Chemistry Reference
In-Depth Information
Figure 2.20 (a) Front view and (b) side view of the calculated Boc-(L-Oxd)
5
-OBnhelix. The
terminally protecting groups have been removed.
Reprinted with permission from Ref. [68c].
Copyright 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
cis
-
trans
isomerism about the tertiary amide bonds. As a consequence of this remarkable
property causing a local constraint, these imido-type oligomers are forced to fold in
ordered conformations, that is, in combination with other kinds of interaction (H bond,
apolar interactions, etc.), lead to the formation of foldamers (Figure 2.20) [68].
Hybrid foldamers, where the Oxd moiety alternates with an a-amino acid, lead to the
formation of different secondary structures. The simplest oligomers of this family that can
be prepared have a general formula Boc-(Gly-
L
-Oxd)
n
-OBn (Gly
¼
glycine; Figure 2.21)
[69].
In order to establish whether these molecules are foldamers, their behavior was ana-
lyzed by IR, CD and
1
H NMR. All the techniques agree in the detection of the formation
of weak hydrogen bonds and, as a result, a secondary structure. To further investigate the
preferred conformations of oligomers of the Boc-(AA-Oxd)
n
-OBn series, Gly was
replaced with
L
-Ala, thus introducing a methyl as a side chain [70].
Again, information on the preferred conformation of the Boc-(
L
-Ala-D-Oxd)
n
-OR
(with
n
Bn, H) oligomers in solution was obtained in structure-supporting
solvents (methylene chloride, deuterochloroform, methanol), by FT-IR absorption,
1
H
NMR and CD techniques. The results obtained from all the techniques demonstrate that,
from the tetramer level, the oligomers assume a folded structure. The most interesting
evidence has been obtained from the
per
-residue CD spectra (Figure 2.22). An ellipticity
increase, associated with a reversal of the Cotton effect, was observed for Boc-(
L
-Ala-D-
Oxd)
5
-OH and more dramatically for Boc-(
L
-Ala-D-Oxd)
6
-OH, thus suggesting the for-
mation of an ordered secondary structure. The CD spectrum of Boc-(
L
-Ala-D-Oxd)
6
-OH,
recorded at different concentrations (not shown), does not show any change as the
¼
1-6 and R
¼
O
O
O
O
N
O
N
OBn
N
H
O
O
O
O
O
2
Figure 2.21 Chemical structure of the longest investigated Boc-(Gly-L-Oxd)
n
-OBn oligomer.
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