therefore be of interest to use the geometrical conformation at a metal ion to induce three dimensional structure (= conformation) in amino acids (Nature also uses metal ions in this context - Box 3).

Examples have been reported of modified peptides in which metal bonding units are attached to different positions of the peptide and induce different structures. Starting from random coil peptides, examples have been reported in which the metal can induce either a-helical- (Figures 1.3.3A or B) or j-sheet-type structures (Figure 1.3.3C).

A helical structure can be stabilized by introducing ligand units to the amino acid side-chains. On addition of appropriate metal ions the a-helix (Figure 1.3.3A) is formed by use of the metal as a ''cross-linking'' unit [5]. Attachment of metal binding sites to the end of well-chosen decapentapeptides and coordination of the ''random coil'' peptide to appropriate metal ions leads to induction of an a-helix

Fig. 1.3.2. Schematic presentation of metal complexes with bidentate ligands bonding to square-planar, tetrahedrally, or octahedrally coordinated metal centers.

7.3.7 Introduction I 33

in the peptide so that - depending on the coordination properties of the metal -tris or quadruple helix bundles result (a schematic representation of a tris-helix bundle is given in Figure 1.3.3B) [6-9]. In such circumstances favorable interaction of non-polar side-chains supports helix formation and the helical twist is induced by the chiral metal centers. Incorporation of a metal binding site in an appropriate linear peptide, and binding of a metal, can lead to a conformational fixing of the peptide which induces an antiparallel ^-sheet-type structure (Figure 1.3.3C) [10].

In structures like A-C rather large peptide domains are conformationally fixed by coordination to metals. Smaller domains can also be obtained, however (Figure 1.3.4) [11]. Extensive NMR studies of the metallomacrocycle D show that an a-helical structure is adopted by the Ac-His-Ala-Ala-Ala-His-NH2 pentapeptide on binding to the (en)Pd-fragment (en = ethylene diamine). The peptidic part of D represents one single turn of the helix [12].

A smaller turn structure is induced in an N,N0-bis(2,3-dihydroxybenzoate)-substituted lysine derivative on coordination to the Mo(VI)O2 fragment E. In E the conformation of the lysine residue is highly constrained, because of intramolecular hydrogen bonding in combination with coordination of the catechol units to the dioxomolybdenum(VI) moiety [13, 14].

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