Helicates, Peptide-Helicates and Metal-Assisted Stabilization of Peptide Microstructures - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 281

Figure 8.9 Part of the NMR spectrum of the dinuclear titanium(IV) complex of ligand 13d
showing the signal of one of the two diastereomeric benzylic protons after 15 h and 14 days
and structure of the dinuclear dianionic complex [(13d) 2 (OMe) 2 Ti 2 ] 2 as found in the crystal.

2. The conformation at the amino acid backbone can be analyzed based on the X-ray
structural data using Ramachandran's method. Therefore the dihedral angles F and C
at the amino acid spacers were set into correlation.
The analysis (Figure 8.10) reveals that the amino acid residues prefer to adopt the
conformation of either a right-handed a-helix or a right-handed twisted b-sheet.
3. Considerations regarding the ligand backbone reveal that two different linkages are
present between the catechol units and the amino acids. Those linkages should be
responsible for stereochemical induction. On the one hand there is the benzyl amide at
the C-terminus with an sp 3 carbon atom within the six membered ring formed by
NH

O hydrogen bonding. Due to the sp 3 carbon this unit is rather flexible and is able
to level out strains, which are built up upon metal coordination. The catecholamide (N-
terminus), on the other hand, forms a planar six-membered ring by hydrogen bonding.
This unit is rigid and transfers stereochemical information from the amino acid to the
metal complex. From the experimental results it can be deduced that S-configuration at
the amino acid induces L at the N-terminal catechol complex.

All the considerations described under 1-3 yield only one possible preferred isomer of
Li 2 [( 13 ) 2 (OR) 2 Ti 2 ]. The titanium complexes have to have different configurations, one L
and one D. In order to adopt a right-handed helical twist at the amino acid, both N-termini
have to bind to the L-configured metal center.