2.1 Materials and Instrumentations
All metal salts, chemicals, reagents and solvents of analytical grade were purchased from commercial sources and used without further purification. 4-tert-Butyl aniline and chloroacetyl chloride were obtained from Sigma Aldrich. Fluorescence active TLC plates (F-2009) were obtained from Merck. Fluorescence spectra were recorded on a Jasco FP-8300 spectrofluorometer (Tokyo, Japan) with EHCS-813. Absorption spectra were recorded using a Jasco V-750 UV-Vis recording spectrophotometer (Tokyo, Japan) with EHCS-750 in the range of 200–800 nm. 1H NMR, 13C NMR, and 13C DEPT spectra were obtained on a Bruker AV- (III) ‐400 MHz spectrometer using a BBFO probe. Mass spectra were recorded on a Micromass Q‐TOF micro mass spectrometer with a capillary voltage of 3000 V and a source temperature of 120°C.
2.2 Synthesis of TCAN2PA
To a solution of thiacalix[4]arene (0.5 g, 0.00069 mol), K2CO3 (0.19 g, 0.0014 mol) in dry acetone, N-(4-(tert-butyl)phenyl)-2-chloroacetamide (0.31 g, 0.0014 mol) was added. The reaction mixture was then refluxed for 8 hrs. The progress of the reaction was monitored through TLC. After completion, the reaction mixture was cooled to room temperature, and the solvent was evaporated to dryness. The pure compound (Scheme 1) was obtained by silica gel column chromatography by using (hexane: ethyl acetate) as eluents (9:1) to get the solid product with 70 % yield.
MS (ESI): m/z for C63H75N2O6S4 Calcd: 1084.5 [M], Found 1123.4 [M + Na]+ (Fig. S8), 1H-NMR (400 MHz, DMSO-d6): δ (ppm): 0.94 (18H, s,-CH3), 1.38(36H, s, -CH3), 5.1(4H, s, -CH2), 8.8(2H, s, -OH) (Fig. S9) 10.1 (2H, s, -NH), 7.5(8H, s, Ar-H), 7.3(4H, s, Ar-H), 2.5(DMSO); 13C NMR (100 MHz, DMSO): δ (ppm): 30.61, 34.69, 76.71, 77.03, 77.35, 120.54, 136.42, 144.71, 155.63 (Fig. S10); M.P: 275.0°C.
2.3 Spectrophotometric and spectrofluorimetric measurements
Stock solutions of TCAN2PA (2 x 10− 4 M) and the nitrate salts (2 x 10− 3 M) of various metal ions [Fe(III), Cu(II), Cd(II), Zn(II), Cr(III), Ca(II), Co(II), Mg(II), Ag(I), Pb(II), Sr(II), Hg(II), Ba(II), K(I), Na(I), Bi(III)] were prepared in acetonitrile (ACN).The absorption spectra were recorded in the range of 200–700 nm, whereas the emission spectra were recorded within the range of 300 to 700 nm. The emission titration experiments were carried out by successive addition of mercury into the solution of TCAN2PA. A Job’s plot study was performed based on the emission studies.
2.4 In silico Insight
Molecular docking is an outstanding tool for computational modeling. Molecular docking can also be used to estimate the stability of host-guest complex that provides the structural descriptions of the host-guest complex to gain insights into the binding conformations, mechanistic information and the elucidation of ligand-induced conformational changes35,36. Design of ligands with specific electrostatic and stereochemical attributes can be developed by molecular docking to procure high receptor binding affinity. It imparts three-dimensional structures for scrupulous inspection of the topology at the binding site for the presence of clefts, cavities and sub-pockets. With molecular docking, careful inspection of the electrostatic properties such as charge distribution can also be done37,38.
Initially, the chemical structures of TCAN2PA and Hg(II) were developed by using the geometry optimization technique. The developed structures of TCAN2PA and Hg(II) were then set as starting host and guest, respectively using the Schrodinger software (Maestro 12.0). 3D knowledge-based shapes from the superimposed pair of the host and guest are utilized to calculate the energy scores for the host-guest complex. The best-docked score of the complex was retrieved by Hex 8.0 software. Identification of the complex possessing the lowest free energy is done. The equation used to calculate the binding energy is as under:
The insights for the possible host-guest interactions are procured by molecular docking, particularly the non-covalent interactions as they are not explained by the spectrofluorimetric results. Accelry’s Discovery Studio visualizer version 16.1 is used to observe the non-covalent interactions that play a major role within the various host-guest interactions. It reveals that ᴨ-lone pair, ᴨ-alkyl, ᴨ-ᴨ stacking and ᴨ-ᴨ T-shape stacking, metal-donor interactions are the main interactions for the host-guest complex formation.
2.5 Molecular dynamics (MD) simulation
MD simulation was carried out using the Desmond v3.6 (Academic version)39 package taking the complex with lowest free energy as an input. It is performed to know the thermodynamic stability of the complex. As per classical mechanics, MD implements Newton’s equations of motion, to acquire the details of speed and position of each atom in the system under investigation. Thus, examination of the trajectory and temporal evolution of host-guest complex can be examined40. In the first step, a particular configuration is attributed to the atoms to replicate the conditions like temperature and pressure of the real system. The position and velocity of each of the atoms at some points later are attainable from the computation of the forces that act on each particle. For the integration of the molecular trajectories for a given time interval, these calculations are repeatedly performed41. The simulation cell was created using the device builder module TIP3P (transferable intermolecular potential with three points) water model and the cubic periodic box containing the Simple Point Charge (SPC) (10×10Å) with the Integrated Liquid Simulations Potential (OPLS) all-atom force field 2005. MD simulation for this case was carried out for a duration of 20 ns with a relation time if 1 ps at a constant temperature of 300 K, along with constant volume and shape (NVT) with a Nose-Hoover thermostat 9, constant volume, Smooth Particle Mesh Ewald (PME) method. After that root mean square deviation (RMSD), root means square fluctuation (RMSF), Hydrogen bond, a radius of gyration (Rg), the histogram for torsional bonds were analyzed for the identification of structural changes with the dynamic role of the selected host-guest complex42,43.