Silk sericin was extracted from the cocoons of three Southern African wild silk moth species, namely Gonometapostica, G. rufobrunnae (Lepidoptera: Lasiocampidae), and Argema mimosae (Lepidoptera: Saturniidae); these three sericin extracts were analysed to determine the relationship that exists between their chemical structures and their functional properties. The relationship was investigated by utilising several methods that include the determination of the amino acid composition, and characterisation of the secondary structures with Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The antibacterial properties of these three sericin extracts were evaluated by an agar well diffusion assay with three Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus, and Staphylococcus epidermidis) as test microorganisms; and, lastly, the antioxidant properties of the three sericin extracts were determined using several scavenging methods that include the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH), 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS˙+), and the ferric reducing antioxidant power (FRAP) assay. The amino acid composition in the silk sericin extracts from G. postica, G. rufobrunnea, and Argema mimosa in terms of the polar/non-polar ratio (P/NP) was found to be 65:35, 56:44, and 59:41, respectively. The FTIR spectra of these three silk sericin extracts showed distinct major bands such as amide A (3265 cm−1), amide B (3062 cm−1), amide I (1644 cm−1), amide II (1538 cm−1), and amide III (1244 cm−1). The XRD patterns of the silk sericin extracts revealed both amorphous and α-helical structures, with small crystalline regions. All three silk sericin extracts presented potent antibacterial efficacy against the three Gram-positive bacteria and were found to have excellent antioxidant activities against the tested free radicals.

This study deals with the fabrication and characterization of sericin-poly(vinyl alcohol) (PVA) composite films from three southern African silkworm cocoons. The sericin-PVA films were achieved by chemically cross-linking poly(vinyl alcohol) (PVA) with pure silk sericin protein using glutaraldehyde (GA) as a cross-linking agent. Fourier transform infrared (FTIR) results confirmed the overall cross-linking of pure silk sericin into PVA-GA networks to form cross-linked sericin-PVA films. This incident was shown by the incorporation of distinct major amide I (ν = 1640–1650 cm –1 ), amide II (ν = 1538–1540 cm –1 ), and amide III (ν = 1238–1244 cm –1 ) peaks. X-ray diffraction (XRD) showed sericin-PVA films to have two features, one representing amorphous and crystalline regions of silk sericin and the other representing sharp high-intensity PVA peaks at around 2θ = 20.2°, demonstrating a high crystallinity in the films as a result of the hydroxyl groups in its side chain. The swelling capacity of the three sericin-PVA films was influenced by the glutaraldehyde content used during the cross-linking process and pH of the aqueous medium into which the films were immersed after a period of time. The water contact angles of the sericin-PVA films were low, at 56.6 ± 0.56 and 60.2 ± 0.86, indicating further their hydrophilic nature. The scanning electron microscopy (SEM) images of the sericin-PVA films showed a rough texture with a granular network pattern on their surface. From the preliminary results, it was observed that the cytotoxicity of three sericin strains ( Gonometa rufobrunnea , Argema mimosae , and Gonometa postica ) had a cell viability percentage of 103, 90, and 80% respectively, demonstrating their biocompatibility in providing a favorable natural microenvironment for cell culture. The characterization results of the three silk sericin-PVA films demonstrated their potential for application in biomedical and biomaterial fields.