Synthesis, Characterization and Biological Activities of Biopolymeric Schiff Bases Prepared with Chitosan and Salicylaldehydes and Their Pd(II) and Pt(II) Complexes

Abstract

In an attempt to enhance chitosan biological activities, biopolymeric Schiff bases of chitosan and different salicylaldehydes and their palladium(II) and platinum(II) complexes were synthesized and tested. The chemical structures of these derivatives were characterized using ¹H-NMR, FTIR spectroscopy and XPRD. Thermal analysis was done through TGA/DTG-DTA. Electronic absorption spectra and surface morphologies were analyzed by SEM-EDAX. Chitosan and its derivatives were evaluated for their in vitro antimicrobial activity against two common bacterial and fungal plant pathogens Pseudomonas syringae pv. tomato and Fusarium graminearum, respectively, and for their antitumor activity against a human breast cancer cell line (MCF-7). It was found that, compared to the nonmodified chitosan, chitosan modified with Schiff bases and their complexes was highly toxic against the MCF-7 cell line and had antibacterial effects against P. syringea. However, the modified chitosan derivatives had less pronounced antifungal effects against F. graminearum compared to the nonmodified chitosan, suggesting different modes of action.

Keywords: Schiff bases; antimicrobial; antitumor; chitosan; complexes.

Scheme 1

Structural representation of the ligands ortho-hydroxyaryl Schiff base of chitosan, palladium and platinum metal complexes.

Figure 1

¹H-NMR spectra of CCh chitosan, Ch chitosan and biopolymeric Schiff bases H-Ch, MeO-Ch and NO₂-Ch. Temperature 70 °C, solvent HCl/D₂O (1%).

Figure 2

TGA/DTG (a) and DTA (b) curves of CCh, Ch, H–Ch, MeO–Ch and NO₂–Ch, under air atmosphere, heating rate of 10 °C min⁻¹, air flow rate 100 mL min⁻¹, sample mass 6 mg ± 0.1, α-alumina sample holder.

Figure 3

TGA/DTG (a) and DTA (b) curves of palladium complexes (I) Pd–Ch, Pd–H–Ch, Pd–MeO–Ch, Pd–NO₂–Ch and platinum complex (II) Pt–Ch, Pt–H–Ch, Pt–MeO–Ch, Pt–NO₂–Ch, both under an air atmosphere, heating rate of 10 °C min⁻¹, air flow rate 100 mL min⁻¹, sample mass 6 mg ± 0.1, α-alumina sample holder.

Figure 4

Scanning electron microscopy (SEM) pictures of (a) Ch; (b) H–Ch; (c) Pd–H–Ch; (d) Pt–H–Ch and X-ray energy-dispersive analysis (EDS) spectra of bright spots (e) Pd–H–Ch and (f) Pt–H–Ch.

Figure 5

Cytotoxicity of chitosan, Schiff bases and Pd(II) and Pt(II) complexes against MCF-7 cells in 96-well plates determined using the MTT assay. (a) Salicylaldehyde Schiff bases and (b) Pd(II) and Pt(II) complexes at concentrations of 5, 30, and 100 μg mL⁻¹. Measurements were conducted for 24 h. Eight replicates were performed for each treatment, and the experiment was carried out three times, from which the standard deviation was calculated.