Synthesis, Characterization, and DNA Binding Studies of Nanoplumbagin

The traditional anticancer medicine plumbagin (PLN) was prepared as nanostructured material (nanoplumbagin, NPn1) from its commercial counterparts, simultaneously coencapsulating with cetyltrimethylammonium bromide or cyclodextrin as stabilizers using ultrasonication technique. Surface morphology of NPn analysed from atomic force microscopy (AFM) indicates that NPn has tunable size between 75 nm and 100 nm with narrow particle size distribution. Its binding efficiency with herring sperm DNA was studied using spectral and electrochemical techniques and its efficiency was found to be more compared to the commercial microcrystalline plumbagin (PLN). DNA cleavage was also studied by gel electrophoresis. The observed results indicate that NPn1 has better solubility in aqueous medium and hence showed better bioavailability compared to its commercial counterparts.


Introduction
Though traditional medicines are generally claimed to be safe and efficacious, in most cases neither the chemical entity nor the molecular mechanism of action is well defined. Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone), a quinonoid constituent isolated from the root of Plumbago zeylanica L., has been shown to exert anticarcinogenic, antiatherosclerotic, and antimicrobial effects [1][2][3]. Oral administration of drugs is the most popular route for the majority of drugs. However when drugs are poorly soluble, they have low bioavailability due to their inherent low solubility and slow dissolution rate primarily in the gastrointestinal tract, which necessitates highly concentrated administration of drugs than normally needed. Many synthesized anticancer drugs exert their cytotoxicity by inhibiting DNA synthesis and cell replication [4]. The side effects of these drugs such as bone marrow suppression and gastrointestinal toxicity are difficult to avoid. In this context, nanoencapsulation of drug particles is a useful technique in controlled and targeted delivery [5][6][7]. During the past few decades, a large variety of biocompatible drug delivery systems such as liposomes, micelles, emulsions, and polymeric micro-and nanoparticles have been developed. Polyelectrolyte capsules were also identified as promising drug delivery vehicles [8,9]. In this context, nanoparticles can be considered as useful platforms as they have unique advantages such as high surface area to volume ratio and allow many functional groups to be attached to a nanoparticle, which can seek out and bind to selected tumor cells. Additionally, the small size of nanoparticles allows them to preferentially accumulate at tumor sites. Dissolution rates may increase by reducing the particle size to increase the interfacial surface area [10].
To overcome the problems of using bulk materials as drugs, nanoparticle based drug delivery approaches were developed [11], in which curcumin is encapsulated in liposomes [12], solid lipid microparticles, such as bovine serum albumin [13] and chitosan [14], or complexed with phospholipids [15] and cyclodextrin [16][17][18][19]. Currently, the sonochemical method has been used extensively to generate novel materials with unusual properties, since they form particles of a much smaller size and higher surface area than those reported by other methods [20,21].
Cetyltrimethylammonium bromide (CTAB) is a wellknown surfactant and has been widely used in the synthesis of many types of nanoparticles with controlled morphologies [22,23]. Using this cationic surfactant, a rich variety of aggregate microstructures can be synthesised even at high dilution and; this can serve as novel organic templates for mesostructured materials with various remarkable morphologies.
Recently, cyclodextrins (CDs) were found to play important roles in self-assembly systems of amphiphiles or surfactants. As an amphiphilic building unit, CDs can be chemically or physically attached to a hydrophobic moiety, where the CD outer surface acts as a hydrophilic moiety and the resultant compound as a classic amphiphile. The amphiphilic CDs can self-assemble in a classic way similar to that of surfactant, where the CD portion acts like the hydrophilic head group of a surfactant [24]. Many amphiphilic CDs were developed and were found to be able to form micelles, monolayers at air/water surface, vesicles, and fibers. Compared to surfactant aggregates, the amphiphilic CD-based aggregates can provide macrocyclic hosting sites on the surface, creating new possibilities for host-guest interaction and recognition.
The principle of ultrasonication is based on formation, growth, and implosion of acoustic cavity and is considered as a green, simple, fast, popular, and reproducible method. In this study, we have used low frequency ultrasonication process for the synthesis of NPn using CTAB or -CD as a surfactant/stabilizers. It was characterized using FT-IR and UV-Vis spectroscopy, AFM and scanning electron microscopy (SEM) and DLS techniques. Their interaction with DNA was also investigated through spectral and electrochemical studies.

Materials and Methods.
Plumbagin, cetyltrimethylammonium bromide (CTAB), and -cyclodextrin ( -CD) were purchased from Sigma Aldrich. pUC18 DNA from SRL, Herring sperm DNA (hs-DNA) from GENEI, and sodium chloride and Tris hydrochloride (Tris-HCl) were purchased from Merck. All other chemicals were used without further purification. Double distilled water was used throughout the experimental studies. All the experimental studies involving the interaction of the plumbagin (PLN) and nanoplumbagin (NPn) with hs-DNA were carried out in aerated buffer (Tris-HCl, pH 7.1) containing NaCl. Gel electrophoresis experiments were performed using pUC18-DNA and 1% agarose gels together with a TAE (Tris-acetic acid-EDTA) running buffer solution. DNA samples were mixed with loading dye (bromophenol blue) and loaded into the slots of the submerged gel and resolved at 50 V. The gels were photographed under UV light.

Physical
Measurements. UV-Vis absorption spectra were recorded using JASCO-spectra manager (V-550) in 1 cm path length quartz cuvette with a volume of 2 mL at 25 ∘ C. Infrared spectra of PLN and NPn were recorded on a JASCO FT-IR-410 (4000-400 cm −1 ) spectrophotometer. The SEM images were obtained using a JEOL JSM-6390 scanning electron microscope. The powder samples were placed over carbon tape and a thin layer of platinum was coated over the samples to avoid charging. Cyclic voltammetric measurements were carried out on a Bioanalytical System (BAS) model CV-50W electrochemical analyzer. The three-electrode cell comprised a reference Ag/AgCl, platinum wire as counter electrode and working glassy carbon (GC) electrodes with surface area of 0.07 cm 2 . The GC was polished with 0.3 and 0.05 mm alumina before each experiment and if necessary the electrode was sonicated in distilled water for 10 min. Dissolved oxygen was removed by purging the solution with pure nitrogen gas for about 15 min before each experiment. A bare cyclic voltammogram has been carried out for a blank solution to check the purity of the supporting electrolyte and the solvent. Nuclear magnetic resonance spectroscopic measurements were recorded using Bruker 300 MHz spectrometer. Circular dichroism spectra were recorded on a JASCO J-810 (200-900 nm) spectropolarimeter. The spectral changes of PLN and NPn during the addition of increasing concentration of DNA were monitored at 25 ∘ C using a quartz cuvette of 1 cm optical path length with a volume of 1 mL at room temperature. Gel electrophoretograms of the samples were investigated by BIORAD Model XI computer controlled electrophoresis power supply.

Preparation of Nanoplumbagin (NPn).
Commercially available microcrystalline plumbagin (0.094 g, 0.5 mM) was dissolved in 5 mL of ethanol and poured into HCl (0.05 M, 20 mL). An aqueous solution of NaOH (0.05 M, 20 mL) was prepared separately in presence of CTAB or -cyclodextrin. Then two different concentrations of 0.2 and 0.5 wt% of CTAB and -cyclodextrin were prepared, the dropwise addition of plumbagin solution into CTAB/ -cyclodextrin solution in NaOH is effected separately under ultrasonic conditions, using an ultrasonic power of 250 w and a frequency of 59 Hz. After sonication for 40 min, yellow precipitate was obtained which was washed with ethanol several times to remove unbounded surfactants (CTAB or -CD) to give pure nanoplumbagin. It was characterized by UV-Vis, absorption, FT-IR, and 1 H NMR spectroscopy techniques. The absorption data clearly showed the enhanced solubility of NPn1 in water, compared to an equivalent concentration of Pn. Further, these spectral data also confirm that NPn does not undergo any structural change during its synthesis from its commercial counterparts. It was characterised by UV-Vis, 1

Preparation of Stock Solution of Nanoplumbagin.
All the measurements were carried out in Tris-hydrochloride buffer at pH 7.1 and double distilled water was used to prepare buffers. The stock solution of nanoplumbagin (1 × 10 −2 M) was prepared by dissolving 0.094 g of nanoplumbagin (NPn1) in 50 mL Tris-hydrochloride buffer at pH 7.1.

Characterization of Nanoplumbagin (NPn)
3.1.1. DLS Data Analysis. The analysis of dynamic light scattering data of NPn1 given in supporting information ( Figure S4) showed that Npn1 has the particle size ranging from 7 nm to 1200 nm with most of the particles lying in the range of 75 nm-110 nm.

Atomic Force Microscopy (AFM).
The effect and the amount of stabilizers were taken into account during the synthesis of NPn, namely, NPn1-NPn4. AFM images and size distribution of the NPn prepared in presence of 0.2 and 0.5 wt% CTAB, namely, NPn1 and NPn2, respectively, are shown in Figure 1. From this data it is clear that in NPn1, the nanoparticles are in the range of 84-110 nm (Figure 1(a)) with the height profile of the three-dimensional (3D) AFM image (Figure 1(b)) of NPn1 found to be about 10.9 nm, whereas in NPn2 the microparticles are in the range of 110-350 nm (Figure 1(c)) with a height of 15 nm (Figure 1(d)) due to the fact that CTAB as stabilizer can be absorbed at the surface to control crystal growth and coagulation.
Similarly AFM images and size distribution of the NPn prepared in the presence of 0.2 and 0.5 wt% -CD, namely, NPn3 and NPn4, respectively, are also shown in From these observations it can be concluded that 0.2 wt% of CTAB, that is, NPn1, is appropriate for the preparation of NPn, which is taken into account for further studies like DNA binding and cleavage studies.

SEM Analysis.
The morphology of the nanocrystalline plumbagin (NPn1) was studied by scanning electron microscopic (SEM) technique. This material consisted of nanospheres with ∼84-110 nm grain size (Figure 3) which is consistent with the data obtained from AFM images and some larger sized particles present in the system are also seen in these images.

XRD Diffraction Pattern of NPn1.
The powder X-ray diffraction pattern of NPn1 is as shown in the Supporting Information ( Figure S5). A definite line broadening of the XRD peaks indicates that the prepared material consists of particles which are in the nanoscale range. The diffraction peaks are observed at 2 values 11.61 ∘ , 13.46 ∘ , 26.72 ∘ , 31.94 ∘ , 45.62 ∘ , 56.63 ∘ , and 75.4 ∘ , from the calculated lattice constants = = 3254Å and = 5202Å. The observed pattern indicates that these NPn1 nanoparticles are present as hexagonal wurtzite phase and the average crystalite size is 14.98 nm.

Electronic Absorption Studies.
The interaction of synthesized nanocrystalline plumbagin with herring sperm DNA was investigated by monitoring the changes observed in the UV-Vis absorption bands ( → * and → * transitions). When increasing the concentration of herring sperm DNA to a constant concentration of nanodrug caused a hypochromism in their UV-Vis absorption bands along with a slight red shift (2-5 nm) ( Figure 4 and Table 1 in SI), which indicates the possibility of deep penetration of NPn1 particles into the core of DNA. In contrast PLN showed no change in its absorption measurements ( Figure 5) due to its poor or no solubility in aqueous medium. These results show that the main action mode of NPn1 binding to DNA is intercalation [25,26].
where [DNA] is the concentration of DNA in base pairs, is the apparent extinction coefficient obtained by calculating obs /[complex], and and correspond to extinction coefficients of the complex in its free and bound forms, respectively [27]. From the plots of is measured by the ratio of slope to intercept as shown in Figures 4 and 5.
The intrinsic binding constants ( ) for NPn1 and PLN with DNA are 2.9 × 10 4 M −1 and 5.4 × 10 3 M −1 . The observed value for PLN closely matches with the previous report [27] and that of NPn1 showed much higher value than PLN and its copper complexes due to its better solubility in aqueous medium than its counterparts. The binding constant values observed from electronic spectral results indicate that NPn1 strongly intercalates into the core DNA compared to its commercial counterparts.

Circular Dichroism Studies.
The changes which occurred in the CD signals of the DNA upon interactions with drugs may often be assigned to the corresponding changes in the DNA structure [28]. The groove binding and electrostatic interaction of a molecules with DNA show less or no perturbations on the base stacking and helicity bands because these two binding modes do not influence the secondary structure of DNA, while the observed intensity change can be attributed to an intercalation mode when small planar molecules interact with DNA [29].
The observed ICD spectrum ( Figure 6) of herring sperm DNA (hs-DNA) consists of a positive band at 275 nm due to base stacking and a negative band at 245 nm due to its helicity, which are characteristic of right-handed B form DNA [30]. Upon addition of PLN to DNA, the CD spectrum exhibits a slight alteration on both negative and positive absorption intensities without much change in its wavelength, indicating a relatively weak intercalation of plumbagin to hs-DNA. In the presence of NPn1, a significant decrease in the peak intensities of both bands was observed which indicates that NPn1 can effectively intercalate the neighbouring base pairs of hs-DNA to reduce the energy of the -electronic transition and also due to strong conformational changes by NPn1 and its binding to DNA mainly by intercalation mode [31].  DNA binding with metal complexes. Cyclic voltammetric behaviour of NPn1 and PLN in the absence and presence of DNA is shown in Figures 7(a) and 7(b). The separation of the cathodic and anodic peak potentials, Δ , and the ratio of cathodic to anodic peak current, / ( Table 2 in SI), indicate that the quasi-reversible redox process was observed for both NPn1 and PLN. The formal potential 0 (or voltammetric 1/2 ), taken as the average of and , in the absence of DNA for NPn1 and PLN is −0.311 and −0.289 mV, respectively, which upon the addition of DNA undergo a slight positive shift, indicating significant association of NPn1 and PLN with DNA [32].

Electrochemical
It is reported by Bard and Faulkner [33] that if the peak potential is shifted negatively when the molecule interacted with DNA, the interaction will be electrostatic in nature. On the contrary, if the peak potential is shifted positively, the interaction will be through intercalation mode. If there is no change of peak potential, then the interaction of the reduced form of the molecule will be the same as that of its oxidized form. In the present study, it is clear that the potential shift occurs in both NPn1 and PLN during the addition of DNA and hence the interactions of the reduced and oxidized forms are not similar. The peak potentials of NPn1 and PLN undergo a slight positive shift with decrease in peak current, which was well distinguished for the NPn1 than PLN. These results indicate that as expected NPn1 were interacted with DNA much efficiently than PLN due to its smaller size which enhanced its solubility. This decrease in current is due to diffusion of an equilibrium mixture of free DNA-bound drug to electrode surface [34].
The Nernst equation for the reversible redox reactions of the free and bound species and the corresponding equilibrium constants for binding of each oxidation state to DNA for an electron process is given as follows: where 0 and 0 are the formal potentials of oxidised/ reduced couple in the bound and free forms, respectively, and ox and red are the binding constants of oxidized and reduced forms to DNA, respectively. The conclusions could Journal of Nanomaterials be drawn in the light of Bard's report that both NPn1 and PLN interacted with DNA relatively through weak intercalation mode.

DNA Cleavage Study.
Gel electrophoresis was used to study cleavage of supercoiled pUC18 DNA by the synthesized NPn1 and commercially available PLN. Gels were run in Tris-HCl/NaCl buffer (pH = 7.1) and analyzed after ethidium bromide staining in the absence of any external reagent or light.
When circular plasmid DNA is subjected to gel electrophoresis, relatively fast migration is observed for the intact supercoil form (form I). If scission occurs on one strand (nicked circular), the supercoil will relax to generate a slowermoving open circular form (form II). If both strands are cleaved, a linear form that migrates between forms I and II will be generated [35][36][37].
From gel electrophoresis studies, plumbagin nanoparticles show the change of DNA form I to form II (Figure 8), due to its nanosize which increases the surface area and solubility in incubation of DNA with plumbagin, the supercoiled form of plasmid pUC18 DNA being lengthened without any obvious unwinding, but the DNA mobility for PLN was slightly reduced (Figure 9) [27]. These observed results are also in accordance with the previously studied spectral and electrochemical investigations. All these results clearly indicate that the present NPn1 interacts more significantly than its original counterparts of PLN due its smaller size and enhanced solubility.

Conclusions
In summary, ultrafine nanoparticles of plumbagin were synthesised successfully by sonochemical methods via controlling the surfactant concentration. They were characterised by UV-Vis, FT-IR, NMR, DLS, AFM, SEM, and XRD techniques and the observed spectral and electrochemical studies indicate that this PLN and Npn1 were partially intercalated to the DNA, in that Npn1 intercalated with DNA more significantly compared to the commercial PLN due to its small size which enhanced its solubility in aqueous medium.