Thiophene-benzothiazole dyad ligand and its Ag(I) complex – Synthesis, characterization, interactions with DNA and BSA

doi:10.1016/j.molliq.2020.114182

Journal of Molecular Liquids

Volume 319, 1 December 2020, 114182

Dedicated to the memory of Professor Jacek Gawroński.

https://doi.org/10.1016/j.molliq.2020.114182 Get rights and content

Highlights

•
TBT ligand and complex [Ag(TBT)₂]⁺ interact with DNA and BSA.
•
TBT tandem ligand and its complex bind biomolecules through the thiophene moiety.
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Intercalation is binding model of Schiff base ligand and its Ag(I) complex with DNA.
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There are greater BSA structure perturbations with [Ag(TBT)₂]⁺ than the TBT ligand.
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Compounds bind in the vicinity of Trp (and not Tyr) residues.

Abstract

The aim of the reported research is to evaluate the significance and potential role of the thiophene moiety in potential DNA and BSA targeting drugs. For this purpose a Thiophene-BenzoThiazole tandem molecule (TBT) and its mononuclear silver(I) complex [Ag(TBT)₂]⁺ was synthesized. The research was carried out using spectroscopic techniques such as circular dichroism (CD), UV–Vis and fluorescence. Based on the presented results the intercalating type of binding to DNA by complex [Ag(TBT)₂]⁺ and ligand TBT was observed. The binding is spontaneous in both cases and the K_b values of both compounds are similar (K_b = 6.40 × 10⁵ and K_b = 5.83 × 10⁵ for ligand TBT and complex [Ag(TBT)₂]⁺, respectively). Yet, this type of interaction was confirmed in ethidium bromide competitive binding experiments and a special emphasis should be put on the higher K_SV value for complex [Ag(TBT)₂]⁺ (5.1 × 10⁴) than ligand TBT (3.4 × 10⁴), since it may be the result of the bisintercalation of the complex. It has found confirmation in the increase of the melting temperature (T_m) of DNA treated with complex [Ag(TBT)₂]⁺ of ~5 °C, while in analogues experiment with ligand TBT T_m was only ~1 °C higher. The bisintercalation of the complex is possible since two TBT ligands are bound to the metallic center with planar thiophene moieties placed on the same site. Furthermore, the complex [Ag(TBT)₂]⁺ caused more significant changes in the secondary structures of the model protein BSA than the ligand TBT as determined by CD (reduction of α-helix content by 95 times in presence of the complex vs. stabilization of BSA structure with TBT). It needs to be emphasized, that both compounds bind to BSA via static quenching mechanism, however [Ag(TBT)₂]⁺ has a higher affinity to it than the ligand itself (as evidenced by the extent of hyperchromism). Both compounds interact in hydrophobic site of the protein, however [Ag(TBT)₂]⁺ exhibits higher Stern-Volmer constant K_SV = 1.66 × 10⁵ in comparison to TBT K_SV = 1.24 × 10⁵. The Scatchard equation allowed one to estimate the compound:BSA binding ratio as 1:1 and 2:3 for ligand and complex, respectively. Moreover, the binding constant K_b is higher for complex (K_b = 4.05 × 10⁷) than ligand (K_b = 3.70 × 10⁵). It confirms that, indeed, both compounds may be distributed by albumins in the body, however the {(BSA)₃ - [Ag(TBT)₂]⁺₂} adduct is more stable. The synchronous fluorescence spectra indicated that both compounds bind better in Trp than Tyr residues, therefore they may serve as potential molecular targets.

Graphical abstract

Introduction

Heterocyclic compounds exhibit pharmacological activity via several mechanisms [1]. Depending on the type of heteroatom present in the molecule, they may show various properties. An example of such a compound having a five-membered ring containing the sulfur element is the thiacyclopentadiene, also referred to as thiophene [2]. It was first isolated by Victor Meyer as the impurity of crude benzene, which was observed by the formation of indophenin in the reaction of isatin with benzene and sulfuric acid [3]. Thiophene derivatives form a wide library of biologically active compounds. They bind to nucleic acids [4], exhibit anticancer [5], anti-inflammatory [6] and anti-bacterial [7] properties. An example of a molecule that inhibits the emergence of inflammation is 1-[1-(2,5-dimethylthiophen-3-yl)ethyl]-1-hydroxyurea. Tioconazole is one of the antifungal drugs currently used in medicine containing the thiophene subunit. It is used as an ointment against vaginitis caused by yeasts, as well as for body ringworm infections treatment. It inhibits the formation of an essential component of the yeast membrane - ergosterol by interacting with the cytochrome P-450 enzyme 14-α demethylase [8]. The scarce information on the complexes of thiazole-comprising molecules has encouraged us to investigate their potential properties. An especially potent metal ion, due to several reasons, is silver(I). First of all, importantly from the design and synthetic viewpoint, it is diamagnetic with d¹⁰ electron configuration. Therefore, it may be characterized by NMR and may adopt various geometries [9,10], coordination number [11] and structural motif [12] as well as the metal ion arrangement may be various [13]. Moreover, silver(I) compounds are known due to their numerous biological activities [14,15]. Sulfadiazine and silver nanoparticles doped drugs are used to heal burns and wounds [16]. In contrast, silver complexes based on phosphine, coumarins, carbenes and polypyridines show good anticancer properties [[17], [18], [19], [20]]. It was even possible to visualize the selective targeting of the latter one to the nuclei of HeLa cells [17]. Furthermore, silver(I) complexes are active antibacterial, antiseptic and anti-inflammatory metallotherapeutics [14,21]. In our study a TBT ligand was synthesized with the aim to evaluate the thiophene moiety influence on compound interaction with DNA and proteins, whereas the well-known biological potential of Ag(I) complexes of several classes of molecules [18,[22], [23], [24]] prompted us to the synthesis of the silver complex with TBT.

DNA is the main target of the anticancer strategies, since targeting this nucleic acid in cancerous cells damages them and blocks their replication, thus leading to their death. Since several mechanisms may lead a synthetic ligand to bind to nucleic acid structure [25], it is clearly necessary to investigate the nature of a molecular recognition, especially in the evaluation of DNA-binding ability of new compounds. To this scope spectroscopic methods including electronic absorption titration [26], fluorescent competitive binding studies with EtBr [27] and CD DNA binding analysis [28] can furnish important details. In general, small molecules can bind to structured DNA in various ways, such as binding to minor or major DNA grooves, electrostatic interactions, intercalation and covalent binding [29]. One of the best known DNA-depending mechanism of action is exhibited by cisplatin, which is one of the most effective anticancer drugs [[30], [31], [32], [33]]. However, patients treated with cisplatin struggle with several severe side effects (nausea, nephrotoxicity etc.) [34,35]. Therefore, nowadays the key task of personalized anticancer therapy is to develop effective drugs that cause fewer side effects and to this aim the exploration of the mechanism of DNA binding of metal-based therapeutics plays a key role. Understanding the type of interaction is crucial for the design of new drugs realized with a given moiety, e.g. thiophene or other aromatic groups.

Another important group of biomolecules that may be a potential target of the metallodrugs are serum albumins. There is an increasing attention paid to albumins due to the recently discovered accumulation of albumin particles in tumors and also in inflammations [36]. Therefore, investigating the mechanism of binding of bioactive compounds with serum albumins is of a clear interest in the design of new anti-cancer molecules, to be used in targeted therapy [37]. Bovine Serum Albumin (BSA) is the most commonly used model albumin in research due to its high homology (76%) to Human Serum Albumin [38].

Aim of this work was the synthesis of the Thiophene-BenzoThiazole tandem molecule (TBT), able to act as a N₂S-donor Schiff base ligand, and its complex [Ag(TBT)₂]⁺. The subsequent step included the investigation of the interaction of the above-mentioned compounds with DNA and BSA using spectroscopic methods. A comparative approach was employed to find out the differences in the mechanism of binding that occurs in the case of thiophene-benzthiazole hybrid and its silver(I) complex. Spectroscopic techniques with high sensitivity [[39], [40], [41]] such as circular dichroism (CD), UV–Vis and fluorescence spectroscopies, were used to shed light on the mechanisms underlying the interactions of our potentially biologically active compounds with calf thymus DNA (CT-DNA) and serum albumin targets.

Section snippets

Materials and physical measurements

CT-DNA, BSA, ethidium bromide, Tris, PBS, metal salt and NaCl were supplied from Sigma Aldrich and used without further purification. ESI mass spectra for MeCN [Ag(TBT)₂]⁺ and MeOH TBT solutions ~10⁻⁴ M were measured using a Waters Micromass ZQ spectrometer. NMR spectra were run on a Varian Gemini 300 MHz spectrometer and were calibrated against the residual protonated solvent signals with chemical shifts represented in ppm. Microanalyses were performed using Vario EL III CHN element analyzer.

Synthesis and characterization

TBT ligand and its Ag(I) complex were obtained by an elegant and efficient synthetic route in view of the investigation of their DNA and BSA binding abilities. All compounds were fully characterized by NMR, ESI-MS, FT-IR spectroscopy, as well as elemental and TG/DTA analyses. The structures of the [Ag(TBT)₂]⁺ complex and of the ligand TBT were confirmed by single X-ray diffraction measurements.

From a synthetic viewpoint, the reaction of the N₂S-donor ligand with Ag(I) salt led to a mononuclear

Conclusion

Our studies suggest that besides the possible electrostatic interactions involving [Ag(TBT)₂]⁺, this complex and the TBT tandem ligand bind biomolecules through the thiophene moiety, which is an important active site. The hanging thiophene “arms” in the complex may facilitate the interactions with Trp residues in BSA and the binders. The results of our DNA binding studies showed that intercalation is a likely binding model, as demonstrated by UV–Vis and the most by fluorescent quenching

CRediT authorship contribution statement

Martyna Szymańska: Investigation, Data curation, Visualization, Writing - original draft, Writing - review & editing, Funding acquisition. Małgorzata Insińska-Rak: Investigation, Validation. Grzegorz Dutkiewicz: Investigation, Writing - original draft. Giovanni N. Roviello: Conceptualization, Investigation, Writing - original draft. Marta A. Fik-Jaskółka: Conceptualization, Investigation, Project administration, Writing - original draft, Writing - review & editing, Funding acquisition. Violetta

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Prof. Artur Ciesielski and Prof. Artur R. Stefankiewicz are gratefully acknowledged for providing the UV–Vis JASCO V-770 and JASCO V-750 spectrophotometers.

MAFJ: the work was supported by the National Science Centre, Poland (grant no. 2017/24/C/ST5/00181).

MSz: the work was supported by grant no. POWR.03.02.00-00-I026/16 co-financed by the European Union through the European Social Fund under the Operational Program Knowledge Education Development.

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Thiophene-benzothiazole dyad ligand and its Ag(I) complex – Synthesis, characterization, interactions with DNA and BSA

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and physical measurements

Synthesis and characterization

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgement

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