Diarylethene derivatives and their applications. Salen derivatives in molecular recognition

摘要

A small series of aromatic water-soluble salophen derivates containing Fe(III), Ni(II) and Ti(IV) were synthesized and tested for their ability to recognize amino acids. udIn the case of the Fe(III)-salophen derivative no binding could be observed for any of the tested amino acids, only for pyrophosphate (PPi) and adenosine-triphosphate (ATP). Interaction of this complex with ATP results in a 1:1 stoichiometry and a relatively high binding constant (lg K = 7.7). ITC measurements for this system could not be performed, due to the poor solubility of the complex in HEPES buffer.udFor Ni(II)-salophen derivative 2, interactions with histidine, histidine methylester and aspartic acid could be detected in the well-plate screening. We concluded from the screening results, that for sufficient binding the donor atoms of the amino acid should have a distance of five atoms and the amino acid should act as a bidentate ligand. The C-terminal part of the amino acid is not essential for the interaction. However, fluorescence measurements showed that the binding constant and the stoichiometry seems to be concentration dependant (different binding behaviour upon changing of added equivalents), whereas the ITC measurements do not confirm these observations. Aggregate formation by pi-pi-interaction was excluded as the origin of the concentration dependency, because the UV spectra remain unchanged at different concentrations (Figure 46). udThe salophen-derivative with Ti(IV) as the metal ion showed no affinity to any of the added analytes of this study. udChiral salen derivatives with an aliphatic bridge between the aromatic moieties show much lower affinities to the same analytes than the non-chiral conjugated salophen-derivatives. However, the Fe(III)-salen complex binds catechols in the µM-range, which allows indicator displacement assays. ITC measurements are prohibited by the poor water solubility of the complex.udFuture studies of salophen complexes should focus on extended aromatic systems, like naphthalene, as the central aryl ring. This may prevent aggregation and will shift the excitation wavelength to longer wavelengths making the compounds more suitable for biological applications. Another issue to address is increasing the water solubility, which would then allow the use of ITC measurements to determine binding interactions.