Chemical, structural and thermal properties of Gonometa postica silk fibroin, a potential biomaterial

Abstract

In the present study, chemical, structural and thermal properties of fibroin from Gonometa postica, a wild silkmoth species were investigated. Silk from Gonometa rufobrunnea and Bombyx mori species were included in this study for comparison. The results indicated that G. postica and G. rufobrunnea silk exhibited similar properties whereas distinct differences were observed with B. mori silk. Amino acid analysis showed that glycine, alanine and serine accounted for more than 70% of the total amino acid content in all species. The amount of polar amino acids in Gonometa fibroin was significantly higher than for B. mori fibroin suggesting increased chemical reactivity of the former. The abundance of basic amino acids in Gonometa fibroin makes it a promising biomaterial in cell and tissue culture. Structural analysis revealed a unique β-sheet structure of Gonometa fibroin which is comprised of both poly-alanine and poly-glycine–alanine sequences. The maximum decomposition temperatures for Gonometa and B. mori fibroin were 350 °C and 320 °C respectively. The influence of amino acid composition on structural and thermal properties of the silks is also discussed.

Introduction

Silk is a natural protein fibre produced by a number of organisms including silkworms and spiders [1]. For centuries, mulberry (Bombyx mori) silk fibre has been the most important and common type of silk. However, wild silkworm species such as Antheraea pernyi and Antheraea assama species have become important sources of silk [2]. The increasing demand for functional biomaterials in biomedical and biotechnology fields has renewed scientific interest in silk due to its unique combination of mechanical strength and bio-properties such as biocompatibility, biodegradability as well as high oxygen and water permeability [3], [4].

Fibroin has thus been successfully tailored into films, 3D scaffolds and nanofibres which have found application in enzyme immobilization [5], vascular grafts [6], supports for cell adhesion and growth [7] and bone regeneration [8]. Although B. mori fibroin has found most applications, noteworthy efforts are being made to utilize wild silk fibroin for biotechnological applications [9]. The attraction to fibroin as a biomaterial stems from its distinctive properties which are attributed to its chemical composition and structure.

Structural and functional studies primarily done on molecules isolated from B. mori species have long established that fibroin is the main silk protein comprising of heavy and light chain polypeptides linked by a disulphide bridge [10]. Fibroin possesses extraordinary mechanical strength due to its highly oriented β-sheet crystalline structure consisting of (Gly-Ala-Gly-Ala-Gly-Ser) repeat units found in the heavy chain. The fibroin amino acid composition is therefore dominated by glycine, alanine and serine.

The specific primary structure (i.e. sequence of amino acids) of fibroin may vary depending on the source species. X-ray diffraction analysis of fibroin from the wild or semi-domesticated Saturniidae silkworms showed existence of polyalanine repeats sequences different to the B. mori glycine–alanine repeat structure [11]. The crystalline region of fibroin is mostly hydrophobic and plays an important role in its physical properties thus influencing properties such as biodegradability and biocompatibility. For example, Acharya et al. reported that A. mylitta films had better support for cell growth as compared to B. mori films [12] because of the arganine–glycine–aspartic acid (Arg-Gly-Asp) tri-peptide sequence in the former which is not found in the fibroin of the latter. An extensive review on the potential of non-mulberry silk fibroin as a biopolymer was recently published [13] and highlights applications of wild silk fibroin in numerous fields along with its advantages over B. mori fibroin. As such, there has been a noticeable surge in the study of wild silks whose chemical and physical properties have not been reported [14].

Gonometa postica and Gonometa rufobrunnea (Lepidoptera: Lasiocampidae) are wild silkmoth species indigenous to Southern Africa which produce quality silk that has been successfully commercialized [15], [16]. Despite similar cocoon features, there are differences between the two Gonometa species such as host plants and geographic location [17] which might influence their characteristics. Unlike B. mori and other wild silks which have been explored as biomaterials, both Gonometa species have not found applications beyond textiles to date. Their potential in biomedical and biotechnological fields can only be understood and explored if characteristics relating to applications are well investigated. These characteristics include procedures involved in the processing of the silks such as degumming and demineralization [18], as well as chemical and physical properties of the silks. Freddi et al. studied the chemical and physical properties of G. rufobrunnea proteins [19]. There has been a report on the chemical composition and rheological behavior of G. postica fibroin extracted directly from the silkworm glands [20]. However, an extensive study of chemical and physical properties of fibroin from the G. postica cocoons has not been reported. It is therefore fundamental to study the chemical and physical characteristics of G. postica fibroin in order to accurately assess its potential functionality and applicability in various fields.

The current study therefore focuses on determination of chemical, structural and thermal characteristics of fibroin from silkworm cocoons of the Southern African species G. postica. A comparison with G. rufobrunnea and the domesticated B. mori species is also presented. This work provides valuable information for further work which focuses on investigating possible utilization of these wild fibroins in a wide range of applications. Also, data from this study will add to and complement existing knowledge on silk proteins.