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10
11
10
10
10
9
H
H
H
H
H
H
O
R
O
H
H
H
H
BocN
N
N
N
N
N
N
N
N
N
OMe
N
N
R
O
R
O
R
O
R
O
R
O
H
H
O
R
O
R
O
R
O
R
O
9
11
12
12
12
Figure 2.13 Example of a so-called hybrid helix, consisting of three individual parts, in
which two b-peptide segments surround a short a
/
b-9/ 11-helix.
mixed 11/13- or 20/22-helices and compared the 13-helix of the hybrid b/g-peptides to
the secondary structure of the native a-peptides. (Figure 2.14).
Gellman
et al.
recently demonstrated that b/g-peptides containing appropriately preor-
ganized subunits adopt the 13-helix in solution and in the solid state [54]. They combined
aconstrained(
R,R,R
)-g-residue with (
R,R
)-
trans
-2-aminocyclopentanecarboxylic acid,
hoping that they would favor the formation of a left-handed b/g-peptide 13-helix, while
the right-handed helix should be favored by residues with
S
configurations. The crystal
structure of b/g-tetrapeptide contains two molecules in the asymmetric unit. Each inde-
pendent molecule forms one 13-atom H-bonded ring, involving the NH group of the second
ACPC residue and the carbonyl of the N-terminal Boc group. Despite this deviation from
the 13-helical H-bonding pattern, the backbone torsion angles for the b-andg-residues
generally fall in ranges predicted by Hofmann
et al.
for the a/b/g-peptide 13-helix.
Finally, we would like to mention the a/d-hybrid peptides that were recently studied by
Sharma and Hoffman [55]. In a concerted theoretical and experimental study, they pre-
sented a/d-hybrid peptides as a novel foldamer class. Taking advantage of the rigidity of
the side chain in the design control, the new d-amino acids, prepared from d-glucose with
a d-xylose furanoside side chain, has been very efficiently utilized in the synthesis of
11-Helix (i i+1)
O
O
O
O
N
N
N
(a)
NH
2
H
H
H
H
O
O
O
O
13-Helix (i+3 i)
C
20
(i i+3)
C
11
(i i+1)
O
O
O
O
N
N
N
(b)
NH
2
H
H
H
H
O
O
O
O
C
22
(i+5 i)
C
13
(i+3 i)
Figure 2.14 H-bonding (a) in 11-helix and 13-helix and (b) in mixed helices: 11/ 13-helix or
20/ 22-helix.
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