C O M M U N I C A T I O N S
Collectively, our X-ray diffraction and NMR data show that the
φ, ψ angles of the (2R,3R)c3diPhe homooligomers are confined to
negative values (right-handedness). Interestingly, in previous
computational analyses of simple (2S,3S)c3diPhe diamides,3a,b
similar results, namely energy minima corresponding to the left-
handedness, were found.
In summary, we have reported unambiguous proofs that the screw
sense of peptide turns and helices may be dictated not only by the
amino acid asymmetric R-carbons but by the topological (chirality)
properties14 of their side-chain â-carbons as well. The peptides based
on c3diPhe studied in this work are related to those formed by the
binaphthyl R-amino acid (Bin) previously described by some of
us,15 in the sense that both residues lack an asymmetric center in
the main chain. However, c3diPhe bears chiral carbons in its side
chains, while Bin, devoid of any side-chain chiral carbon, is overall
dissymmetric (axially chiral). Our next step in this research on the
role of side-chain chiral centers in 3D structure will be a comparison
between peptides based on diastereomeric L-Ile versus L-alloIle
residues.
Figure 1. X-ray diffraction structure of molecule A in the asymmetric unit
of Boc-[(2R,3R)c3diPhe]2-NHiPr with heteroatoms colored and numbered.
The intramolecular CdO‚‚‚H-N hydrogen bond is represented by a dashed
line. The structures of the two other independent molecules B and C
(deposited) are not shown as they are very close to that of molecule A.
Side-chain chiral carbons are starred.
Acknowledgment. W.D.B. thanks the Research Foundation-
Flanders (FWO-Vlaanderen) for the postdoctoral fellowship. Fi-
nancial support from the Ministerio de Educacio´n y Ciencia
(CTQ2004-5358), DGA (predoctoral fellowship for S.R.), and CAI
(travel grant for S.R.) is acknowledged.
Supporting Information Available: Experimental details on the
synthesis, characterization, and NMR data of the three homooligomers,
and X-ray diffraction data of Boc-[(2R,3R)c3diPhe]2-NHiPr (CCDC
254024) (PDF, CIF). This material is available free of charge via the
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Figure 2. (A) Stereomodel of the right-handed 310-helix (ref 5) of Boc-
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solvent exposed), as apparent from a temperature study, indicates
the most populated helix is the 310-type. The right-handedness of
the helix was deduced from the observation of an NOE cross-peak
between the same CâH proton of residue 1 and a CâH proton of
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