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Approaches Used to Stabilize Bioactive Conformations at Peptides by Metal Coordination

Protein-protein interactions usually occur on the surface of proteins where mainly loops or turns are present [24]. It is, therefore, of interest to stabilize specific peptide turns and test them for their biological activity [25-28]. As discussed, metal coordination seems to be a powerful tool for obtaining macrocycles with a "bent" peptidic domain simply by mixing metal ions and peptide-bridged ligands.

Examples of naturally occurring bioactive cyclopeptides are Segetalins A and B (Figure 1.3.11), which were isolated from Vaccaria segetalis (Caryophyllacea) and were shown to have estrogen-like activity. The seeds of Vaccaria segetalis are used

1.3.4 Approaches Used to Stabilize Bioactive Conformations at Peptides by Metal Coordination I 41

Scheme 1.3.3. Stabilization of a loop-type structure at a tripeptide by coordination of ligand 3 to the MoO22+ fragment.

Segetalin A

Segetalin B

Fig. 1.3.11. Segetalin A and Segetalin B isolated from Vaccaria segetalis (Caryophyllacea).

Segetalin A

Segetalin B

Fig. 1.3.11. Segetalin A and Segetalin B isolated from Vaccaria segetalis (Caryophyllacea).

in Chinese folk medicine to activate blood flow, to promote milk secretion, and to treat amenorrhea and breast infections [29-31].

That Segetalin A and Segetalin B have the Try-Ala-Gly-Val (WAGV) sequence in common indicates this is the biologically active part of the molecules [32]. NMR studies indicate, however, that the orientation of the Val residue is different in the molecules. The effect of this amino acid on the activity of the compounds might not be very important [29-31].

The WAG sequence can easily be introduced as a spacer between two catechol moieties by following the FMOC strategy for the preparation of peptides. An N-terminal FMOC-protected amino acid is attached to an amine using HBTU as coupling reagent. The protecting group is removed and another FMOC-protected amino acid can be attached, to elongate the peptide chain. By use of this procedure, then cleavage of the methyl ethers by BBr3, the ligand 4-H4 is prepared in eight steps (Scheme 1.3.4) [33].

Reaction of the ligand 4-H4 with (acac)2MoO2 should lead to the macrocyclic complex in which the biologically active WAG-sequence of the cyclopeptidic Sege-talins A/B is stabilized in a loop-type fashion.

Clipping of the linear derivative 4-H4 to fix the WAG-sequence in a bent conformation proceeds smoothly by addition of O2Mo(acac)2 and K2CO3 in methanol. The cis-dioxomolybdenum(VI) complex K2[4MoO2] is characterized by spectroscopic methods like NMR or ESI-MS. The results obtained indicate that only one of the two possible stereoisomers is formed. This shows that the amino acids of the WAG sequence are able to induce chirality perfectly at the metal complex unit. The good resolution and signal dispersion in the 'H NMR spectrum indicate the peptide turn adopts a well defined conformation. From the present results, however, it cannot be deduced if this conformation is the biologically active one [34].

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