Introduction to Unnatural Foldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 65

H

N O
O H N

O H

N O

O H

N O

O

O

O

N N

O O H O O H O

Figure 2.18 An oligomer of a-aminoxy acids. The a N - O turns, leading to an eight-
membered-ring hydrogen-bond, are reported.

depicted in Figure 2.18) [64]. Both FT-IR and 1 H NMR spectroscopies showed that intra-
molecular hydrogen bonds are formed between the amide NH groups at the i 2 position

and the carbonyl oxygen atoms at the i position. Furthermore, the CD curves of the oligo-
mers in 2,2,2-trifluoroethanol were almost superimposable, indicating that their secondary
structures are very similar. The same results were observed in the crystal structures.
The exploration of the conformations of hybrid peptides containing a-amino acids
and a-aminoxy acids showed that, in peptides of alternating D -a-amino acids and
L -a-aminoxy acids, the seven-membered-ring intramolecular hydrogen bond (i.e., g-turn)
is initiated by a succeeding a N - O turn [65]. It signifies a new strategy to induce a g-turn at
specific sites of short peptides by incorporating an a-aminoxy acid immediately after the
particular a-amino acid of interest.

þ

2.2.7 Foldamers Containing Amido Groups

Interesting new structures may be prepared, replacing the proline moieties with pseudopro-
lines (cPro) [6]. This term was introduced recently to indicate synthetic proline analogues
which are usually obtained by cyclocondensation of the amino acids cysteine, threonine or
serine with aldehydes or ketones. Five-membered cycles that contain in the ring a nitrogen
and a carboxy unit close one to the other may be ascribed to the family of pseudoprolines: all
these compounds share all the same properties because the nitrogen near to a carbonyl group
behaves as a rigid spacer, owing to the presence of the endocyclic carbonyl, which strictly
imparts a trans conformation to the adjacent peptide bond. This effect is due to the tendency
of the two carbonyls to lie apart one from the other (Figure 2.19) [66].
Our group has extensively studied the conformational behavior of oxazolidin-2-one
homo-oligomers [67]. These compounds lead to a new type of helical structure, similar to
that adopted by poly-( L -Pro) n with trans tertiary peptide bonds (type II). Both repeating
systems generate left-handed, ternary (3 1 symmetry) helices, but the oxazolidin-2-one
system is rigid, whereas the ( L -Pro) n system is remarkably more flexible, due to the

O

R

n

R = H, Me
X = CH 2, O
n = 0, 1

X N O

O

Figure 2.19 Preferential conformation of the imidic bond.