Interaction of Biomimetic Oligomers with Metal Ions - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 393

Figure 11.18 Crystal structures showing (a) the P D M 10 and (b) M L M 10. (c) Top and side
views of the overlay of fragments of the above complexes showing two D Cu þ1 complexes
(in gray) and the first two quinoline residues of an M helix (in red) and of a P helix (in blue), as
reported by the groups of Nitschke and Huc. Side chains, BF 4 ions, and included solvent
molecules are omitted for clarity. Reprinted with permission from Ref. [40] (Nitschke's and
Huc's groups). Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
bearing only two helix-iminopyridine ligands and two hydroxide counter ions bound
directly to the metal center. In this case, the relative orientation of the two helices
imparted by the octahedral geometry of Fe 2þ is almost parallel. The two hydroxide lig-
ands that connected to Fe 2þ appear to play a role in this orientation as they seem to pre-
vent the helices from folding back on the iminopyridine moieties. The structure of the
complex formed upon binding to an Fe 2þ ion further validates metal-directed dynamic
assembly as an efficient approach to generate helically folded, aromatic-amide oligomers
and to precisely set their relative orientation.
11.4.2 Metal Coordination in Peptidomimetic Foldamers
Peptidomimetics are synthetic oligomers akin to natural peptides and polypeptides, which
were designed to adopt three-dimensional structures by a controlled disposition of func-
tional groups along their spine. Examples of metallo-peptidomimetics involve two types
of oligomers: b-peptides and “peptoids” - a class of a-peptides. The secondary structure
of both types has been well characterized [41], therefore metal coordination is anticipated
to induce further stabilization and/or create higher-ordered structures. One advantage of
peptidomimetics is their facile preparation achieved by efficient solid-phase synthesis
[42]. This method can be automated, enabling the generation of several compounds in
parallel. Moreover, it allows ambient temperatures, (typically 25 C), faster synthesis and
fewer purification steps.
b-Peptides are well characterized non-natural oligomers, which were developed within
the last decade [41a]. Systematic studies on b-peptides were conducted in solution and in
the solid state, revealing their potential to adopt well defined ordered conformations
[41b,43]. Recently, Seebach [44] investigated the ability of Zn 2þ complexation to fortify