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14-Helix (i+3 i)
O
O
N
N
H
H
(a)
O
O
9-Helix (i+2 i)
C 14 (i+3 i)
C 12 (i i+2)
O
O
O
N
N
N
(b)
H
H
H
O
O
O
C 22 (i i+3)
C 24 (i+5 i)
Figure 2.10
H-bonding in (a) 9-helix and 14-helix and (b) in mixed helices: 14/ 12-helix or
24/ 22-helix.
Compared to b-peptides, the introduction of a supplementary carbon in the backbone
can be a source of structural diversity [45]. It is likely that other new structural features or
properties will emerge by the development of original amino acid building blocks. Poten-
tially interesting results can be expected in the field of wider helices, as they were pre-
dicted by Hofmann to be very stable.
2.2.4 Hybrid Foldamers
Recently, several examples of foldamers have been reported, containing mixtures of
a- and b-, a- and g-orb- and g-amino acids in alternating order [46]. These foldamers
allow the formation of new kind of helices, besides those that may be obtained from a-, b-
and g-peptides, as shown in Figure 2.11.
Figure 2.11 Helices with 4!1 hydrogen-bond patterns in a , b , g and hybrid peptides.
Reprinted with permission from Ref. [46b]. Copyright 2011 American Chemical Society.
 
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