Structural evidence for the intramolecular charge-transfer interaction involving an indole ring in ternary copper(II) complexes with L-tryptophan and aromatic diamines

doi:10.1016/S0020-1693(00)83068-6

Inorganica Chimica Acta

Volume 180, Issue 1, 1 February 1991, Pages 73-79

https://doi.org/10.1016/S0020-1693(00)83068-6 Get rights and content

Abstract

With a view to understanding the precise binding mode and strength of the stacking interaction in the ternary copper(II) complexes comprising an aromatic diamine such as 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) and an aromatic amino acid such as L-phenylalanine, L-tyrosine and L-tryptophan (L-trp), the crystal structure of [Cu(bpy)(L-trp)]ClO₄ and the circular dichroism (CD) and absorption spectra of [Cu(bpy)(L-trp)]ClO₄ and [Cu(phen)(L-trp)]ClO₄ have been investigated. The complex [Cu(bpy)(L-trp)]ClO₄ crystallizes in the monoclinic space group, P2₁, with two molecules in a unit cell of dimensions a=13.022(1), b=7.753(1), c=10.533(1) Å, and β=91.18(1)°. The Cu(II) ion is five-coordinate square-pyramidal, with the two nitrogen atoms of bpy and the nitrogen and oxygen atoms of the amino acid coordinated at the equatorial positions in a slightly distorted square-planar form and the carboxylate oxygen atom of the neighboring molecule at the axial position. The most interesting structural feature of the complex is the existence of the intramolecular stacking interaction between the aromatic rings of L-trp and bpy with the average spacing of 3.67 Å from the vacant axial position. The CD spectra in the d-d region for [Cu(bpy)(L-trp)ClO₄ and [Cu(phen)(L-trp)]ClO₄ in aqueous solution showed a large negative peak at 587 and 598 nm, respectively, and the magnitudes were greatly reduced in dioxane-water, which indicates that the aromatic ring stacking interaction is weakened in a hydrophobic environment. The absorption bands due to the charge transfer (CT) interaction between the indole ring and the aromatic diamine have been observed in the difference spectra in the near ultraviolet region. The strength of the stacking interactions has been demonstrated by the CT band intensity and the distance between the stacked rings to be in the order [Cu(phen)(L-trp)]ClO₄>[Cu(bpy)(L-trp)]ClO₄ both in solution and in the solid stare.

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Synthesis, crystal structure, DNA interaction, DFT analysis and molecular docking studies of copper(ii) complexes with 1-methyl-L-tryptophan and phenanthroline units
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The fraction of unpaired electron density located on the copper ion i.e. the value in-plane sigma bonding parameter α2 (molecular orbital coefficient) was estimated from the expression, α2 = (A||/P) + (g|| – 2.0023) + 3/7 (g⊥ – 2.0023) + 0.04, (where A|| is the parallel coupling constant expressed in cm−1, and the value of P = 0.036). The α2 values for the copper(II) complexes found in the range 0.73–0.75, supported the covalent nature of these complexes [59]. The binding of present mixed-ligand copper(II) complexes (1–3) was investigated with CT–DNA in Tris–HCl/NaCl medium.
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As one can see form Table 3, the copper(II) ternary complexes with aromatic residues present a slightly more positive formal redox potential values than those with aliphatic residues. This small difference could be probably ascribed to weak interligand stacking interactions, which are known to be present in these systems [27,58]. Copper(II) ternary complexes with aminoacidate carrying positively charged residues also show more positive formal redox potentials.
Ternary copper(II) complexes with 1,10‑phenanthroline and the aminoacids l‑arginine, l‑aspartic acid, l‑histidine, l‑glutamic acid, l‑glutamine, l‑leucine, l‑lysine, l‑methionine, l‑phenylalanine, l‑tryptophan, l‑tyrosine, l‑valine, were studied in aqueous solution by means of UV–Vis-NIR spectrophotometry, EPR spectroscopy either at room or at low temperatures, and Square Wave Voltammetry. From the experimental data it is possible to conclude that most of these ternary complexes show a pseudo-octahedral geometry with a CuN₃O in plane chromophore and two oxygen atoms coming from water molecules perpendicularly bound to the equatorial plane. An exception to this general behaviour is given by the ternary copper(II) complex with 1,10‑phenanthroline and histidine at pH value near the neutrality because of the terdentate nature of histidine when it coordinates by means of its histamine-like mode. In this case, evidence for a probable square-based pyramidal stereochemistry is given in support. At pH values around 5 the histidine behaves as bidentate ligand coordinating by its glycine-like mode, so as the copper(II) ternary complex with 1,10‑phenathroline shows the pseudo-octahedral geometry found for all the ternary complexes with the other aminoacids. Moreover the ternary complex species with histidine at pH 5 and 7 are in equilibrium with each other as a function of the aqueous solution pH value and the temperature. In fact, the examination of low temperature EPR spectra at pH near 7 revealed not only a square-based pyramid complex but also products of decomposition. These results were also confirmed by the trend found in the formal redox potentials by the voltammetric measurements on many of these ternary complexes.
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The most important mode of enzyme inactivation is thermal inactivation. Immobilization technology is an efficient approach to elongate the life-time of enzymes. d-lactate dehydrogenase (D-LDH) was stabilized at high temperatures with immobilization on CNT and fCNT. The kinetic and thermodynamic parameters, optimum temperature and pH, and the intrinsic fluorescence of free and immobilized enzymes were examined in the present study. Also, an attempt was made to investigate the effect of CNT and fCNT on the adsorption and conformation of d-lactate dehydrogenase using molecular dynamics (MD) simulations. In comparison with free enzyme, the immobilized enzyme displayed an improved stability at high temperatures and, therefore, the immobilized enzyme is suitable for use in the industry because most reactions in the industry happen at high temperatures. Results of the present study showed that the adsorption of enzyme on CNT is mediated through the van der Waals and π-π stacking interactions, whereas in the adsorption of enzyme on fCNT in addition to hydrophobic interactions, the hydrogen bonding between enzyme and functional groups of fCNT is involved. Moreover, RMSD, RMSF and secondary structure analysis indicate that the fCNT protects the conformation of enzyme more than CNT. Therefore, D-LDH can be efficiently immobilized upon the fCNT compared to the pristine CNT.
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Herein, we report the synthesis, spectroscopy, crystal structure and theoretical studies of a new Cu(II) complex with formula [Cu(l-trp)₂(H₂O)₄](pic)₂·4H₂O (1), which exhibits an unusual tryptophan coordination to copper(II) via carboxylate monodentate binding. The complex has been prepared using the mixed ligand approach. Single crystal X-ray diffraction structural analysis of 1 revealed that, in the complex, the Cu(II) site shows a distorted octahedral geometry, and the tryptophan (trp) ligand is coordinated through the carboxylate group in a monodentate fashion. The picric acid (pic) co-ligand, which is present in the anionic form, resides outside the coordination sphere forming a charge-separated complex. Density Functional Theory (DFT; including time-dependent DFT) has been employed to calculate the equilibrium geometry of the complex as isolated species in dimethylsulfoxide (DMSO) solution, and also its electronic spectrum, which was used to understand details of the experimentally obtained ultraviolet-visible (UV-Vis) spectrum. The structural analysis was complemented by additional spectroscopic studies [infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies], and the thermal stability of the complex evaluated by thermogravimetric analysis (TGA).
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In a very recent experimental study, Park et al. [11] reported the successful application of Cu/DNA based catalysts in a unimolecular Friedel–Crafts reaction (FC) by using a single reactant molecule with 2-acyl imidazole part, coordinating to Cu(II), and indole moiety (Scheme 1). This system provoked our interest due to the narrower range of possible initial configurations of the system, and the availability of some experimental information on the structure of similar metal complexes and their interaction with DNA [13,14]. Ternary Cu(II) amino-acid complexes were studied previously as DNA cleavage agents by combining experiment and docking simulation [13].
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Structural evidence for the intramolecular charge-transfer interaction involving an indole ring in ternary copper(II) complexes with L-tryptophan and aromatic diamines

Abstract

Inorg. Chim. Acta

Inorg. Chim. Acta

Adv. Protein Chem.

Biobchim. Biophys. Acta

Adv. Enzymol.

J. Chem. Educ.

J. Am. Chem. Soc.

Proc. Natl. Acad. Sci. U.S.A.

Pure Appl. Chem.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

Nippon Kagaku Kaishi