Properties of the indole ring in metal complexes. A comparison with the phenol ring

Phenol (bio)isosteres in drug design and development

Due to their versatile properties, phenolic compounds are integral to various biologically active molecules, including many pharmaceuticals. However, their application in drug design is often hindered by issues such as poor oral bioavailability, rapid metabolism, and potential toxicity. This review explores the use of phenol bioisosteres-structurally similar compounds that can mimic the biological activity of phenols while potentially offering improved drug-like properties. We provide an extensive analysis of various phenol bioisosteres, including benzimidazolones, benzoxazolones, indoles, quinolinones, and pyridones, highlighting their impact on the pharmacokinetic and pharmacodynamic profiles of drugs. Case studies illustrate the successful application of these bioisosteres in enhancing metabolic stability, receptor selectivity, and overall therapeutic efficacy. Additionally, the review addresses the challenges associated with phenol bioisosterism, such as maintaining potency and avoiding undesirable side effects. By offering a detailed examination of current strategies and potential future directions, this review serves as a valuable resource for medicinal chemists seeking to optimize phenolic scaffolds in drug development. The insights provided herein aim to facilitate the design of more effective and safer therapeutic agents through strategic bioisosteric modifications.

Binding characteristics of the major kratom alkaloid, mitragynine, towards serum albumin: Spectroscopic, calorimetric, microscopic, and computational investigations

Mitragynine (MTG) is a prominent indole alkaloid that is present abundantly in Mitragyna speciosa, commonly referred to as kratom. MTG has garnered significant attention due to its selective agonistic characteristics towards opioid receptors and related analgesic effects. In the circulatory system, the in vivo efficacy of MTG is dictated by its interaction with plasma proteins, primarily human serum albumin (HSA). In the present study, we utilized a broad methodology that included spectroscopic, calorimetric, microscopic, and in silico approaches to characterize the interaction between MTG and HSA. Alterations in the UV absorption spectrum of HSA by the presence of MTG demonstrated a ground-state complexation between the protein and the ligand. The Ka values obtained for the MTG-HSA interaction were in the range 103-104 M-1 based on analysis of fluorescence and ITC data, respectively, indicating an intermediate binding affinity. The binding reaction was thermodynamically favorable as revealed by ΔH, ΔS, and ΔG values of -16.42 kJ mol-1, 39.97 J mol-1 K-1, and -28.34 kJ mol-1, respectively. Furthermore, CD spectroscopy results suggested MTG binding induced minimal effects on the structural integrity of HSA, supported by computational methods. Changes in the dimensions of HSA particles due to aggregation, as observed using atomic force microscopy in the presence of MTG. Competitive drug displacement results seemingly suggested site III of HSA located at subdomain IB as the preferred binding site of MTG, but were in inconclusive. However, docking results showed the clear preference of MTG to bind to site III, facilitated by hydrophobic (alkyl and pi-alkyl) and van der Waals forces, together with carbon hydrogen bonds. Additionally, the MTG-HSA complexation was demonstrated to be stable based on molecular dynamics analysis. The outcomes of this study shed light on the therapeutic potential of MTG and can help in the design of more effective derivatives of the compound.

Indole-Containing Metal Complexes and Their Medicinal Applications

Indole is an important element of many natural and synthetic molecules with significant biological activity. Nonetheless, the co-presence of transitional metals in organic scaffold may represent an important factor in the development of effective medicinal agents. This review covers some of the latest and most relevant achievements in the biological and pharmacological activity of important indole-containing metal complexes in the area of drug discovery.

Residue-specific binding of Ni(II) ions influences the structure and aggregation of amyloid beta (Aβ) peptides

Alzheimer's disease (AD) is the most common cause of dementia worldwide. AD brains display deposits of insoluble amyloid plaques consisting mainly of aggregated amyloid-β (Aβ) peptides, and Aβ oligomers are likely a toxic species in AD pathology. AD patients display altered metal homeostasis, and AD plaques show elevated concentrations of metals such as Cu, Fe, and Zn. Yet, the metal chemistry in AD pathology remains unclear. Ni(II) ions are known to interact with Aβ peptides, but the nature and effects of such interactions are unknown. Here, we use numerous biophysical methods-mainly spectroscopy and imaging techniques-to characterize Aβ/Ni(II) interactions in vitro, for different Aβ variants: Aβ(1-40), Aβ(1-40)(H6A, H13A, H14A), Aβ(4-40), and Aβ(1-42). We show for the first time that Ni(II) ions display specific binding to the N-terminal segment of full-length Aβ monomers. Equimolar amounts of Ni(II) ions retard Aβ aggregation and direct it towards non-structured aggregates. The His6, His13, and His14 residues are implicated as binding ligands, and the Ni(II)·Aβ binding affinity is in the low µM range. The redox-active Ni(II) ions induce formation of dityrosine cross-links via redox chemistry, thereby creating covalent Aβ dimers. In aqueous buffer Ni(II) ions promote formation of beta sheet structure in Aβ monomers, while in a membrane-mimicking environment (SDS micelles) coil-coil helix interactions appear to be induced. For SDS-stabilized Aβ oligomers, Ni(II) ions direct the oligomers towards larger sizes and more diverse (heterogeneous) populations. All of these structural rearrangements may be relevant for the Aβ aggregation processes that are involved in AD brain pathology.

π-π Stacking Interaction of Metal Phenoxyl Radical Complexes

π-π stacking interaction is well-known to be one of the weak interactions. Its importance in the stabilization of protein structures and functionalization has been reported for various systems. We have focused on a single copper oxidase, galactose oxidase, which has the π-π stacking interaction of the alkylthio-substituted phenoxyl radical with the indole ring of the proximal tryptophan residue and catalyzes primary alcohol oxidation to give the corresponding aldehyde. This stacking interaction has been considered to stabilize the alkylthio-phenoxyl radical, but further details of the interaction are still unclear. In this review, we discuss the effect of the π-π stacking interaction of the alkylthio-substituted phenoxyl radical with an indole ring.

Recent Advances in One-Electron-Oxidized CuII -Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)-phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

Peptidylglycine monooxygenase activity of monomeric species of growth hormone

C-terminal α-amidation of peptides is an important event in the course of pro-hormone and neuropeptide processing; it is a modification that contributes to the biological activity and stability of about 25 peptides in neural and endocrine systems. This laboratory has shown that bovine growth hormone (bGH) also has a catalytic function, i.e. peptidylglycine monooxygenase activity, which is the first step in the alpha-amidation of glycine-extended peptides. We report here that the peptidylglycine monooxygenase activity of monomeric bovine pituitary GH, in the presence of ascorbate, is stimulated by combination with oligomeric forms of bGH one of which is a hetero-oligomer with metallothionein. Three species of recombinant monomeric GH (bovine, human and chicken) also catalyze this monooxygenase reaction. Tetrahydrobiopterin also functions as a reductant - with a significantly greater turnover than achieved with ascorbate. These findings clarify the role of GH in peptidylglycine monooxygenation and provide an explanation for earlier observations that peptide amidation is not totally obliterated in the absence of ascorbate, in cultured pituitary cells or in vivo. The evolution of bifunctional GH is also discussed, as are some of the significances of the peptidylglycine monooxygenase activity of human GH in relation to peptides such as oxytocin, glucagon-like peptide-1 and peptide PYY.

The effect of π-π stacking interaction of the indole ring with the coordinated phenoxyl radical in a nickel(ii)-salen type complex. Comparison with the corresponding Cu(ii) complex

In order to gain new insights into the effect of the π-π stacking interaction of the indole ring with the phenoxyl radical moiety as seen in the active form of galactose oxidase, we have prepared Ni(ii) complexes of a methoxy substituted salen-type ligand containing a pendent indole ring on the dinitrogen chelate backbone and characterized their one-electron oxidized forms. The X-ray crystal structure analysis and the other physicochemical experiments of the Ni(ii) complex revealed no significant intramolecular interaction of the indole ring with the coordination plane. On the other hand, the X-ray crystal structures of the oxidized Ni(ii) complex exhibited the π-π stacking interaction of the indole ring mainly with one of the two phenolate moieties. While the phenoxyl radical electron was delocalized on the two phenolate moieties in the Ni(ii)-salen coordination plane, the phenolate moiety in close contact with the indole moiety was considered to be the initial oxidation locus, indicating that the indole ring interacted with the phenoxyl radical by π-π stacking. The UV-vis-NIR spectrum of the oxidized Ni(ii) complex with the pendent indole ring was different from that of the complex without the side chain indole ring, but the differences were rather small in comparison with the oxidized Cu(ii)-salen complexes with the π-π stacking interaction of the indole ring. Such differences are due to the electronic structure difference, the localized radical electron on one of the phenolate moieties in the oxidized Cu(ii) complexes being more favorable for the π-π stacking interaction.

π-π Stacking Interaction in an Oxidized CuII -Salen Complex with a Side-Chain Indole Ring: An Approach to the Function of the Tryptophan in the Active Site of Galactose Oxidase

In order to gain new insights into the effect of the π-π stacking interaction of the indole ring with the Cu^II -phenoxyl radical as seen in the active form of galactose oxidase, we have prepared a Cu^II complex of a methoxy-substituted salen-type ligand, containing a pendent indole ring on the dinitrogen chelate backbone, and characterized its one-electron-oxidized forms. The X-ray crystal structures of the oxidized Cu^II complex exhibited the π-π stacking interaction of the indole ring mainly with one of the two phenolate moieties. The phenolate moiety in close contact with the indole moiety showed the characteristic phenoxyl radical structural features, indicating that the indole ring favors the π-π stacking interaction with the phenoxyl radical. The UV/Vis/NIR spectra of the oxidized Cu^II complex with the pendent indole ring was significantly different from those of the complex without the side-chain indole ring, and the absorption and CD spectra exhibited a solvent dependence, which is in line with the phenoxyl radical-indole stacking interaction in solution. The other physicochemical results and theoretical calculations strongly support that the indole ring, as an electron donor, stabilizes the phenoxyl radical by the π-π stacking interaction.

Characterization of the one-electron oxidized Cu(II)-salen complexes with a side chain aromatic ring: the effect of the indole ring on the Cu(II)-phenoxyl radical species

To gain insights into the role of the proximal indole ring in the redox-active metal center as seen in galactose oxidase, we prepared the Cu(II)-salen-type complexes having a pendent indol-3-ylmethyl (1), methyl (2) or benzyl (3) group substituted on the ethylenediamine moiety and investigated the structures and redox properties by various physicochemical methods and theoretical calculations. Neutral complexes 1, 2, and 3 showed no significant difference in the UV-Vis-NIR and EPR spectra. One-electron oxidation of 1, 2, and 3 by addition of 1 equiv. of thianthrenyl radical gave [1]SbCl ₆ , [2]SbCl ₆ , and [3]SbCl ₆ , respectively, which could be assigned to relatively localized phenoxyl radical species. The cyclic and differential pulse voltammograms of [1]SbCl ₆ showed two redox waves with a large separation between the first and second redox potentials compared with the separations observed for [2]SbCl ₆ and [3]SbCl ₆ . This suggests that [1]SbCl ₆ is more stabilized than [2]SbCl ₆ and [3]SbCl ₆ . The NIR band of [1]SbCl ₆ showed a larger blue shift than that of [2]SbCl ₆ and [3]SbCl ₆ . The EPR spectrum of [2]SbCl ₆ exhibited an intense signal at the g value of 2 due to partial disproportionation to form the EPR active two-electron oxidized complex [2] ²⁺ , while the EPR intensity of [1]SbCl ₆ was much weaker than that of [2]SbCl ₆ . These results indicate that the pendent indole moiety stabilizes the Cu(II)-phenoxyl radical in [1]SbCl ₆ most probably by stacking with the phenoxyl moiety, which is further supported by DFT calculations.