Introduction to Unnatural Foldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 58

trans (anti) gauche gauche

HN

O

HN NH

ψ

φ

ω θ

N N

2 Re H

H 2 Si

H 2 Re

2 Si H

C 3

O

C 3 C 3

3 Re H

H 3 Si

3 Re H

H 3 Si

3 Re H

H 3 Si

O O

H 2 Si

H 2 Re

O

(a) (b)

Figure 2.7 (a) Definition for the torsional angles in b-peptides. (b) Rotamers for b-alanine
regarding the u dihedral.

C2,C3-Disubstituted amino acids are even more conformationally constrained and favor
gauche conformers when the substituents are anti . Gauche-type torsion angles are even
more strongly promoted when these atoms are included in a cyclohexane or cyclopentane
ring [37]. The ring size determines the precise C2-C3 torsional preference, which in turn
influences the b-peptide helix type [38].
Five distinct helices have so far been identified in the field of b-peptides: the 14-, a
12-, a 10-, the 12/10- and an 8-helix, which are defined by the sizes of 8-, 10-, 12-, and
14-membered hydrogen-bonded rings (Figure 2.9).
Besides helical arrangements, b-peptides, like the a-peptidic prototypes, can attain
nonaggregating extended chain structures, assemble to pleated sheets, fold to hairpin
turns and stack in crystals. Extended arrangements have also been identified, namely par-
allel and antiparallel pleated sheets, a nonaggregating linear structure and stacks. NMR
spectroscopy has emerged as the most useful tool in determining the secondary structures
of b-peptides, although some samples suitable for single-crystal or powder X-ray diffrac-
tion structure determination have been obtained. For an exhaustive review on helices and
other secondary structures of b-peptides see Seebach et al. [39].

O O

θ

Unsubstituted residue:
conformationally flexible

NHR I

R H

R 3

O O

3 -Residue:
favors gauche conformation

β

NHR I R H

O O

2 -Residue:
favors gauche conformation

β

NHR I R H

R 2
R 3 O O 2,3 -Residue anti configuration:

strongly favors gauche conformation

β

NHR I R H

R 2

R 3 O O

2,3 -Residue syn configuration:
strongly favors gauche conformation

β

NHR I

R H

R 2

2,3 -Residue cyclic, trans configuration:
strongly favors gauche conformation

β

CONHR I

RCONH

Figure 2.8 Effect of substituents on the torsional angle u.