Spectroscopic studies on the interaction of morin–Eu(III) complex with calf thymus DNA

Abstract

The interaction between morin–Eu(III) complex and calf thymus DNA in physiological buffer (pH 7.4) was investigated using UV–vis spectrophotometry, fluorescence spectroscopy, viscosity measurements and DNA melting techniques. Hypochromicity and red shift of the absorption spectra of morin–Eu(III) complex were observed in the presence of DNA, and the fluorescence intensity of morin–Eu(III) complex was greatly enhanced with the addition of DNA. Moreover, fluorescence quenching and blue shift of the emission peak were seen in the DNA–ethidium bromide (EB) system when morin–Eu(III) complex was added. The relative viscosity of DNA increased with the addition of morin–Eu(III) complex, whereas the value of melting temperature of DNA–EB system decreased in the presence of morin–Eu(III) complex. All these results indicated that morin–Eu(III) complex can bind to DNA and the major binding mode is intercalative binding. The 3:1 morin:Eu(III) complex (estimated binding constant = 2.36 × 10⁶ L mol⁻¹) is stabilized by intercalation into the DNA. The calculated binding constants of morin–Eu(III) complex with DNA at 292, 301 and 310 K were 7.47 × 10⁴, 8.89 × 10⁴ and 1.13 × 10⁵ L mol⁻¹, respectively. The thermodynamic parameters were also obtained: ΔH^θ was 20.14 kJ mol⁻¹ > 0 and ΔS^θ was 161.70 J mol⁻¹ K⁻¹ > 0, suggesting that hydrophobic force plays a major role in the binding of morin–Eu(III) complex to DNA.

Introduction

The interaction of drug molecules with DNA has become an active research area in recent years [1]. Because the intracellular target for a wide range of anticancer and antibiotic drugs is DNA [2], [3], [4], illustrating the binding between small molecules and DNA can greatly help understand drug–DNA interactions and design new and promising drugs for clinical use. Generally, a variety of small molecules interacts reversibly with DNA, primarily through three modes: (i) intercalative binding that small molecules intercalate into the base pairs of nucleic acids [5]; (ii) groove binding in which the small molecules bound on nucleic acids are located in the major or minor groove [5]; (iii) long-range assembly on the molecular surfaces of nucleic acids so that the small molecules are not related to the groove structure of the nucleic acids [6]. The intercalative binding is stronger than other two binding modes because the surface of intercalative molecule is sandwiched between the aromatic, heterocyclic base pairs of DNA [7], [8].

Morin(2′,3,4′,5,7-pentahydroxyflavone, Fig. 1), a flavonoid compound, is present in tea, coffee, cereal grains, fruits, vegetables and many traditional Chinese herbal medicines [9]. It has been shown in various biological and pharmacological activities, including antiinflammatory, antioxidant, anticancer and cardiovascular protection [10], [11], [12].

Morin is known to complex with many metal ions to form stable compounds. In recent years, tremendous interest has been drawn to interactions between transition metal complexes of morin and nucleic acids due to potential applications of the metal complexes as anticancer drugs or as complexes with other biological functions [13], [14]. Some authors reported that morin, Zn(II) and Cu(II) complexes of morin can bind to DNA, respectively, but the binding mode is different. The complexes bind to DNA mainly by intercalating mode, while morin binds in a non-intercalating mode [15], [16]. Moreover, the complexes show higher antitumor activities than that of morin [17]. Rare earth metals not only have more physiological activities, but also their toxicities are decreased after coordinating with a ligand [18]. Zhou et al. found that the antitumour activities of rare earth metal complexes of quercetin with La(III), Eu(III) and Gd(III) are superior to quercetin, the major binding mode of quercetin–La(III) complex with DNA is intercalative binding [19]. Rare earth metals and their complexes as chemical nucleases are superior to transition metals and their complexes because they can bind to nucleic acid more efficiently by hydrogen bonding and hydrolyzed mode [20]. Whereas, so far the interactions between rare earth metal complexes of morin with DNA have seldom been reported.

A number of techniques have been employed to study the interaction of drugs with DNA, including fluorescence spectroscopy [21], UV-spectrophotometry [22], electrophoresis [23], nuclear magnetic resonance [24], electrochemical methods [25], etc. UV–vis absorption and fluorescence spectroscopy are regarded as effective methods among these techniques because they are sensitive, rapid and simple [26].

In this work, we used UV–vis absorption, fluorescence spectroscopy, viscosity measurements and DNA melting techniques to explore the interaction between morin–Eu(III) complex and calf thymus DNA. We believe this will be helpful to further understand the mechanism of interactions between DNA and morin’s rare earth metal complexes as well as further understand morin’s pharmacological effects. The knowledge gained from this study should be useful for the development of potential probes for DNA structure and new therapeutic reagents for tumours and other diseases.