Dinuclear Coordination Compounds from Amino Acid-bridged Dicatechol Ligands: Induction of a Right- or a Left-handed Conformation at a Single Amino Acid Residue

The amide bond is the dominating structural feature in peptide chemistry. Because of resonance between the CO re-system and the lone pair in the re-orbital at the nitrogen the amide moieties in peptides are planar (or close to planar) with s-trans orientation of the NH and the CO units. Thus, the conformation of the amino acid residue of a protein mainly is defined by the dihedral angles F (C(O)-N-Ca-C(O)) and C (N-Ca-C(O)-N) which were introduced by Ramachandran (Figure 1.3.5) [15]. Typical dihedral angles for an amino acid located in the a-helical part of a protein are in the region C = —47° and F = —57°, angles for an antiparallel ¿-sheet are close to C = 135° and F = —139°. In general, amino acids prefer a dihedral angle, F, with a negative sign, because here a right-handed helical conformation is adopted by the amino acid residue. Such a right-handed conformation at L-amino

1.3.2 Dinuclear Coordination Compounds from Amino Acid-bridged Dicatechol Ligands I 35

acids (with the exception of glycine) is favored over the left-handed, because of minimization of the steric interaction between the substituent at the a-carbon atom and the amino acid (peptide) backbone [1].

A starting point for investigation of the effect of metal coordination on the conformation of a single amino acid is the synthesis of amino acid-bridged ligands with two metal binding sites and study of their coordination chemistry [13, 14]. The synthetic sequence used to obtain amino acid-bridged dicatechols 1a-1e-H4 is shown in Scheme 1.3.1 [16].

Scheme 1.3.1. Preparation of the amino acid-bridged dicatechol ligands 1a-1e-H4.

Standard peptide coupling chemistry is performed (Box 25). The acid component is transformed to an activated ester derivative and then treated with the amine [1719]. This procedure is repeated twice. In the final step the ligands 1a-1e-H4 are deprotected by aryl ether cleavage with BBr3. By use of this simple reaction sequence derivatives with glycine, alanine, phenylalanine, valine, leucine, and other amino acids as spacers were obtained [16].

The thus prepared ligands 1a-1e-H4 are introduced into coordination studies with titanium(IV) ions in the presence of base (M2CO3: M = Li, Na, K) with methanol as solvent. Negative ESI-MS (electron spray ionization mass spectrome-try) of the obtained orange-red solids in methanol reveals that coordination compounds of general formula M2[12(OCH3)2Ti2] are formed (Scheme 1.3.2) [20, 21].



Scheme 1.3.1. Preparation of the amino acid-bridged dicatechol ligands 1a-1e-H4.

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