A wide set of novel, functionalized cage complexes was synthesized, and corresponding synthetic methodologies were developed.
The following several types of novel cage complexes were obtained:
• Hexahydrazide clathrochelate complexes with high-valent first row transitional metals (Fe(IV), Mn(IV)) and Co(III) as ions incorporated into macrobicyclic cage;
• Heterofunctionalized Fe(II) clathrochelate complexes to facilitate targeting of protein macromolecules and connection of reporter units;
• Fe(II) dihalogenoclathrochelates with inherent halogen substituents for performing of Suzuki–Miyaura and Sonogashira reactions;
• Fe(I) cage complex with high stability towards strong H-acids;
• Co(II) pseudoclathrochelates with a rigid cholesteryl substituent , and a very large magnetic anisotropy;
• Hybrid metallo(IV)phthalocyaninate-capped Co(III) complexes.
Novel cage complexes were characterized by appropriate set of physico-chemical and analytical methods, including elemental analysis, mass spectrometry (ESI-MS and MALDI-TOF), UV-VIS, fluorescent, CD, IR, ATR-FTIR, NMR, EPR, spectroelectrochemistry and X-ray analysis. Single crystals of several M(II)/M(III)/M(IV) clathrochelate complexes have been successfully obtained and studied by means of X-ray diffraction structure analysis.
Original experimental data resulted from the redox reactivity characterization of hexahydrazide clathrochelate complexes with high-valent first row transitional metals (Fe(IV), Mn(IV)). The mentioned cage complexes on action of strong oxidants (cerium ammonium nitrate, lead(IV) oxide) can be oxidized to pentavalent state, while in the presence of strong and moderate reductants (sodium dithionite, ascorbic acid), a reduction to trivalent species occurs.
Of importance, the phenomena of chirality induction upon binding with proteins was discovered for inherently achiral iron(II) clathrochelates, due to this clathrochelates are considered as prospective CD reporters for protein studies. The interactions of cage metal complexes with proteins were characterized by a wide range of spectral methods (CD, UV-VIS, fluorescence), ESI-MS and ITC.
It was shown that carboxyphenyl ribbed-functionalized iron(II) clathrochelates are able to induce a pronounced CD (ICD) response upon supramolecular interactions with a macromolecular chiral inductor, such as a protein. The induced CD signals are observed in the spectral range characteristic to the clathrochelates MLCT absorption bands. The binding to a protein was found to be determined by the nature of clathrochelate terminal groups. Further, we have shown that iron(II) clathrochelates able to discriminate between serum albumins of relative structure (human and bovine albumins) by giving distinct ICD spectra. Besides, by the variation of the shape and intensity of CD bands, these cage metal complexes reflect alterations of the tertiary structure of albumins. The interactions of constitutional isomers of iron(II) clathrochelates with different globular proteins (serum albumins, lysozyme, -lactoglobulin (BLG), trypsin, insulin) had shown that a highly-intensive ICD output of the clathrochelates was observed upon their association with albumins and BLG. Based on the docking simulations, the binding of the clathrochelate molecule to the main BLG binding site is suggested.
For clathrochelates, which carry a single carboxylic acid, the anticancer activity was observed on a series of cancer cells, whereas normal cells were found to be unaffected. The studies of the mechanism of cytotoxicity have revealed that the compounds are efficiently uptaken by cancer cells via an active mechanism. In cells, they first induce the ER stress due to the accumulation of unfolded proteins as evidenced by the activation of the protein ubiquitination pathway and the increase of the amount of intracellular ubiquitinated proteins. The initial effect on the ER leads later on to the decrease of mitochondrial membrane potential, induction of oxidative stress including formation of mitochondrial ROS, upregulation of nitric oxide as well as growth arrest in the G1 phase and inhibition of DNA and RNA synthesis. All these factors cause cell apoptosis and partially necrosis.
