Methyl-blocked dimeric α,γ-peptide nanotube segments: Formation of a peptide heterodimer through backbone-backbone interactions

Article abstract of DOI:10.1002/anie.200501555

Cyclic peptides can dimerize through β-sheet-like hydrogen bonding. Heterodimerization is favored over homodimerization, which creates interesting combinatorial possibilities without detriment to the functionalization of amino acid side chains. These dime

Full text of DOI:10.1002/anie.200501555

Communications
Peptide Nanostructures
DOI: 10.1002/anie.200501555
Methyl-Blocked Dimeric a,g-Peptide Nanotube
Segments: Formation of a Peptide Heterodimer
through Backbone–Backbone Interactions**
Roberto J. Brea, Manuel Amorín, Luis Castedo, and
Juan R. Granja*
In recent years, considerable effort has been devoted to the
synthesis of organic and inorganic nanotubes.[1] Among a
number of other nanotube design concepts, self-assembling
peptide nanotubes (SPNs)[1–6] have attracted attention for
some time because of the probable ease with which they may
be endowed with structural and functional properties of
interest for applications in biology and materials science.[7]
SPNs are formed by stacking cyclic peptides whose amino
acid residues have configurations that impart the peptide ring
=
backbone with an essentially flat shape and that orient the C
Figure 1. Design for self-assembling peptide nanotubes composed of
cyclo[(d-Aa-(1R,3S)-g-Aa)3-] or cyclo[(d-Aa-(1R,3S)-g-Aa)4-] units,involv-
ing two different (a–a and g–g) Hydrogen-bonding patterns (Aa=
amino acid). Dimer models of the two hydrogen-bonding patterns are
shown at the bottom of the figure. Amino acid side chains have been
omitted in the nanotube and dimer structures for clarity.
O and NH groups roughly perpendicular to the ring plane.
This allows b-sheet-like hydrogen bonding between antipar-
allel rings (Figure 1). In particular, SPNs based on cyclic
a,g peptides (a,g CPs) such as 1a and 2a, in which the
requisite all-trans conformation is achieved by the alternation
of (1R,3S)-3-aminocyclohexanecarboxylic acids (g-Ach) with
d-a-amino acids,[8] hold considerable promise for the design
of nanotubes with novel structural and internal cavity proper-
ties.
The results reported herein support the possibility of
constructing SPNs by using a,g CPs of type 3, in which the g-
amino acid is not g-Ach but (1R,3S)-3-aminocyclopentane-
carboxylic acid (g-Acp).[9] We show that heterodimerization
between g-Ach-based and g-Acp-based a,g CPs is not only
possible, but is favored over homodimerization for the
compounds used herein. In nature, heterodimerization of
peptides is common[10–12] and appears to constitute an
economical means toward structural diversity and functional
versatility; the same benefits may be expected to accrue from
the development of a battery of a,g CPs that are capable of
forming hetero-oligomers with each other. However, whereas
in all known natural cases the selectivity for hetero- rather
than homodimerization is mainly due to interactions between
the side chains of the component monomers, interactions of
this kind are not necessary for preferential heterodimeriza-
tion of the a,g CPs studied herein. This is fortunate, because it
means that the combinatorial advantages of heterodimeriza-
tion are available without detriment to the possibility of
exploiting side chains for other purposes.
The construction of an SPNfrom a,g CPs involves two
different sets of b-sheet-like hydrogen bonds: one exclusively
=
involves the NH and C O groups of the g-amino acid (g–g
bonding, Figure 1) and the other, those of the a-amino acid
(a–a bonding). In previous work with hexameric and
octameric g-Ach-based a,g CPs, we found that a–a bonding
was stronger than g–g bonding.[8] Herein, we accordingly
focused on a–a bonding with the preparation of a,g dimers in
which g–g bonding was prevented by N-methylation of all the
g-amino acids as in 1c–3c to avoid complications that would
be irrelevant to this preliminary work.[13]
(1R,3S)-N-Boc-g-Acp (5) was prepared from Vinceꢀs
lactam (4)[14] by hydrogenation, acidic hydrolysis, and N-
protection (Scheme 1) followed by successive recrystalliza-
tions from chloroform containing 0.7 equivalents of (+)-a-
phenylethylamine, and was obtained with an enantiomeric
purity greater than 97%[15]. The N-methylated amino acid 6a
[*] R. J. Brea,M. Amorín,Prof. Dr. L. Castedo,Dr. J. R. Granja
Departamento de Química Orgµnica e Unidade Asociada ó C.S.I.C.
Facultade de Química,Universidade de Santiago
15782 Santiago de Compostela (Spain)
(
MeN-g-Acp) was then obtained by treatment of 5 with sodium
hydride and iodomethane in THF, and its fluorenyl-protected
ester 6b by reaction of 6a with fluorenylmethanol.
Fax: (+34)981-595012
E-mail: qojuangg@usc.es
The a,g CP cyclo[(d-Leu-(1R,3S)-MeN-g-Acp)3-] (7a,
Figure 2) was synthesized by standard procedures and char-
acterized by NMR spectroscopy and mass spectrometry (see
[**] This work was supported by the Ministerio de Ciencia y Tecnología
(SAF2001-3120,SAF2004-01044) and the Xunta de Galicia
(PGIDT02BTF20901PR,PGIDT04BTF209006PR). M.A. and R.J.B.
thank the Ministerio de Ciencia y Tecnología for their FPU grants.
1
Supporting Information). Its H NMR spectra in polar and
nonpolar solvents (CCl4, CDCl3, MeOH, DMSO) are well-
defined, reflect a high degree of symmetry, and show a JNH,Ha
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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a b-sheet-like array of six hydrogen bonds with interatomic
N···O distances of 2.88–2.98 Š (Figure 3a,b). The drum-
shaped dimer has an approximate van der Waals internal
diameter of 5.4 Š, and its cavity is occupied by a partially
disordered chloroform molecule. The body-centered struc-
Scheme 1. Synthesis of (1R,3S)-Boc-MeN-g-Acp-OH (6a) and (1R,3S)-
Boc-MeN-g-Acp-OFm (6b) from Vince’s lactam (4); a) H2,Pd/C,
MeOH,99%; b) HCl (10%),90%; c) (Boc) 2O, N,N-diisopropylethyl-
amine,H 2O/dioxane,89%; d) resolution: ( +)-a-phenylethylamine,
CHCl3/hexanes: [a]D =À16.8 (c=1.0 in MeOH), >97% ee; e) NaH,
MeI,THF,98%; f) FmOH,1-(3-dimethylaminopropyl)-3-ethylcarbodi-
imide,1-hydroxybenzotriazole,4-(dimethylamino)pyridine,CH 2Cl2,
92%. Boc=tert-butoxycarbonyl,Fm =fluorenylmethyl.
coupling constant of 9.4 Hz, which is typical of the all-trans
backbone conformation required for the flatness of the
peptide ring (which, in turn, favors the formation of hydro-
gen-bonded assemblies such as the SPNshown in Figure 1). In
nonpolar solvents, the formation of a dimer (87a) is reflected
by the downfield shift of the NH signal of Leu. The fact that
the location of this signal (d = 8.23 ppm) remains constant at
concentrations as low as 1 ” 10À4 m, regardless of methanol
composing up to 30% of the solvent or heating at 323 K,
shows that the association constant for dimer formation must
be at least 105 mÀ1. FTIR spectra recorded in chloroform
display bands at 1626, 1534, and 3304 cmÀ1 which, like
analogous bands observed for other all-trans cyclic peptides
linked by hydrogen bonds in dimers or SPNs,[2,8,13] resemble
the amide I, amide IIII, and amide A bands typical of protein
b sheets.
X-ray crystallography of a colorless prismatic crystal
obtained from a solution of peptide 7a in chloroform by
vapor-phase equilibration with hexane confirmed the a–a
dimerization of essentially flat, antiparallel rings by means of
Figure 3. a) Views from the sides (a,c,e) and down the axes (b,d,f)
of a,b) homodimers 87a and e,f) 10[8a],and of cd, ) heterodimer 117a–9 in
the solid state; chloroform molecules in the central cavities have been
removed for clarity.
Figure 2. Homo- and heterodimerization of cyclo[(d-Leu-(1R,3S)-MeN-g-Acp)3-] (7a), cyclo[(d-Phe-(1R,3S)-MeN-g-Acp)3-] (7b) and cyclo[(d-Phe-
(1R,3S)-MeN-g-Ach)3-] (9).
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Communications
ture of the crystal forms channels by stacking the dimers along
between CPs in the crystals (see Supporting Information) and
À
the crystallographic a axis, each dimer making van der Waals
contacts with the next through its g face carbonyl and N-
methyl groups.
by those calculated from the N H stretch frequencies (n˜ =
3304, 3309, and 3300 cmÀ1 in 87a, 10 and 117a–9, respectively;
see Supporting Information). However, further studies might
clarify this issue.
The g-Ach-based 7a analogue cyclo[(d-Phe-(1R,3S)-MeN-
g-Ach)3-] (9) also forms stable homodimers 10 in nonpolar
solvents, with an association constant estimated to be at least
105 mÀ1, as in the case of 7a (Figure 2).[8] The possibility of
heterodimer formation was confirmed upon the addition of
0.8 equivalents of 7a to a solution of 9 (2.1 mm) in chloroform,
which resulted in the appearance of a new species with a
1H NMR spectrum that did not correspond to either of the
possible homodimers. The dimeric nature of the new species,
117a-9, was shown by NOE cross-peaks between the signals of
HgAcp (at d = 4.92 ppm) and HaAch (at d = 2.62 ppm). Defin-
itive evidence of heterodimerization in the solid state was
obtained by X-ray crystallography of the colorless prismatic
single crystals recovered from a 1:1 solution of 7a and 9 in
chloroform. As expected, the monomeric cyclic peptides form
heterodimers in which the two essentially flat rings are
antiparallel, with their a faces linked in b-sheet fashion
through six hydrogen bonds with N···O interatomic distances
of 2.82–2.90 Š (Figure 3). The dimers stack closely along the
a axis, each making van der Waals contacts with its neighbor
with g-face carbonyl and N-methyl groups. The resulting
channel contains two chloroform molecules per unit, one in
the cavity of the heterodimer and a second at variable sites
between successive dimers.
In conclusion, NMR and FTIR spectroscopy and X-ray
diffraction data show conclusively that cyclic a,g hexapep-
tides containing three g-Acp units can form stable cylindrical
dimers in which antiparallel peptide rings are linked by a b-
sheet-like set of six hydrogen bonds. Even greater stability is
shown by the heterodimers of these g-Acp a,g CPs with g-Ach
a,g CPs. Potential applications for nanotubes developed from
appropriately functionalized a,g CPs include their use as logic
gates, biosensors, catalysts, molecular receptors, and molecule
containers.
Received: May 6, 2005
Revised: June 7, 2005
Published online: August 4, 2005
Keywords: amino acids · dimerization · nanotubes · peptides ·
.
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