Profiling of Silk Sericin from Cocoons of Three Southern African Wild Silk Moths with a Focus on Their Antimicrobial and Antioxidant Properties.

Kanono Comet Manesa,Simiso Dube,Temesgen Girma Kebede,Mathew Muzi Nindi

doi:10.3390/ma13245706

Abstract

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.

Highlights

Silk protein is a natural fibre produced by the silkworm to form a cocoon for protection during the pupal stage against various threats
Twenty L-amino acid standards, Dabsyl-Cl, and Whatman syringe filters polyvinylidene fluoride (PVDF) membrane used in this study were purchased from Sigma-Aldrich (Steinheim, Germany).Hydrochloric acid, sodium carbonate, sodium hydrogen carbonate, sodium acetate, sodium hydroxide pellets were all purchased from Merck (Darmstadt, Germany), while 2,2-diphenyl
The results of amino acid chemical composition of the sericin protein derived from G. postica, G

Summary

Introduction

Silk protein is a natural fibre produced by the silkworm to form a cocoon for protection during the pupal stage against various threats. Silk fibre consists of two distinct proteins, namely the fibroin and sericin. Silk sericin is a glue-like protein, which binds the fibroin fibres into an intact cocoon. It has been found to possess important biochemical properties [2]. It has 18 amino acids that mainly contain polar side-chains made of hydroxyl, carboxyl, and amino groups. These functional groups allow sericin protein to attain easy cross-linking, blending, and copolymerisation with other natural and synthetic polymers to produce biomaterials with enhanced properties. The high content of polar amino acids, namely serine, aspartic acid, and glutamic acid

Objectives

Methods

Results

Conclusion