ChemComm
Communication
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at the interface with bulk water, interacting with the anionic
carboxylate of the proline headgroup of surfactant 2.24 This
electrostatic interaction would be negligible in water because of
the solvation of the ions in this medium but is rather relevant in
the confined volume constituted by the interfacial region of the
micellar aggregate for two reasons: first the polarity of the
environment is lower than that of bulk water, second the local
concentration of the ions is much higher.24 The interaction
between the chiral headgroup of the proline-based surfactant
would induce the preferred helical sense of the peptide in a
similar fashion to what happens when N-t-butoxycarbamoyl
amino acids (Boc-AA-OH) are added to helical Aib/dehydroamino
acid-based sequences reported by Inai,10,12 although in this case
chloroform, a much less competitive solvent, is used. The key role
played by the free amino group is supported by the failure to
observe any deracemization when we use Z-protected 1, although
it must be pointed out that the protected peptide is much less
soluble under the conditions used. The selection of a right-
handed helix with (L)-2 (and, conversely, a left-handed helix with
(D)-2) is in full accord with that reported for chloroform by adding
Boc-AA-OH to an achiral helical sequence.10,12,25
In conclusion, by using a chiral micellar aggregate we were
able to deracemize a 310 helical octapeptide made exclusively of
the achiral amino acid Aib and to record for the first time its CD
spectrum. This constitutes the CD signature of a 310 helix
devoid of the contribution of any chiral amino acid. Micellar
2 solubilizes little polar 1 in a local environment of lower
polarity than water and contributes the chiral milieu for the
deracemization of the two enantiomeric helices, likely through
an interaction of the surfactant carboxylate and peptide ammo-
nium groups. In view of the claimed fundamental role of
amphiphilic self-assemblies in the origin of life,26 including
the development of homochiral life, we believe that our obser-
vation could lend further support to such a hypothesis.
`
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11 J. Sola, M. Helliwell and J. Clayden, J. Am. Chem. Soc., 2010, 132, 4548.
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18 For a recent reference see: T. D. Clark, L. Bartolotti and R. P. Hicks,
Biopolymers, 2013, 99, 548.
PS is indebted to Prof. C. Toniolo and F. Formaggio for
disclosing unpublished material from their own laboratory and
for fruitful discussion. Financial support from MIUR (Rome)
contract 2010JMAZML is gratefully acknowledged.
´
19 M. Hebrant, P. Burgoss, X. Assfeld and J.-P. Joly, J. Chem. Soc., Perkin
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micellar environment hampered our attempts to investigate the
peptide helix formation using NMR. Other microscopic techniques
such as TEM or AFM proved also unsuitable on this regard.
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 10133--10135 10135