Disruption of the Metal Ion Environment by EDTA for Silk Formation Affects the Mechanical Properties of Silkworm Silk.

Qingsong Liu,Xiaoyin Tan,Qingyou Xia,Haonan Dong,Yi Li,Xiaoqian Xie,Xinning Li,Xin Wang,Ping Zhao

doi:10.3390/ijms20123026

Abstract

Silk fiber has become a research focus because of its comprehensive mechanical properties. Metal ions can influence the conformational transition of silk fibroin. Current research is mainly focused on the role of a single ion, rather than the whole metal ion environment. Here, we report the effects of the overall metal ion environment on the secondary structure and mechanical properties of silk fibers after direct injection and feeding of silkworms with EDTA. The metal composition of the hemolymph, silk gland, and silk fiber changed significantly post EDTA treatment. Synchrotron FTIR analysis indicated that the secondary structure of silk fiber after EDTA treatment changed dramatically; particularly, the β-sheets decreased and the β-turns increased. Post EDTA treatment, the silk fiber had significantly decreased strength, Young’s modulus, and toughness as compared with the control groups, while the strain exhibited no obvious change. These changes can be attributed to the change in the metal ion environment in the silk fibroin and sericin in the silk gland. Our investigation provides a new theoretical basis for the natural silk spinning process, and our findings could help develop a method to modify the mechanical properties of silk fiber using metal ions.

Highlights

Silk fiber is a natural polymer material [1]
All these results demonstrated that the Ethylenediaminetetraacetic acid (EDTA) had little effect on the morphological characteristics of cocoons and single silk fiber
We investigated the effects of metal ions on the mechanical properties of silk fiber through directly disturbing the ionic environment for silk fiber formation by EDTA

Summary

Introduction

Silk fiber is a natural polymer material [1]. Recently, silk fiber has received extensive attention because of its outstanding mechanical properties such as high fracture strength, high toughness, and exceptional extensibility [2]. Modifying the mechanical properties of silk fiber in order to expand their application in various fields has attracted the interest of many researchers. Various methods have been used to improve the mechanical properties of silk fiber. A number of researchers have reported that the mechanical properties of silk fiber can be modified by transgenic silkworm overexpression with the spider gene [3,4,5,6]. Recent evidence suggests that metal ions, pH, and shear forces are involving in the conformational transition of silk proteins, which will affect the mechanical properties of silk fiber [11,12]. Injection of certain metal salt solutions such as CaCl2, KCl, CuCl2, and FeCl3 can improve the mechanical properties of silk fiber [16,17,18]. The mechanical properties of silk fiber can be modified by genetically disrupting the Ca2+ content during silk fiber formation [19]

Methods

Results

Conclusion