
Thus, the upfield Aib NH should be assigned to the Aib(3)’s.
The other NH groups observed were less dependent on tem-
perature, thus being shielded as a result of intramolecular
hydrogen bonding (Figure 2). Although the ∆ZPhe(2) NH was
not clearly found, peptides 1-m were shown to adopt a 310-
helical conformation. The influence of (CD3)2SO on NH
chemical shifts8f,g,i,10a-d,14e,j,o,p,32 was also investigated in 1-2
(Figure S6, Supporting Information). Among the NH resonances,
the Aib(3) NH undergoes remarkable shift to a low magnetic
field. In contrast, the six NHs of ∆ZPhe(4)-Aib(9) are es-
sentially maintained by the hydrogen-bond acceptor, also
supporting the occurrence of a 310-helix. Similar temperature
dependence of NH chemical shifts in a 310-helix was seen in
peptide 2 (Figure 2).
point density functional theory (DFT) [B3LYP/6-31G(d,p)] com-
putation to improve energy precision.33,35,36
The structures and energies are summarized in Figure 337
and Table 2. In all cases, energy minimization from a 310-helix
maintained 310-helical structures. Their structural data (torsion
angles and hydrogen bond parameters) are summarized in Table
S1, Supporting Information. The three peptides of 1-m have
almost the same average values of (φ, ψ). In each peptide, 310-
helical hydrogen bonds are formed for every ith CO and (i +
3)th NH pair. Here average hydrogen-bonding parameters of
1-m are also similar to each other. Thus the structural features
of the 310-helix are not affected by chain length in the current
sequences but originate from conformational properties of the
Aib and ∆AA residues.
Energy minimization from an R-helix yielded essentially
R-helices, but a 310-helical hydrogen-bonding manner was found
Computational Simulation of Helical Structures. The helical
propensity of 1-m was also investigated from a theoretical point
of view. Since ∆ZPhe- and Aib-containing peptides often take
not only a 310-helix but an R-helix,14 both helices were
considered as initial conformers. Here two orientations of the
biphenyl moiety were used, i.e., positive or negative torsion
angles (ꢁ6) about the phenyl-phenylene linkage. Energy mini-
mization was carried out with Gaussian 0333 by the semiempirical
molecular orbital (MO) method (AM1),33,34 followed by a single-
(33) (a) Gaussian 03, ReVision C.02 Frisch, M. J. et al. Gaussian, Inc.:
Wallingford CT, 2004. (b) For the manual and full references for Gaussian 03,
see also: The Official Gaussian 03 Website (http://www.gaussian.com/).
(34) For the AM1 method, see: Dewar, M. J. S.; Zoebisch, E. G.; Healy,
E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902–3909.
(35) For the B3LYP method, see: (a) Becke, A. D. J. Chem. Phys. 1993, 98,
5648–5652. (b) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623–11627. (c) Lee, C.; Yang, W.; Parr, R. G. Phys.
ReV. B. 1988, 37, 785–789.
(36) Foresman, J. B.; Frisch, Æ. Exploring Chemistry with Electronic
Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996; Chapters
6-7.
(37) For molecular graphics in the present paper, see: Thompson, M. A. ArgusLab
4.0.1; Planaria Software LLC: Seattle, WA (http://www.arguslab.com).
(32) Pitner, T. P.; Urry, D. W. J. Am. Chem. Soc. 1972, 94, 1399–1400.
J. Org. Chem. Vol. 74, No. 4, 2009 1433

Ousaka and Inai
FIGURE 3. Right-handed helical structures of 1-m energy-minimized from a 310-helix: (A, B) 1-6, (C, D) 1-4, and (E, F) 1-2. M and P stand for
the two orientations (ꢁ6) of the biphenyl groups.
helical propensities are often switched by chain length.38
TABLE 2. Energy and Dipole Moment of Peptides 1-m
Energy-Minimized from a 310-Helix or an r-Helixa
However, in all cases, the 310-helices are more stable by 0.8-1.2
kcal mol-1 per residues than the R-helices. Such a strong
energy difference
(kcal mol-1
)
preference for 310-helix should be attributed to the conforma-
tional nature of Aib and ∆AA residues.14 The biphenyl
orientation of M or P seems not to influence the stability of
helical structures substantially.
type of
biphenyl
per
per
dipole moment
(debye)
peptide initial helixb orientationc molecule residues
1-6
1-4
1-2
3
3
10-helix
10-helix
P
0.00
0.04
13.66
15.20
0.00
0.00
0.00
0.80
0.89
0.00
0.00
0.87
0.99
0.00
0.02
0.96
1.19
64.7
64.7
67.9
67.8
47.3
47.3
48.5
48.4
30.2
30.2
30.0
29.8
M
M
P
M
P
M
P
M
P
UV Absorption Patterns. ∆ZPhe-based peptide 3 in chlo-
roform showed a strong absorption band at around 280 nm.10a
In the ∆ZBip-based peptide 2 in chloroform, a marked absorption
band was observed at longer wavelengths (ca. 310 nm) (Figure
S8, Supporting Information). Absorption bands of peptides 1-m
should occur through combination of both chromophoric
residues. In fact, the absorption maxima of 1-m was between
280 and 310 nm in chloroform (Figure 4), indicating partial
overlap of the two absorption bands. A peak shoulder or tail
above 320 nm in 1-m is not derived from the ∆ZPhe residues,
as judged from absorption spectrum of 3 (Figure S8, Supporting
Information). In other words, CD signals above 320 nm are
essentially defined as chiral information of the ∆ZBip-based
segment.
R-helix
R-helix
3
3
10-helix
10-helix
0.05
R-helix
R-helix
11.29
12.93
0.00
0.22
8.66
3
3
10-helix
10-helix
R-helix
R-helix
M
P
10.74
a These structures are provided in Figure 3 (310-helix) and Figure S7,
Supporting Information (R-helix). The energy minimization was carried
out by the AM1 method.34 Each structure obtained was subjected to
single-point DFT computation to yield its energy and dipole moment.
c
b For the initial conformation. Negative (M) and positive (P) ꢁ6 values
of the two biphenyl groups in the initial conformation (for the detail, see
the experimental section).
in the N-terminal sequence (Figure S7, Supporting Information).
As shown in Table 2, the energy difference per residue between
structures energy-minimized from a 310-helix and an R-helix
seems to become somewhat smaller with increasing chain length.
This prediction might be related to the view that the 310-/R-
(38) (a) Pavone, V.; Benedetti, E.; Di Blasio, B.; Pedone, C.; Santini, A.;
Bavoso, A.; Toniolo, C.; Crisma, M.; Sartore, L. J. Biomol. Struct. Dyn. 1990,
7, 1321–1331. (b) Karle, I. L.; Flippen-Anderson, J. L.; Gurunath, R.; Balaram,
P. Protein Sci. 1994, 3, 1547–1555. (c) Fiori, W. R.; Miick, S. M.; Millhauser,
G. L. Biochemistry 1993, 32, 11957–11962. (d) Basu, G.; Kuki, A. Biopolymers
1992, 32, 61–71.
1434 J. Org. Chem. Vol. 74, No. 4, 2009

Transfer of Chiral Information in a Helical Peptide
originating in the complex structure spreads over the helical
backbone. A similar mechanism for the chiral induction should
be applied to the present case, on the basis of the preceding
results. Consequently, the external chiral molecule stimulates
the N-terminal sequence to generate a helical sense in the
optically inactive backbone. Herein chiral information at the
N-terminus reaches the C-terminal ∆ZBip-based region.
Helical Arrangement of Transition Moments. The TDM
of ∆ZPhe absorption band at around 280 nm was estimated to
lie approximately at the phenyl-carbonyl line.17 In a 310-helical
segment of repeating (X-∆ZPhe) units, the nearest TDM pairs
are twisted clockwise or counterclockwise. Such a spatial twist
enables us to determine the helical sense by the exciton chirality
method.18
Here, we predicted the TDM for each ∆ZBip or ∆ZPhe residue
incorporated in peptide 1-6 adopting the right-handed 310-helices.
Each ∆AA geometry was extracted from Figure 3 to convert
into Ac-∆ZBip-NMA or Ac-∆ZPhe-NMA. For these N- and
C-protected amino acids, the ZINDO/S computation was carried
out with Gaussian 0333,41 to yield their electronic transition
parameters of oscillator strength (f) and TDM.
FIGURE 4. Induced CD spectra of peptides 1-m with Boc-L-proline
in chloroform: [Boc-L-proline] ) 65 mM. The dotted lines indicate
those without chiral additive: [1-6] ) 6.3 × 10-2 mM; [1-4] ) 9.0 ×
10-2 mM; [1-2] ) 0.13 mM. The bottom panel stands for the
corresponding absorption spectra.
The ∆ZBip residues showed large f values at around 295-298
nm, while marked f values were seen at around 274-277 nm
for the ∆ZPhe residues. These predicted wavelengths were close
to the ones experimentally found in 2 and 3 (Figure S8,
Supporting Information), ensuring the validity of the prediction.
Each TDM obtained in the velocity form is superimposed on
the corresponding right-handed 310-helix of 1-6 (Figure 5). The
neighboring TDM pairs are twisted counterclockwise along the
right-handed helical axis.42 Since the energy gap between the
∆ZBip- and ∆ZPhe-transition states of each fmax is not so large,
the exciton chirality method will be valid for the present case.18
The helical arrangement of TDM will generate split-type CD
patterns with a negative peak at longer wavelengths, thus
indicating that a right-handed helix is induced by Boc-L-proline.
Induction or stabilization of a right-handed helix by Boc-L-amino
acidhasbeendemonstratedinN-terminalfreehelicalpeptides.10-12
Therefore both the ∆ZPhe- and ∆ZBip-based segments adopt a
310-helical conformation with the same sense.
Simulation of Theoretical CD Spectra. The preceding
analysis reveals that chiral information reaches the ∆ZBip-based
region. Recently, theoretical CD simulation on ∆ZPhe-containing
helical peptides provides deep insights into their conforma-
tions.17d In the present section, we report similar CD simulation
of 1-m and also attempts to discriminate between the 310-helix
and R-helix in Table 2.
Electronic transition-state and chiroptical parameters of 1-m
in the right-handed helix were computed by the time-dependent
(TD) method of ZINDO/S with Gaussian 03.33,41-44 The use
of these parameters in the velocity form produced the CD spectra
as well as absorption profiles (Figure 6).
Induction of Helix Sense through Noncovalent Chiral
Interaction. The achiral sequence 1-m originally shows no CD
signals in the absence of chiral stimuli. In contrast, CD signals
of 1-m were induced at around 270-320 nm by chiral Boc-
amino acid. Figure 4 displays CD spectra of 1-m with Boc-
proline in chloroform. In each peptide, a mirror CD pattern was
observed between the added enantiomers (Figure S9, Supporting
Information). It is obvious that the CD signals are generated
from external chiral stimulus. All of the CD spectra, which are
essentially identified as exciton splitting patterns, possess clear
signals at over 320 nm. Thus chiral induction occurs at both
the ∆ZPhe- and ∆ZBip-based regions.
N-Terminally protected peptides 1-m, Boc-(Aib-∆ZPhe)m-
(Aib-∆ZBip)2-Aib-OMe (m ) 2, 4, and 6), yielded no marked
CD signals in the presence of a large excess of Boc-L-proline
(Figure S10, Supporting Information). On the other hand, the
induced CD intensity of 1-m increased with increasing concen-
tration of Boc-L-proline (Figure S11, Supporting Information).
From the plots of ∆ε against the additive concentration, the
apparent binding constants (Kapp) for 1-m were estimated to be
substantially similar to each other.39 These additional facts
support that the chiral molecule binds the N-terminal sequence
to induce the preference for a one-handed helix.
We have proposed the mechanism of NCDE in H-ꢀ-Ala- and
H-Gly-capped 310-helical peptides.10b,c Hydrogen-bond-free
amide NH and amino groups appear inherently in the N-terminal
helical motif to participate in the three-point coordination to
chiral Boc-amino acid.10b,c,40 Terminal asymmetric information
(41) For the ZINDO/S method, see: (a) Ridley, J. E.; Zerner, M. C. Theor.
Chim. Acta 1973, 32, 111–134. (b) Ridley, J. E.; Zerner, M. C. Theor. Chim.
Acta 1976, 42, 223–236. (c) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-
Westerhoff, U. T. J. Am. Chem. Soc. 1980, 102, 589–599. (d) Thompson, M. A.;
Zerner, M. C. J. Am. Chem. Soc. 1991, 113, 8210–8215.
(42) A similar helical arrangement of each TDM in the length form was
obtained; see Figure S12, Supporting Information.
(43) For the ZINDO-based CD simulation, see: (a) Telfer, S. G.; Tajima,
N.; Kuroda, R. J. Am. Chem. Soc. 2004, 126, 1408–1418. (b) Ankai, E.;
Sakakibara, K.; Uchida, S.; Uchida, Y.; Yokoyama, Y.; Yokoyama, Y. Bull.
Chem. Soc. Jpn. 2001, 75, 1101–1108. (c) Kawai, M.; Nagai, U.; Inai, Y.;
Yamamura, H.; Akasaka, R.; Takagi, S.; Miwa, Y.; Taga, T. Biopolymers 2005,
80, 186–198. (d) Fujii, T.; Shiotsuki, M.; Inai, Y.; Sanda, F.; Masuda, T.
Macromolecules 2007, 40, 7079–7088.
(39) Nonlinear fitting of the ∆ε values at 0-100 mM gave the corresponding
Kapp value: 2.5 × 10 (1-6), 2.0 × 10 (1-4), and 1.9 × 10 (1-2). Similar treatment
was done in ref 11c. Likewise, the estimated Kapp value was 1.7 × 10 (3: for
0-102 mM). For these plots, see Figure S11, Supporting Information. While
the relationship between ∆ε of 3 and the concentration of chiral additive was
reported,10a the updated data were analyzed. The Kapp values ranging between
17 and 25 indicate similar interaction for the chiral induction.
(40) For the nature of helix termini in proteins and peptides, see: (a) Aurora,
R.; Rose, G. D. Protein Sci. 1998, 7, 21–38. (b) Baldwin, R. L.; Rose, G. D.
Trends Biochem. Sci. 1999, 24, 26–33. (c) Harper, E. T.; Rose, G. D. Biochemistry
1993, 32, 7605–7609. (d) Seale, J. W.; Srinivasan, R.; Rose, G. D. Protein Sci.
1994, 3, 1741–1745. (e) Karpen, M. E.; DeHaseth, P. L.; Neet, K. E. Protein
Sci. 1992, 1, 1333–1342.
J. Org. Chem. Vol. 74, No. 4, 2009 1435

Ousaka and Inai
FIGURE 5. Spatial arrangement of each ∆AA residue TDM (in the velocity form) along the right-handed 310-helix of 1-6: (A) and (B) correspond
to Figure 3A and 3B with P- and M-orientations of the ∆ZBip side chains. The right-handed helix produces a left-handed twist of the neighboring
moments with respect to the helical axis.
FIGURE 6. Electronic transition parameters and simulated CD spectra of 1-m in the 310-helices (Figure 3) with (A) M- and (B) P-orientations of
the two ∆ZBip side chains.
When peptides 1-m assume the right-handed 310-helical
structures, the simulated spectra (Figure 6) resemble the
experimental ones (Figure 4) in the following points: (i) Peptides
1-m produced essentially split-type CD patterns with negative
signals at longer wavelengths. (ii) The CD amplitude per
molecule monotonically increased with the chain length. (iii)
Both maximal and minimal positions of the split CD patterns
were shifted to shorter wavelengths by the increasing chain
length. The corresponding absorption profiles underwent a
similar blue shift. These spectral changes are readily understood
in terms of increasing contribution of the ∆ZPhe-based transition
states. (iv) There appeared to be an isodichroic point at a positive
∆ε value of a second Cotton effect.
(44) The TD SCF MO method for simulation of CD spectra has been recently
advanced: (a) Stephens, P. J.; McCann, D. M.; Devlin, F. J.; Cheeseman, J. R.;
Frisch, M. J. J. Am. Chem. Soc. 2004, 126, 7514–7521. (b) Furche, F.; Ahlrichs,
R.; Wachsmann, C.; Weber, E.; Sobanski, A.; Vo¨gtle, F.; Grimme, S. J. Am.
Chem. Soc. 2000, 122, 1717–1724. (c) Autschbach, J.; Ziegler, T.; Gisbergen,
S. J. A.; Baerends, E. J. J. Chem. Phys. 2002, 116, 6930–6940. (d) Diedrich, C.;
Grimme, S. J. Phys. Chem. A 2003, 107, 2524–2539. (e) Brown, A.; Kemp,
C. M.; Mason, S. F. J. Chem. Soc. 1971, 751–755.
The biphenyl orientation of M or P seems not to influence
CD patterns at over 250 nm essentially. Similarities between
the experimental and theoretical CD spectra in the 310-helices
(Figure 6) disappear in the R-helices (Figure 7). That is, the
R-helices of 1-m mostly produced not a simple split pattern but
1436 J. Org. Chem. Vol. 74, No. 4, 2009

Transfer of Chiral Information in a Helical Peptide
FIGURE 7. Electronic transition parameters and simulated CD spectra of 1-m in the R-helices (Figure S7, Supporting Information) with (A) M-
and (B) P-orientations in the two ∆ZBip side chains.
a more complex one with positive-positive-negative signals for
shorter to longer wavelengths (Figure 7). In addition, the CD
intensities of the R-helices were considerably smaller than those
of the 310-helical cases. A marked difference between the CD
intensities of the 310-helix and R-helix indicates that the
transformation from 310-helix to R-helix does not occur in the
three peptides. They all adopt a 310-helical structure in solution
as displayed in Figure 3. We have also checked the validity of
the current CD simulation.45
Similarly, CD spectra were simulated for peptides 2 and 3
that take 310-helical or R-helical conformations (Figures S14-S17,
Supporting Information). The absorption profile of 2 was shifted
to a longer wavelength by ca. 20 nm compared with that of 3,
supporting the experimental trend of the absorption spectra
(Figure S8, Supporting Information). The right-handed 310-helix
of 2 and 3 showed a split CD pattern with negative signals at
longer wavelengths. The pattern agrees with that induced by
Boc-L-proline, which thus leads to a right-handed helix of 2 as
commonly observed in 1-m, 3, and other analogous peptides.10,12
Peptide 1-6 forms a 310-helical backbone characterized by
about five turns and a total length of ca. 33 Å [Aib(1)
N-Aib(17) N]. An increase in peptide chain length often
switches a 310-helix to an R-helix.38 In fact, a 310-helix is usually
present as a short segment in proteins.23,40e Oligopeptides based
on R,R-disubstituted residues or ꢀ-substituted R,ꢀ-didehy-
droresidues also form a 310-helical structure (in some cases,
R-helix).14 Whereas a long 310-helical chain is found in
sequences of 10 residues or more,46 little is known about the
case of 16 residues or more.47 Thus we have proposed that the
17-mer sequence composed of Aib and ∆AA residues favors a
310-helix in solution.
Energies and CD Spectra for Helix Inversion. The preced-
ing CD simulation supports that chiral information received at
the N-terminus reaches the 16th position. However, achiral
residues such as Aib and ∆ZPhe can take equally both screw
senses to reverse helical sense. It has been demonstrated that
such helix inversion occurs at an achiral residue along a peptide
sequence.2a-e,13a For instance, a right-handed R-helical motif
in proteins is often accompanied with its C-terminal Gly residue
taking a left-handed conformation.2a-e,j,k Recently, we discov-
ered that chiral/achiral block sequence undergoes helix inversion
at around the boundary to generate a heterochiral helix.13a
Correspondingly, the helix sense of 1-6 might be switched
at the ∆ZBip-based region. To clarify the point, several
heterochiral helices were generated as follows. In the initial
conformers, the position for starting helix inversion (PHI) was
set to the 13th, 15th, or 16th residue from the N-terminus.
Energy minimization from each conformer yielded the corre-
sponding heterochiral structure (Figure S18, Supporting Infor-
mation). Their energies and dipole moments obtained through
single-point DFT computation are provided in Table 3. It also
contains the most stable 310-helix shown in Table 2 (the helix
inversion at the C-terminal Aib is defined as PHI ) 17th). The
heterochiral helices of PHI ) 13th, 15th, and 16th were less
(45) CD simulation at the semiempirical MO level (ZINDO/S) has been
verified through comparison with CD simulation at a higher level. Here TD DFT
was applied to three smaller fragments of 1-2: Ac-∆ZPhe-Gly-∆ZPhe-NMA, Ac-
∆ZPhe-Gly-∆ZBip-NMA, and Ac-∆ZBip-Gly-∆ZBip-NMA (Ac ) acetyl; NMA
) N-methylamide) were produced by using the atomic coordinates extracted
from the 310-helical structures of 1-2 (Figure S13, Supporting Information). CD
spectra of these short helical fragments, obtained from both the DFT and
ZINDO/S methods, yielded split-type CD patterns essentially (Figure S13,
Supporting Information). Split CD patterns were shifted to a longer wavelength
with increasing the aromatic size of ∆AA residue. At the DFT method, there is
no essential difference between the velocity- and length-based CD spectra of
which all are identified as split-type patterns. Meanwhile, at the ZINDO/S method,
the velocity-based CD spectra are regarded as split types, but such split patterns
are distorted in the length-based spectra of 1-4 and 1-2. As a result, CD profiles
at the DFT level were essentially obtained by using velocity-based parameters
at the ZINDO/S level. This supports the validity of the current CD simulation
of 1-m.
(46) For instance, see: (a) Toniolo, C.; Crisma, M.; Bonora, G. M.; Benedetti,
E.; Di Blasio, B.; Pavone, V.; Pedone, C.; Santini, A. Biopolymers 1991, 31,
129–138. (b) Gessmann, R.; Brückner, H.; Petratos, K. J. Pept. Sci. 2003, 9,
753–762. (c) Rudresh; Gupta, M.; Ramakumar, S.; Chauhan, V. S. Biopolymers
2005, 80, 617–627. (d) Ramagopal, U. A.; Ramakumar, S.; Mathur, P.; Joshi,
R.; Chauhan, V. S. Protein Eng. 2002, 15, 331–335. (e) Pavone, V.; Di Blasio,
B.; Santini, A.; Benedetti, E.; Pedone, C.; Toniolo, C.; Crisma, M. J. Mol. Biol.
1990, 214, 633–635.
(47) For instance, see: Malcolm, B. R.; Walkinshaw, M. D. Biopolymers
1986, 25, 607–625.
J. Org. Chem. Vol. 74, No. 4, 2009 1437

Ousaka and Inai
TABLE 3. Energy and Dipole Moment of 1-6 Energy-Minimized
In the 17-mer (1-6), the first to 16th residues are separated
by a distance of more than 30 Å, which is regarded as a long-
range allosteric effect in a synthetic unimolecule.19a Our model
might propose a basic element for artificial molecular motors
or machines based on helical gears.22a,48 External chiral
manipulation of the terminal gear can control the direction and
power of the internal rotation of the opposite remote gear.
The noncovalent chiral information can govern the helical
sense of the optically inactive peptides, even if they have a
considerably long chain. From the thermodynamic viewpoint,
peptide 1-6 adopts right-handed and left-handed 310-helices in
equilibrium of solution. In our previous works,10-12 CD intensity
induced in N-terminal-free peptides based on -(∆ZPhe-X)4-
depends on their N-terminal residue, chiral additive, its con-
centration,andenvironmentalfactors(solventortemperature).10-12
Our present findings confirm that change in the induced CD
intensity is responsible for a varying ratio of the two helical
contents, because each helix does not reverse up to the 16th
residue.
from Several Helix Inversionsa
energy difference
d
(kcal mol-1
)
position for
starting helix
biphenyl
per
per
dipole moment
(debye)
peptide inversion (PHI)b orientationc molecule residues
1-6
17thd
13th
15th
13th
15th
16th
16th
P
0.00
5.28
5.92
6.10
6.53
9.67
9.98
0.00
0.31
0.35
0.36
0.38
0.57
0.59
64.7
63.6
63.0
63.3
63.0
62.6
62.7
M
M
P
P
M
P
a Energy minimization from 310-helices containing a helical reversal
point was carried out by the AM1 method.33,34 Each structure obtained
was subjected to single-point DFT computation to yield its energy and
dipole moment.33,35 b PHI at initial conformations. c Phenyl-phenylene
twist in the two biphenyl groups. d From the lowest energy in Table 2.
stable than the 310-helix of PHI ) 17th but energetically more
favorable than the R-helices in Table 2. The preference for PHI
) 17th might be relevant to the fact that the C-terminal residue
of Aib-OMe often tends to switch the screw sense.14d,h,15b
CD spectra simulated for these heterochiral helices are
displayed in Figure S19, Supporting Information. They showed
positive or small negative signals at 310-330 nm. Positive CD
peaks of PHI ) 13th, 15th, and 16th appeared at shorter
wavelengths (260-280 nm) with smaller intensities compared
with the case of PHI ) 17th. The decrease in CD intensity will
be responsible for the opposite chiral information between the
∆ZPhe- and ∆ZBip-based sequences. CD spectra of 1-6 should
discriminate between PHI ) 13th-16th and PHI ) 17th. The
analysis of the theoretical CD spectra confirms that the same
helix sense reaches the 16th residue in 1-6.
Experimental Section
Synthesis. Synthetic route and characterization of peptides 1-m
and 2 are provided in Supporting Information.
Spectroscopy. Spectroscopic measurements were performed
basically according to refs 11d and 13a. CD data were acquired at
ambient temperature on a CD spectrometer. Data smoothing was
done for noise-reduced CD spectra. UV absorption spectra were
obtained on the CD spectrometer or a UV-vis spectrometer.
Sample solution was prepared as follows.11c Concentration of
1-m, 2, and 3 was determined by using their maximal molar
extinction coefficients (εmax) at 270-310 nm of analogous and
model compounds: εmax of 1-6 ) 15.8 × 104 (from Boc-protected
1-6 at 292 nm); εmax of 1-4 ) 11.4 × 104 (from Boc-protected 1-4
at 293.5 nm); εmax of 1-2 ) 7.9 × 104 (from Boc-protected 1-2 at
297 nm); εmax of 2 ) 2.6 × 104 per ∆ZBip (from Boc-Aib-∆ZBip-
Aib-OMe at 296 nm); εmax of 3 ) 1.8 × 104 per ∆ZPhe (from
analogous peptides10b,17d).
Conclusions
The end-to-end transfer of chiral information along a peptide
chain not only gains deep insight into propagation of a one-
handed helicity in proteins but also provides a primitive model
for allosteric process in biological structure and function.21
Concerning the latter issue, a remote control along a single chain
has been elegantly studied, but a long-range allosteric effect is
rarely found in a synthetic unimolecule.19 For a unique example,
the asymmetric reaction for one terminal moiety of an achiral
segment is controlled by chiral moiety covalently bound to the
opposite terminus.19a Here, both the termini are separated by a
distance of over 25 Å.19a As another example, when a chiral
residue is covalently incorporated into one terminus of an achiral
peptide, CD signals are induced in a chromophoric group at
the opposite end.8f In addition, chiral information transfers along
duplex chains of peptide nucleic acids.8a In contrast, little is
known about the precise tracing of noncoValent chiral informa-
tion in a synthetic unimolecule.19b
A chloroform solution of an N-terminal free peptide was prepared
at a given concentration. Then a prescribed volume of the solution
was added to an amount of chiral molecule to give its desirable
concentration. Here, the added volume was assumed not to change
throughout the mixing or further handling processes. Basically, the
∆ε value was calibrated with (1R)-10-camphorsulfonic acid am-
monium salt (CSAAS) purchased from Aldrich (Milwaukee, WI).
[On the other hand, the calibration factor of ∆ε was somewhat
smaller when commercially available (1S)-CSAAS (Sigma Aldrich
Japan; Tokyo, Japan) was used for CD calibration.49 If using the
latter standard, the ∆ε value should be multiplied by a factor of
ca. 0.93.] In addition, the sample solutions of 1-2, 1-4, and 1-6
were prepared with the same solvent to minimize CD spectral
changes caused from different conditions.
1
Basically according to ref 13a, H NMR spectra were obtained
on 600- or 200-MHz 1H NMR spectrometers. 2D-NOESY spectra
were acquired on the 600-MHz NMR spectrometer by using the
pulse program (“noesytp”50) usually with a mixing time of 0.4 or
Here we have synthesized ∆ZPhe-based peptides anchoring
a pair of the ∆ZBip residues to the C-terminal region. CD
spectroscopy combined with exciton chirality method18 and
theoretical simulation17d has effectively traced the route. That
is, a right-handed helix is induced in the optically inactive
peptides of 9-mer to 17-mer by a Boc-L-proline stimulus to their
N-terminal sequence. The dynamic chiral information reaches
substantially their C-terminal region [16th position in the 17-
mer (1-6)]. This provides clear evidence for the NCDE, in which
an external molecule binding to the N-terminal region can
control the chiroptical signals and helical sense of the C-terminal
region.
(48) (a) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005,
105, 1281–1376. (b) Feringa, B. L. Acc. Chem. Res. 2001, 34, 504–513. (c)
Kinbara, K.; Aida, T. Chem. ReV. 2005, 105, 1377–1400. (d) Kelly, T. R. Acc.
Chem. Res. 2001, 34, 514–522.
(49) Absolute CD values of (1S)- and (1R)-CSAAS were assumed to be the
same. For the calibration with (1S)-CSAAS, see the spectroscopic manual of
JASCO J-820. See also: Takakuwa, T.; Konno, T.; Meguro, H. Anal. Sci. 1985,
1, 215–218.
(50) Bodenhausen, G.; Kogler, H.; Ernst, R. R. J. Magn. Reson. 1984, 58,
370–388.
(51) Wu¨thrich, K. NMR of Proteins and Nucleic Acids; John Wiley and Sons:
New York, 1986.
1438 J. Org. Chem. Vol. 74, No. 4, 2009

Transfer of Chiral Information in a Helical Peptide
0.2 s. (For NMR guidance of biopolymers, see ref 51.) NMR
processing and analysis were done with the Bruker software [Bruker
BioSpin, Karlsruhe, Germany; Tsukuba, Japan] or SPINWORKS.52
FT-IR absorption and MALDI-TOF mass spectra were obtained
essentially according to ref 13a.
Conformational Energy Calculation. The helical structure of
1-m (m ) 2, 4, and 6) was predicted by energy calculation. The
initial backbone (φ, ψ, ω) of the 2nd-16th residues was set to a
right-handed 310-helix (-60°, -30°, 180°)53 or R-helix (-57°,
-47°, 180°).54 (Although the right-handed helix was commonly
chosen here, it must be energetically identical to the enantiomeric
left-handed helix due to the achiral sequence.) The C-terminal Aib-
OMe adopted the corresponding left-handed helix, because such a
helical reversal at the C-terminus is widely found in natural or
synthetic sequences.2a,b,14d,f,h,15b
used for the ZINDO/S computation. That is, the total AO number
of 1-6 (755) yielded 204 (higher occupied) × 204 (lower unoc-
cupied) for the CI number and similarly, 160 × 160 for 1-4 (591)
and 116 × 116 for 1-2 (427). In each case, the low-energy transition
states of 160 were predicted together with the respective R and f.
Gaussian 03 offers R and f values in the length-based and
velocity-based forms.33 We employed the velocity-based parameters
of R and f due to the origin-independent nature.44 In fact, the
velocity-based CD spectra of 310-helical peptides 1-m (Figure 6)
yielded split patterns that well reproduced their experimental ones,
whereas the split patterns were distorted in the length-based spectra.
Simulated CD spectra were drawn with a Gaussian-type function
based on wavelength scale by using each R value and its transition
position.17d,18b,43,44 A similar Gaussian function based on wave-
length scale was used with the velocity-based f to estimate the
corresponding absorption profile, of which the vertical scale was
expressed in arbitrary unit.18b,43d In all cases, a half Gaussian (1/
e)-bandwidth (∆/2) was assumed to be 20 nm, being larger than
14 nm in our previous work.17d The use of such a large (∆/2), as
a result, yielded absorption profiles that visually reflected experi-
mental ones.
According to the theoretical CD analysis,17d the ∆ZPhe phenyl
planes along a 310-helical backbone of (Aib-∆ZPhe)n prefer an
orientation roughly parallel to the helical axis. Thus the ꢁ2 angle
of the ∆ZPhe and ∆ZBip residues was set to -40° for a right-handed
helix.55 The two ∆ZBip residues took the same side-chain orienta-
tion of ꢁ6 ) -45° (M) or +45° (P).56 Energy minimization of these
initial conformations57 was carried out with Gaussian 03 at the AM1
method,33,34 in which all structural parameters varied. For each
optimized structure, single-point computation at the B3LYP/6-
31(d,p) level was performed to improve the energy accuracy.33,35,36
Heterochiral helical structures of 1-6 were also simulated. The
initial conformation of PHI ) ith took a right-handed 310-helix
(-60°, -30°, 180°) for the second to (i - 1)th residues, and a
left-handed 310-helix (+60°, +30°, 180°)53 for the ith to 17th
residues. In each initial conformer, the ꢁ2 of ∆ZBip residues in the
left-handed helical segment was set to 40°, while the other ∆ZPhe
and ∆ZBip residues retained the original ꢁ2 value (-40°). The ꢁ6
of the two ∆ZBip residues was also set to -45° (M) or +45° (P).56
As mentioned above, each heterochiral helix was energy-minimized
at the AM1 level to re-evaluate its energy at the single-point DFT
with Gaussian 03.33-36
Computation of Theoretical CD Spectra. CD spectra of helical
conformations were simulated basically according to ref 17d and
43d. A combination of semiempirical MO (ZINDO/S) and TD
methods, by using the keyword of “zindo” and “td” in Gaussian
03, gave electronic-transition and chiroptical parameters such as
the rotary strength (R) and f.17d,33,41,43d Under limited computational
time and memory, higher occupied and lower unoccupied MOs were
considered for the configuration interaction (CI). The CI number
was calculated from 0.27 × the total atomic orbital (AO) number
Furthermore, CD simulation at TD DFT [B3LYP/6-31(d,p)] level
was carried out for small fragments of 1-2 on Gaussian 03 as
mentioned in ref 45. Here R and f parameters of 40 low-energy
states were estimated to reproduce CD spectra in a manner similar
to the above method.
Acknowledgment. This work was partially supported by the
projects (nos. 19550163 and 16550142) of the Ministry of
Education, Culture, Sports, Science, and Technology of Japan
(MEXT).
Supporting Information Available: FT-IR absorption spec-
tra of 2 and 3; 1H NMR and 2D NOESY spectra of 1-m; solvent-
composition dependence of NH chemical shifts of 1-2; average
values of selected torsion angles and hydrogen-bonding param-
eters of 1-m in 310-helix; right-handed helical structures of 1-m
energy-minimized from an R-helix; CD spectra of 2 or 3 with
and without Boc-L-proline; induced CD spectra of 1-m with Boc-
D-proline or Boc-L-proline; CD spectra of Boc-protected 1-m
with Boc-L-proline; relationship between the CD intensity (∆ε)
of 1-m and 2 and the concentration of Boc-L-proline; spatial
arrangements of each ∆AA residue TDM (in the length form)
along the right-handed 310-helix; electronic transition state
parameters and simulated CD spectra of Ac-∆ZPhe-Gly-∆ZPhe-
NMA, Ac-∆ZPhe-Gly-∆ZBip-NMA, and Ac-∆ZBip-Gly-∆ZBip-
NMA; right-handed helical structures of 2 and 3 energy-
minimized from a 310-helix or from an R-helix; electronic
transition state parameters and simulated CD spectra of 2 and
3 in the 310-helix and R-helix; heterochiral helical structures of
1-6 energy-minimized with PHI ) 13th, 15th, and 16th;
electronic transition state parameters and simulated CD spectra
of 1-6 in the heterochiral helices; MALDI-TOF mass spectra
of 1-m and 2; synthetic route and characterization of 2; synthetic
route and characterization of 1-m; additional references and
notes. This material is available free of charge via the Internet
at http://pubs.acs.org.
(52) Marat, K. SpinWorks; University of Manitoba: Canada, 1999-2006
(http://www.umanitoba.ca/chemistry/nmr/spinworks/index.html).
(53) (a) Paterson, Y.; Rumsey, S. M.; Benedetti, E.; Ne´methy, G.; Scheraga,
H. A. J. Am. Chem. Soc. 1981, 103, 2947–2955. (b) Venkatachalam, C. M.
Biopolymers 1968, 6, 1425–1436.
(54) (a) Arnott, S.; Dover, S. D. J. Mol. Biol. 1967, 30, 209–212. (b) Arnott,
S.; Wonacott, A. J. J. Mol. Biol. 1966, 21, 371–383. (c) IUPAC-IUB Commission
on Biochemical Nomenclature. Biochemistry 1970, 9, 3471-3479. For the
definition of torsion angles for peptide conformation, see ref 54c.
(55) (a) Ajò, D.; Casarin, M.; Granozzi, G. J. Mol. Struct.
(THEOCHEM)1982, 86, 297-300. (b) For the side chain conformation (ꢁ2) of
∆AA residues, see also ref 25c.
(56) (a) Ha¨felinger, G.; Regelmann, C. J. Comput. Chem. 1987, 8, 1057–
1065. (b) Tsuzuki, S.; Tanabe, K. J. Phys. Chem. 1991, 95, 139–144.
(57) For the modeling, see ref 13a. For the replacement of ∆ZPhe-phenyl
group by the biphenyl group, see: GaussView 3.09, Dennington, R., III.; Keith,
T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. Semichem, Inc.:
Shawnee Mission, KS, 2003.
JO801686M
J. Org. Chem. Vol. 74, No. 4, 2009 1439