Visible light-driven photocatalysts, quantum chemical calculations, ADMET-SAR parameters, and DNA binding studies of nickel complex of sulfadiazine

A 3D-supramolecular nickel integrated Ni-SDZ complex was synthesized using sodium salt of sulfadiazine as the ligand and nickel(II) acetate as the metal salt using a condensation process and slow evaporation approach to growing the single crystal. The metal complex was characterized for its composition, functional groups, surface morphology as well as complex 3D structure, by resorting to various analytical techniques. The interacting surface and stability as well as reactivity of the complex were carried out using the DFT platform. From ADMET parameters, human Intestinal Absorbance data revealed that the compound has the potential to be well absorbed, and also Ni-SDZ complex cannot cross the blood–brain barrier (BBB). Additionally, the complex's DNA binding affinity and in-vivo and in-vitro cytotoxic studies were explored utilizing UV–Vis absorbance titration, viscosity measurements, and S. pombe cells and brine shrimp lethality tests. In visible light radiation, the Ni-SDZ complex displayed exceptional photo-degradation characteristics of approximately 70.19% within 70 min against methylene blue (MB).

Preparation of Ni-SDZ complex.The complex was synthesized using the reflux method at 60-70 °C.
The aqueous nickel acetate (1 mmol) solution was added dropwise to a methanol-water solution of sodium salt of sulfadiazine (2 mmol) with constant stirring.The formed precipitate was separated from the solution by filtration, washed, and dried in a desiccator over CaCl 2 .The complex was insoluble in water and in most of the common organic solvents but soluble in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF).The solid precipitates were dissolved in 3-methyl pyridine and left to crystallize.After four weeks, greenish square crystals were obtained and picked up for single-crystal X-ray diffraction.Light Green solid yield (70%).

Computational analysis.
The Hirshfeld surfaces of the molecule are generated by Crystal Explorer 3.1 43 .
The two-dimensional fingerprint plot for Ni-SDZ molecule, SDZ, and 3-methyl pyridine (Solvent) displays the contributions from different contacts: All, H…H, O…H, C…H and N…H, which investigates the intermolecular contacts and gives a quantitative summary of nature and type of intermolecular contacts qualified by the molecules in the crystal.The fingerprint plot is based on the d e (distance from the point to the nearest nucleus external to the surface) and d i (the distance to the nearest nucleus internal to the surface).The value of dnorm is depending on the intermolecular contacts being less than, greater than, and equal to the van der Waals separations and is shown by the surface with a red spot representing shorter contacts, white areas representing contacts around the van der Waals separation, and blue regions are devoid of close contacts.The surface shape index and curvedness highlighted the C-H…π and π…π intermolecular interactions respectively 44,45 .All the calculations of an optimized structure of Ni-SDZ molecule were performed using the Jaguar program as built in the Schrödinger package software at B3LYP/LAV2P* level of theory [46][47][48][49][50]  Cytotoxic studies.The cellular level bioassay was carried out using S. pombe cells.The cells were grown in 50 mL yeast extract media in a 150 mL Erlenmeyer flask.The flask was incubated at 30 °C on a shaker at 150 rpm till the exponential growth of S. pombe was obtained (24-30 h).The cell culture was treated with different concentrations (2, 4, 6, 8, 10 mg mL −1 ) of complexes, free ligands, and DMSO (control) and incubated for 16-18 h.Brine shrimp lethality assay is an important tool for the preliminary cytotoxicity assay based on the ability to kill a laboratory-cultured larva (nauplii).Experiments were concluded with control and different concentration of the complexes.The whole set was triplicated to get an accurate result.The LC 50 was determined from the best-fit line, a graph of % mortality against the concentration.The LC 50 value was obtained from the antilogarithm of log 19 at 50% mortality (LC 50 ).All data were collected from three independent experiments [59][60][61] .

Photocatalytic degradation of MB.
The photocatalytic activities of Ni-SDZ complex-based photocatalysts were calculated by the photodegradation of MB dye under a 24 W lamp irradiation (visible light) in the open air and at room temperature.The distance between the light source and the beaker containing the reaction mixture was fixed at 3 cm.The Ni-SDZ (0.1 g) metal complex photocatalysts were dispersed into 100.0ml MB (10 PPM) aqueous solutions.Before irradiation (visible light), all three suspensions were magnetically stirred and the atom numbering scheme are shown in Fig. 1.The preliminary and intensity data of crystallography are given in Table 1.  2.
The stereochemistry of the sulfur atom is as usual a slightly distorted tetrahedral geometry with the bond parameters involving sulfur lying within the range quoted in the literature 63,64 66 .The C-H…Π and Π…Π Interactions are presented in Fig. 2. The molecular structure shows the heterocyclic ring is planar.The nickel is in a similar plane as the four coordination 3-methyl pyridine nitrogen.The angles to the least square plane through nickel, sulfonamide, and heterocyclic nitrogen [N1-Ni-N1 i ] are 1.2(4)°, and 180(4)° respectively and the angle between sulfonamide and 3-methyl pyridine nitrogen is almost perpendicular [N1-Ni1-N5 = 91.2(2)°](Fig. 3).

FT-IR spectral analysis.
To clarify the mode of bonding and the effect of the nickel ion on the SDZ ligand, the IR spectra of the SDZ ligand and their nickel complex are calculated and assigned based on a careful comparison of their spectra with that of the SDZ ligand.The infrared spectra (FT-IR) of the nickel complex taken in the region 4000-400 cm -1 are compared with the SDZ ligand.Based on some general references 33,34 and earlier studies of complexes with sulfonamide derivatives [67][68][69][70] , a tentative assignment of the best important bands is given in Figure S2.The symmetric and asymmetric bands assigned to ν(NH 2 ) in SDZ ligand (3496 and 3317 cm −1 ) are  shifted to wave numbers in the nickel complex (3490 and 3382 cm −1 ), representing the NH 2 group are modified to the free SDZ ligand.These modifications are probably owing to the hydrogen bonding between complexes involving the NH 2 and SO 2 groups.The scissoring vibrations for the amino (-NH 2 ) group seem at 1625 cm −1 and the peak due to the phenyl ring is at 1530 cm −1 .The peaks at 1339 cm −1 assigned to ν asym (SO 2 ) and those at 1154 cm −1 to ν sym (SO 2 ) show important changes upon complexation.The 1066 cm −1 band in the ligand is assigned to ν(S-N) 71 at higher frequencies.These shifts to higher frequencies are in accord with the shortening of the S-N bond lengths which have been observed in the respective crystal structure of the complex.Also, the presence of 406 cm −1 in Ni-SDZ spectra indicates metal-ligand binding which is absent in the spectra of SDZ ligand 60,72 .
Magnetic behavior studies.The room temperature X-band EPR spectrum of the nickel complex is shown in Fig. 4. The EPR spectrum of the Ni-SDZ complex is exhibiting both parallel and perpendicular g tensor values which are 2.371 for g parallel and 1.942 for g perpendicular .The experimental g values were g parallel > g perpendicular > 2.0023 which indicates that the nickel has a ground state characteristic for octahedral or square planar geometry.Also, we compute the g avg = 2.085 with the help of equation g 2 avg = 1/3[2g 2 perpendicular + g 2 parallel ].The g parallel > g perpendicular value suggests the octahedral environment of the nickel complex of SDZ 73,74 .
The magnetic behavior of the complex was studied at room temperature (293 K) (Figure S3).The summary of the results on the magnetic behaviour of the nickel complex was given by Figgis and Nyholm 75 .The experimental values of the magnetic moment for nickel complexes are generally revealing of the coordination geometry of the metal.Nickel must exhibit a magnetic moment higher than that expected for two unpaired electrons in octahedral (2.80-3.20 B.M) and tetrahedral (3.40-4.20 B.M.) complexes, while its square planar complexes would be diamagnetic.The magnetic moment observed for the nickel(II) complex is 2.83 B.M which is consistent with the octahedral stereochemistry of the complex.Also, The nature of the graph of magnetic susceptibility: χ = M/H with a slope of 2.227 × 10 -7 , reveals that the complex possesses paramagnetic properties.

Hirshfeld surface analysis.
Hirshfeld surface analysis and their correlated 2D fingerprint plots 76 have been performed to study the nature of the intermolecular interactions and their measurable contributions towards the crystal packing.The Hirshfeld surfaces (shape index, curvedness, di, and de) of the Ni-SDZ molecule are depicted in Fig. 5.The shape index and curvedness plots are significant indicators for the C-H…π interactions and π…π stacking within the crystal lattice 77 .There are touching pair of triangles (blue and red) for the shape index and the curvedness surfaces display broad, relatively flat regions characteristic of π-stacking of molecules.One of the significant interactions in the molecule is between C…H/H…C atoms with the distances of 1.08 Å and 1.45 Å for H to di surface and C to di surface respectively and so, the total distance of C…H/H…C interactions is 2.53 Å.The dominant interactions N-H…O and C-H…O is observed on the de surface.The distance of O…H (N-H…O) is 2.55 Å and the distances of O to de and de to H are 1.438 Å and 1.11 Å respectively.The normalized contact distance (dnorm) is based on both de and di.The surface represented by a deep red spot is illuminating hydrogen bonding interactions (Fig. 5) 58,78 .The dominant interactions are H…O can be seen in di and de surface plots as the bright red spots and deep red spots in maps are showing strong interactions in molecules.The column chart shows the percentage contributions of various contacts of the molecule (Ni-SDZ molecule), SDZ, and 3-methyl pyridine (Solvent) in Fig. 6.The red spot in the dnorm surface (Fig. 6) is attributed to weak hydrogen bonds involving the acceptor oxygen and nitrogen atom from the sulfonyl group and the hydrogen atom from the pyrimidinyl ring.The 2D fingerprint plots can be deconstructed to focus on particular atom pair contacts.At the top left and bottom right of the fingerprint plot, there are characteristic ''wings'' which are identified as a result of C…H contacts and O…H interactions are represented by a spike (Fig. 7).The 2D fingerprint plots of the Ni-SDZ molecule, SDZ, and 3-methyl pyridine shows a variety of contacts (H…H, C…H, O…H and N…H) in the crystal structure.The percentages contributions of H…H (37%) of SDZ is higher than Computational analysis.Optimized geometry.An optimized molecular structure of the Ni-SDZ is calculated at B3LYP/LAV2P level by using the Jaguar program in Schrödinger software 80 .The experimental data are  In the phenyl ring, all the carbon atoms have negative charges except C24 (0.2426).This suggests that atom C24 is acting as the center for charge transfer between the NH 2 substituent and the phenyl ring 81 .
HOMO-LUMO energy.Homo and Lumo energy calculated by B3LYP/LAV2P* method is depicted in Fig. 9 Homo (− 5.407 eV) and Lumo (− 1.279 eV) energy represents an ability to donate and accept the electron.
The electronic absorption agrees with the transition from the ground to the first excited state which is largely described by one electron excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital 46,82 .The wide energy difference between Homo and Lumo points towards the stability of the molecules and this energy gap (4.128 eV) reflects the chemical activity of the molecule.The UV-Vis spectra report the absorption maxima (265 nm) and optical energy band gap (4.683 eV).The energy gap between the HOMO and LUMO orbital has been found 4.128 eV as calculated theoretically.The computational (HOMO-LUMO) and experimental (UV-Vis) energy gap is measured in different conditions of the environment, therefore smaller deviation has been observed in it 78 .By using Homo and Lumo energy values of a molecule, the global chemical reactivity descriptor of molecules such as Electronegativity (χ), global hardness (η), global softness (S), and electrophilicity (ω) are calculated using Natural bond orbital analysis (NBO).Natural bond orbital (NBO) analysis is useful to investigate the stability of the molecule (Ni-SDZ) arising from charge delocalization.NBO analysis provides an essential method for understanding the interaction among bonds that played a vital role in the stabilization of the molecule.The second-order Fock matrix has been carried out to evaluate the donor-acceptor interactions in the NBO analysis.www.nature.com/scientificreports/E (2) which represents the stabilization energy associated with i(donor)-j(acceptor) delocalization is estimated from the second-order perturbation approach.For each donor NBO (L) and acceptor NBO (NL), the stabilization energy E (2) associated with delocalization is given as: where q l is the donor orbital occupancy, E(NL) and E(L) are diagonal elements (orbital energies) and F(L, NL) is the off-diagonal NBO Fock matrix element.The higher value of E (2) shows the more intense the interaction between electron donors and electron acceptors 46 .The LP(2) N7(donor), LV(1) C18(acceptor) interactions have the highest E (2) value of 131.55 kcal/mol.The donor and acceptor orbitals played the most dominant role in stabilizing the molecule (Table 3).The charge transfer interaction has been formed by the orbital overlap between bonding and antibonding orbital.The types of charge transfer in molecules are found to be n → σ * , n → π * and π → π * .The co-ordination covalent bond of non-lewis nickel(acceptor) with lewis type of one 3-methyl pyridine (solvent) nitrogen and two sulfonamide nitrogen (Ni-N) have the E (2) values of 17.65, 7.08, and 16.57kcal/mol respectively.
ADMET-SAR parameters.The Caco-2 cellular permeability of the chemical was predicted by the absorption (A) study to be medium.The results in Table 4 indicate that the compound's capability to bind to plasma proteins was modest.Usually, P-glycoprotein binders facilitate the excretion of chemicals from the cells through the drug efflux mechanism and increase drug resistance.The results revealed that the compound was neither a substrate nor an inhibitor of P-glycoprotein.Human Intestinal Absorbance data revealed that the compound has the potential to be well absorbed.The compound has lower MDCK permeability.Drug distribution (D) analysis predicted to the compound cannot cross the blood-brain barrier (BBB).In the analysis of liver microsomal metabolism, the 2 key cytochrome P450s (CYP450s) which are involved in drug metabolism are CYP2D6 and CYP3A4; these two enzymes together perform the majority of the liver microsomal phase-I drug metabolism.
The results showed that the compound was an inhibitor of CYP2D6 and CYP3A4; not a substrate for CYP3A4 and CYP2D6.Toxicity parameters of the compound reflected that it has a medium risk for hERG inhibition 85 .
CT-DNA interaction studies.Polycyclic aromatic molecules can interact with DNA either through forming covalent bonding with DNA or by some non-covalent interaction like groove binding, electrostatic interaction, and intercalation.In groove binding, the molecules locate themselves in the major or minor groove of the DNA, and forces like hydrogen bonding, van der Waals, or hydrophobic interaction stabilizes the molecule-DNA www.nature.com/scientificreports/conformation.Intercalation occurs when molecules of a reasonable size and chemical nature fit in between the stacks of DNA base pairs through associative π-stacking interaction and these often make a good nucleic acid stain 86 .Some intensively studied DNA intercalators are ethidium bromide, daunomycin, doxorubicin, etc. Doxorubicin and daunorubicin are used in the treatment of some tumors.DNA interaction study is very useful in developing a novel class of antitumor agents.UV-visible absorption spectroscopy and relative viscosity measurement studies were used to determine the binding ability of Ni-complex with DNA.The measurement of change in electronic spectra of the metal complex upon successive addition of DNA gives valuable information about the binding mode and strength.The effect of the stoichiometric addition of calf thymus DNA on the UV-visible absorption spectra of Ni-complex is shown in Fig. 10.The increase in the concentration of CT DNA resulted in a hypochromic with bathochromic shift, which suggests that the complex interacts with DNA most likely through a mode that involves a stacking interaction between the aromatic chromophore and the base pairs of DNA.The columbic force between metal ions and the phosphate of nucleotide could favor these interactions.Using absorption titration measurements, a linear plot of [DNA]/(ε f − ε a ) versus [DNA] (Fig. 10, inset) is obtained.Assuming all the molecules of complexes were bound with DNA, the experimental K b was obtained by substituting the absorbance into Beer's law.The K b value derived from the plot for the complex is 3.8 × 10 4 M -1 .The bathochromic shift, hypochromic, and large binding constant value obtained for Ni-complex suggests that the binding mode is intercalation 21,[87][88][89][90][91] .
The change in relative viscosity of DNA with increasing concentration of any compound is directly associated with the mode of interaction.The intercalation of compounds in between the base pair stacks lengthens the DNA size, and so the viscosity of the DNA solution increases.While the partial intercalation or non-classical interactions can bend the DNA, and hence the size of the DNA chain is reduced and so is the viscosity.So, the relative viscosity measurement results are the least ambiguous in finding the mode of interaction between DNA.The relative viscosity of the DNA solution increases with an increase in the concentration of Ni-complex, which is the result of an increase in the size of the DNA chain due to intercalation (Figure S5).Similar kinds of results are observed in the case of reported metal complexes and classical intercalator, EtBr and the data are supporting the UV-visible absorption titration study 55,87,88,92 .Table 4. Computational studies of a molecule.BBB = blood-brain barrier (high absorption CNS > 2.0, middle absorption CNS 2.0-0.1, low absorption to CNS < 0.1, Caco2 (high permeability > 70, middle permeability 4-70, low permeability < 4, %HIA (human intestinal absorbance) (well-absorbed compounds 70-100%, moderately absorbed compounds 20%-70%, poorly absorbed compounds 0-20%), %PPB (plasma protein binding) (strongly bound > 90%, weakly bound < 90%), MDCK (higher permeability > 500, medium permeability 25-500, lower permeability < 25).formed between two or more constituent molecules.Currently, it is actively used in virtual screening, lead optimization, biological activity prediction, binding site identification, drug-biomolecular interaction, etc.So, it has gained enormous interest for researchers as a tool for drug discovery, nowadays.The computational strategy allows for permeating all aspects of drug discovery today.Our interest to find out the primary site of interaction of Ni-complex with DNA was accomplished by molecular docking studies on DNA duplex of self-complementary sequenced (ACC GAC GTC GGT ) 2 by Hex software 8.1.The final binding orientation among all possible conformation having optimal energy is considered as a docked pose with minimum energy, which shows that minor groove mode plays a predominant role in the interaction (Fig. 11) 45,[93][94][95] .The relative binding energy of the docked structure was found − 213.45 kJ mol −1 .The lower binding energy implies a more potent binding affinity between the receptor (DNA) and a metal complex.These data are consistent with the UV-visible absorption titration study and relative viscosity measurement.
Cytotoxicity and cell viability studies.Cytotoxicity is the most important study among all biological evaluation studies.The compound may have different cytotoxicity mechanisms such as cell membrane destruction, prevention of protein synthesis, irreversible receptor binding, etc.So, the determination of cell death requires cheap, reliable, and reproducible short-term cytotoxicity and cell viability assays, for which a broad spectrum of cytotoxicity assays are currently used.The brine shrimp lethality assay correlates reasonably well with cytotoxic and anti-tumor properties.In the present study, the brine shrimp lethality of the metal complex was determined using the procedure of Meyer et al.The maximum mortality of brine shrimp was observed at a 30 µM concentration of Ni-complex.The LC 50 value of the metal complex was obtained by a plot of the percentage of the shrimp nauplii killed against the concentrations of the metal complex and the best-fit line was obtained from the data using regression analysis.This significant lethality of the metal complex (LC 50 = 7.44 µM) to brine shrimp is indicative of the presence of potent cytotoxic components which warrants further investigation.The proportion of viable cells in a cell population was estimated by the simplest and most widely used dye exclusion method.In the dye exclusion method, viable cells exclude dyes, but dead cells do not exclude them.In vivo, cytotoxic study of Ni(II) complex at a cellular level was carried out using eukaryotic S. pombe cell by trypan blue assay.The percentage viability of cells treated by complex after 17 h treatment is 92%, 87%, 81%, 79%, and 74%, for series of complex concentrations 2, 4, 6, 8, and 10 µg/mL, respectively.The data clearly shows the potent cytotoxic nature of the complex.

Photo-catalytic degradation
The photocatalytic degradation of MB was conducted to investigate the efficiency of photocatalysts.The photocatalytic activities of Ni-SDZ were monitored from the variation of the color in the reaction system by measuring the maximum absorbance intensity of MB.The absorption spectra of a sample (20 min time interval) and MB is shown in Fig. 12a.The absorption spectra of the MB solution decreased with visible light irradiation time.Under visible irradiation in a period of 70 min, the Ni-SDZ complex can degrade over 70.19% of the MB dye (Fig. 12a).The reduction in the intensity of the maximum absorption peak (662 nm) for MB suggests the The introduction of an electrophilic .OH group at the unpaired electron of the S heteroatom results in a change of its oxidation state from − 2 to 0. However, the transition from C-S +.C to C-S( .O)-C must maintain the preservation of the double bond conjugation, which in turn triggers the unfolding of the central aromatic ring that encompasses both heteroatoms (S and N).The necessary hydrogen atoms for the formation of C-H and N-H bonds may be derived from the reduction of protons due to electrons generated by light.Among the intermediate byproducts observed in the degradation of MB are 2-amino-5-(N-methyl formamide) benzene sulfonic acid, 2-amino-5-(methyl amino)-hydroxybenzene sulfonic acid, benzenesulfonic acid, and phenol (Fig. 12e).
where C 0 and C is the initial and apparent concentration of the MB and k is the kinetic rate constant.The values of k were obtained from the slope and the intercept of the linear plot.Figure 12d shows a linear relationship between ln(C/C 0 ) and the irradiation time for MB degradation.The rate constants for MB photodegradation with Ni-SDZ under visible light are − 0.01634 min −1 .The values of k indicated that the activity of photocatalyst Ni-SDZ is better than that of reported data 58,91,[98][99][100] .

Conclusion
In conclusion, the Ni(II) complex of sulfadiazine has been successfully synthesized.The magnetic measurements (EPR and magnetic susceptibility) data reveal the complex coordination is distorted octahedral.By crystal data results, the sulfadiazine ligand is bidentate and the geometry of the Ni-SDZ molecule is suggested to be octahedral, involving two nitrogen atoms of the sulfonamide group and two nitrogen atoms from the 3-methyl pyridine ligand.The experimental results were complemented with DFT calculations showing a good agreement between calculated and experimental geometrical parameters.The NBO analysis reveals that the intra-molecular interaction orbital overlaps providing the maximum stability to the molecule.Hirshfeld surface analysis reveals intermolecular O•••H and H•••H contacts are the most significant interactions in the crystal.The DNA interaction studies as performed using relative viscosity measurement and UV-visible absorption titration suggest intercalation of the complex in between the stacks of DNA base pairs.The molecular docking study suggests the minor groove as the first site of interaction of complex for DNA followed by intercalation.The cytotoxic and cell viability analysis of the complex suggests the potent cytotoxic nature of the complex.The molecular docking study complemented the DNA binding studies of the complex.The percentage of degradation of methylene blue dye was found to be 70.19%within 70 min of visible light radiation.

Figure 1 .
Figure 1.ORTEP View of Ni-SDZ molecule showing the numbering scheme of their displacement ellipsoids at the 50% probability level.

Figure 6 .
Figure 6.Hirshfeld surface is visualizing dnorm surface and the column chart shows the percentage contributions of various contacts of molecule Ni-SDZ molecule, SDZ, and 3-methyl pyridine (Solvent).

Figure 11 .
Figure 11.Molecular docked model of a complex located within the hydrophobic pocket of DNA (PDB ID: 1BNA).

( 1 )Figure 12 .
Figure 12.(a) Absorption spectra of MB solution with Ni-SDZ complex function of visible light irradiation time, (b) Photocatalytic degradation rate of MB in the presence of Ni-SDZ complex, (c) The liner fitting of -in C/C o and irradiation time and (d) Starting and final degradation % V/S time and (e) Degradation pathway of MB dye via Chromophoric group degradation.

Table 1 .
Preliminary crystallographic data and refinement parameters.