Abstract
An ability of the ribbed-functionalized iron(ii) clathrochelates to induce a CD output in interactions with a protein, covalent bonding or supramolecular interactions with a low-molecular-weight chiral inductor, was discovered. The interactions of CD inactive, carboxyl-terminated iron(ii) clathrochelates with serum albumin induced their molecular asymmetry, causing an appearance of strong CD signals in the range of 350-600 nm, whereas methyl ester and amide clathrochelate derivatives remained almost CD inactive. The CD spectra of carboxyl-terminated clathrochelates on supramolecular interactions or covalent bonding with (R)-(+)-1-phenylethylamine gave a substantially lower CD output than with albumin, affected by both the solvent polarity and the isomerism of clathrochelate's ribbed substituents. In supramolecular assemblies, the bands were most intensive for ortho-substituted carboxyl-terminated clathrochelates. The ortho- and meta-phenylethylamide cage complexes in tetrachloromethane inverted the signs of their CD bands compared with those in acetonitrile. It was suggested that the tris-dioximate metal clathrochelates possess a Russian doll-like molecular system. Because of the distorted TP-TAP geometry, their coordination polyhedron had no inversion centre and possessed an inherent chirality together with the equiprobability of its left(Λ)- and right(Δ)-handle twists. The selective fixation of one of these C3-distorted conformations resulted in the appearance of the CD signal in the range of their visible metal-to-ligand charge transfer bands. Calculations by DFT methods were used to illustrate the possible conformations of the macrobicyclic molecules, as well as the intramolecular interactions between the cage framework and optically active distal substituents responsible for the chirality induction of the metal-centred coordination polyhedra.
Highlights
Circular dichroism (CD) spectroscopy is considered to be among the most efficient techniques for both stereochemical analysis and the study of chiral biopolymers.[1]
Clathrochelates[9,10,11] are three-dimensional macropolycyclic complexes with an encapsulated metal ion, and previously, we have suggested that these polyazomethines have the ability of to give a CD response upon their supramolecular binding with chiral inductors, such as biomacromolecules, because of the specific geometry of MN6-coordination polyhedra, which is an 1036 | Dalton Trans., 2018, 47, 1036–1052
A selective fixation of one of the C3-distorted conformations may result in an appearance of a CD signal in the spectra of these clathrochelates in the range of their visible metal-to-ligand charge transfer (MLCT) bands (400–600 nm)
Summary
Clathrochelates[9,10,11] are three-dimensional macropolycyclic complexes with an encapsulated metal ion, and previously, we have suggested that these polyazomethines have the ability of to give a CD response upon their supramolecular binding with chiral inductors, such as biomacromolecules, because of the specific geometry of MN6-coordination polyhedra, which is an 1036 | Dalton Trans., 2018, 47, 1036–1052. The dioximate iron(II) monoand bis-clathrochelates are reported[12,13,14,15,16] to be very efficient (including submicromolar) inhibitors in in vitro transcription system of T7 RNA polymerase Such compounds are found[17] to affect the pathway of protein fibrilization reaction, leading to changes in its kinetics as well as in the morphology of the aggregates formed.[18] Formation of the supramolecular assemblies of the ribbed-functionalized iron(II) complexes with globular proteins (in particular, those with serum albumins) has been studied[19,20] using various spectral techniques. The ab initio calculations with DFT functionals (PBE and PBE0) were conducted to illustrate the possible conformations of these macrobicyclic molecules, as well as the intramolecular interactions between the cage framework and the optically active distal substituents, which are responsible for chirality induction of the metal-centred coordination polyhedra
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