1.3.2 Dinuclear Coordination Compounds from Amino Acid-bridged Dicatechol Ligands | 37
internal catecholate oxygen atoms are formed whereas in a helical arrangement two of those bonds have to be broken. Thus stabilization by intramolecular hydrogen bonding is significantly greater for non-helical arrangement of the ligands (meso relation between the complex units). The chiral helical complexes formed at the beginning should lead to high optical rotation [a]D values, whereas in the non-helical compounds the strong metal complex chromophores have a meso-type arrangement which reduces the ''chirality introduced by the metal centers'' (the expression ''meso-type'' corresponds to the configuration at the metal centers while chirality is still introduced by the spacers).
Finally, the structure of the thermodynamically favored isomer of [12(OCH3)2Ti2]2~ was deduced from conformational analysis of X-ray structural data of some of the complexes using Ramachandran's method. The dihedral angles F and C of the amino acid residues observed in the X-ray structures were determined and were correlated in a Ramachandran diagram.
Four X-ray structures were obtained for the complexes [12(OR)2Ti2]2~ (R = Me, H) with alanine, phenylalanine, leucine, or valine residues in the spacer, representing a total of three different isomers. (For valine the structure of the meso complex [(ld)(1d')(OCH3)2Ti2]2~ (ld' = R-ld) was obtained, because of epimeri-zation of the ligand during its synthesis.) The Ramachandran plot in Figure 1.3.7 shows that the derivatives with alanine and leucine as spacer adopt conformations which are typical for amino acid residues of right handed a-helical peptides. One of the valine spacers and one of the phenylalanine spacers adopt structures typical of a right handed twisted sheet.
Surprisingly, two of the data points obtained (Phe-1, R-Val) are found in the region of left-handed helical conformations. For the R-valine bridged ligand this is not a surprise. It is, however, very unusual for the S-phenylalanine derivative. As already mentioned, amino acids prefer to adopt a right-handed helical conformation. In the left-handed arrangement there is repulsive steric interaction between the substituent on the a-C and the amino acid backbone. A closer look at the X-ray structures reveals that all ligands with the favored right-handed helical twist are bound with their more rigidly connected N-terminal ligand moiety to the A-configured metal center of the meso-type complex. The N-termini of the left-handed twisted ligands, on the other hand, bind to a A-configured metal. Thus, A-configuration at the N-terminus induces the more favored right-handed helicity at the ligand whereas A-configuration at the N-terminus induces the unfavored left-handed twist.
In the thermodynamically favored species, therefore, both ligand strands must bind with their N-termini to the A-configured metal center of the meso-type dinu-clear complex to adopt the sterically more favored right-handed helical conformation. Thus, under thermodynamically controlled conditions the dominating species in solution is isomer I (Figure 1.3.8). The results and considerations discussed show that stereochemical communication between a metal center and an amino acid residue can control the microstructure at the amino acid. In the example presented this leads, in a thermodynamically controlled system, after initial formation of a complex mixture, to only one final dominating species [20, 21].
1.3.3 Peptide-bridged Dicatechol Ligands for Stabilization of Linear I 39
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