Chemistry Reference
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Figure 8.1 A short peptide possessing different metal binding sites a-e and selected examples
of natural (A, B) and non-natural (C) metal-binding sites.
site d). Direct backbone coordination is exemplified in A , showing binding of the terminal
carboxylate and amine as well as deprotonated amide units. Related structures are fre-
quently found with copper(II) ions as the respective metal center [8].
An ideal anchoring unit for metals can be the side chain of the amino acid building
blocks of the peptide. In this context, the natural occurring amino acid histidine B is the
raw model for the development of artificial metal coordinating amino acids as building
blocks for peptides. Modification of the imidazole nitrogen donor-binding site results in
amino acids like the 2,2 0 -bipyridine derivative C [9]. In addition to C, the corresponding
terpyridine derivatives have also been prepared [10]. Although the nitrogen donor systems
are the most widely used ones, significant work has been devoted towards the introduction
of phosphorous [11] or oxygen donors [12] as amino acid side chains.
Figure 8.2 shows selected examples of bipyridine-substituted amino acid or peptide
metal conjugates 1-3 .In 1 , a bipyridine unit is incorporated into a short peptide by the
attachment of Val-Val units to the 3- and 5-position of the ligand unit. Bipyridine is able
to bind ruthenium(II) bis bipyridine, while cobalt(III)pentaamine can be coordinated to
the carboxylate terminus. This arrangement allows investigation of electron transfer pro-
cesses between the metal centers along the short Val-Val b-sheet [13].
In compound 2 , a central trisamide-linked bis(bipyridine) coordinates the redox- and
photoactive ruthenium(II) bipyridine fragment [14]. In a similar fashion, cobalt(II) fixes a
Gly-Gly-Gly-Gly loop in 3 . It coordinates to two bipyridine units, forming a tetrahedral
complex. The amino acid termini of the organic ligand are brought together by the corre-
sponding conformational change and the through space interaction of those units by lumi-
nescence quenching has been investigated [15].
An alternative to the side chain functionalization of peptidic amino acid building
blocks is the attachment of ligand units to the N- or C-terminus of peptides. This will be
discussed in more detail below. However, Figures 8.3 and 8.4 show examples
4 ,
6 and
7
with metal complexes attached to the terminus of peptide strands.
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