Preparation, Characterization and Antibacterial Evaluation of Soy Protein Isolate Biopolymeric Films Loaded with Nalidixic Acid

Abstract

Glycerol plasticized soy protein isolate (SPI) films at different contents (1 to 5% w/w w.r.t SPI) of nalidixic acid (NA) were fabricated by solution casting method. SPI films and NA incorporated SPI films were structurally and mechanically characterized by Fourier transform infrared spectroscopy (FTIR) and mechanical properties, respectively. Thermomechanical and water uptake studies were also carried out for NA incorporated SPI films. The results from FTIR study indicated the presence of NA in SPI as evidenced by generation of two additional peaks at 1259 cm^− 1 and 800 cm^− 1 attributed to C–C stretching vibration and the deformation of the heterocyclic aromatic ring, respectively. At 4% NA, the tensile stress and tensile modulus of SPI films increased from 5.18 MPa to 8.57 MPa and 57.14 MPa to 93.90 MPa, respectively. Antibacterial activity of NA incorporated SPI films against E. coli and L. monocytogenes were studied. The antibacterial studies of NA incorporated SPI film showed the clear zone of inhibition against both the bacteria. Docking studies were performed for SPI in order to evaluate their binding affinity with NA and similar type of aromatic acids such as mandelic and benzilic acid with which SPI films have been prepared earlier by our group. After docking, the ligands in form of aromatic acids were evaluated according to their binding energy and the interaction pattern between SPI and aromatic acids. The docking results indicated that nitrogen containing heterocyclic ring moiety remained intact during interaction of SPI and NA. This work will be helpful in fabricating NA incorporated SPI film from the renewable resources with significant antibacterial properties and reasonable mechanical properties.