Control of Chiral Assemblies Through Hydrogen Bondings for Helical Architectures - Molecular Self-Assembly: Advances and Applications - page 82

3.3.2 Chiral Assemblies of Dipeptidyl Urea
Urea functionality has been utilized to create highly organized
hydrogen-bonded molecular assemblies [28]. Combination of a urea
and peptide unit is envisioned to provide stable hydrogen-bonded
molecular assemblies [29]. Among the numerous artificial self-
assembly systems through hydrogen bonding, construction of stable
hydrogen-bonded molecular duplexes is one of the important targets
of current research [30]. The crystal structure of the dipeptidyl
urea

composed of two dipeptide chains bearing (-L-Ala-L-Pro-
NH-2-Py) reveals that two molecules of

13

are held together by six
intermolecular hydrogen bonds to form a hydrogen-bonded duplex
(Figs. 3.16 and 3.17a,b) [31]. It should be noted that this hydrogen-
bonded duplex adopts a right-handed helical conformation. The
propensity to form the chiral helicity is likely to be induced by the
chirality of the peptide chains. Furthermore, each hydrogen-bonded
duplex is connected by continuous intermolecular hydrogen bonds
between urea CO and C-terminal amide NH to form a double helix-
like arrangement as shown in Fig. 3.17c. A shuttle-like dynamic
process based on the recombination of hydrogen bonds is observed
in a solution state (Fig. 3.18) [31]. The activation energy of this
process is calculated as 9.4 kcal/mol from the Arrhenius equation.

13

O

N

N

N N N N

H H

O O

O O

CO(- L -Ala- L -Pro-NH-2-Py) 2 13

Figure 3.16 The dipeptidyl urea 13 composed of two dipeptide chains

(-L-Ala-L-Pro-NH-2-Py).

3.4 Conclusion

A variety of molecular scaffolds are designed and incorporated
into peptides to induce highly ordered structures of peptides for
helical architectures. With molecular scaffolds, the attached peptide
strands are regulated in the appropriate dimensions. The utilization
of molecular scaffolds is a potential strategy for the organization
of peptide structures, which allows the control of intramolecular
interaction of peptides or peptidomimetic strands. A further
noteworthy feature of the peptide bioconjugates is their strong