Study Of Soy Protein Isolate-Silver Composite And Graft Copolymer By Atom Transfer Radical Polymerization

Soy protein isolate (SPI) is a kind of natural macromolecule rich in kinds of amino acids, and soy protein isolate based materials are expected to be environment-friendly, biodegradable and reproducible products. We focused on novel strategies to create soy protein isolate-silver nanoparticle composite and soy protein isolate-polymer hybridsA simple route for the synthesis of soy protein isolate-silver nanoparticle composite using soy protein isolate (SPI) has been proposed in this work(PART 1). Well defined size distribution of silver nanoparticles are obtained by the method of UV irradiation of an aqueous solution containing silver nitrate (AgNO3) and SPI in the absence of any reducing agents and protecting agents. The purified composite solution exhibit a yellow color, indicating the formation of silver nanoparticles. Transmission electronic microscopy (TEM) revealed that the silver nanoparticles were well dispersed with an average diameter about 13 nm, and X-ray diffraction (XRD) analysis of the freeze-dried powder of SPI-silver nanoparticle composite confirmed the formation of metallic silver. UV-Vis spectrum showed that the purified solution yielded the maximum absorbance at about 430 nm due to surface plasmon resonance of the silver nanoparticles. Infrared spectroscopy confirmed that the polypeptide backbone of SPI was not cleaved during the conjugation process and some active amino groups were oxidized. The formation of silver nanoparticles might be related with the reduction and photochemistry between SPI and UV light. The size and size distribution of the resulting silver nanoparticles were dependent on pH and UV irradiation time. SPI-Ag nanoparticle composite showed excellent antibacterial activity against two representative bacteria, Staphylococcus aureus (Gram positive) and Escherichia coli (Gram negative) in the presence of SPI. Apart from ecofriendliness and easy availability, this work will give us an alternate method to synthesize silver nanoparticles.In part 2 of this paper, we described the synthesis of poly(2-hydroxythyl methacrylate) grafted soy protein isolates(SPI-g-PHEMA) using ATRP. Poly(2-hydroxythyl methacrylate) (PHEMA) is one of most extensively studied biocompatible polymer, which has been used for applications such as contact lenses. The scheme of the work was that the soy protein isolate macroinitiator (SPI-Br) was synthesized first by reacting the active amino groups with 2-bromoisobutyryl bromide, and SPI-g-PHEMA was prepared by atom transfer radical polymerization of HEMA in aqueous solution.The soy protein isolate macroinitiator (SPI-Br) was prepared by condensation reaction of the active amino groups and 2-bromopropionyl bromide, and the resulting secondary bromide could be used as initiator for HEMA monomer in atom transfer radical polymerization. FTIR and ¹³C-NMR spectra confirmed the conversion from SPI sequence to SPI-Br.Atom transfer radical polymerization of 2-hydroxythyl methacrylate (HEMA) on SPI was carried out in aqueous solution using SPI-Br as initiator, copper chloride/bipyridine (CuCl/bpy) as catalyst. FTIR spectrum was used to characterize the successful polymerization of HEMA onto SPI globulin. The graft chain was cleaved from SPI-g-PHEMA by degradation of hydrochloric acid for purpose of the measurement of molecular weight of graft chain by GPC. The changes in the spectra of SPI, SPI-Br, SPI-g-PHEMA and grafted chain confirmed successive steps in the synthesis of SPI-g-PHEMA hybrids. A series of comparative experiments were taken with different reaction mediums(H₂O, H₂O/ n-propanol, n-propanol), and higher molecular weight result was gained in aqueous solution by atom transfer radical polymerization. The number average molecular weight was about 11.6×10³ and a polydispersity was about 1.12, so the water medium system was chosen for further study. The molecule weight and the polydispersity of graft chain cleaved from SPI-g-PHEMA prepared via ATRP initiated by soy protein isolate macroinitiator was similar to those of PHEMA prepared by normal ATRP using 2-bromoisobutyryl bromide without SPI in solution under similar reaction conditions.The solution properties of SPI-g-PHEMA were characterized by UV and Fluorescent spectrophotometer. The results indicated that with increasing of pH value, the solubility of SPI and SPI-g-PHEMA increased, and the hydrophobicity decreased. The zeta-potential of SPI and SPI-g-PHEMA solutions were also studied by zeta-potentiometer. The static charges on the surface of SPI molecules increased clearly after graft modification. And the values of the zeta potential of SPI and SPI-g-PHEMA solutions decreased with the increase of pH values. The TEM images showed that the diameter of SPI-g-PHEMA was beyond that of SPI, and the morphologies between SPI and SPI-g-PHEMA were different.