Hejazi, Sondos (2024) Poly-γ-Glutamic Acid Based Bioplastic. [Tesi di dottorato]

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Abstract

Inspired by the microorganism Bacillus species bacteria, this project explores the extraordinary world of poly-γ-glutamic acid (γ-PGA), a polypeptide with remarkable properties for diverse applications. An eco-friendly purification method of γ-PGA, based on an ultrafiltration process, was developed, and extensive characterization of low molecular weight purified fractions of γ-PGA (R1 and R2), revealing their remarkable properties and potential applications, was carried out. The use of purified R1 and R2 for biodegradable film preparation, using both casting and thermal pressing, was investigated, leading to the development of novel biomaterials. Building upon this success, an innovative blending approach between the anionic γ-PGA fractions and cationic chitosan (CH) was performed, employing a simple complexation method based on the physical electrostatic interaction between the two biopolymers. The resulting biomaterials were obtained as hydrogel in a hydrated form and a glass-like state in a dehydrated form. The novel biomaterials produced through this approach were characterized for their physicochemical features and their potential as bio-glue was explored. In this project fermentation techniques to produce high molecular weight γ-PGA, using two genetically modified Bacillus subtilis strains able to hyper-produce γ-PGA, were also investigated, and the resulting biopolymer was characterized. Subsequently, taking advantage of the crosslinking method previously exploited for the R1 and R2 fractions, the γ-PGA produced by the modified bacteria was used for the preparation of hydrogels thanks, once again, to its blending with CH, a biocompatible polymer derived from chitin, the second most abundant polysaccharide occurring in Nature. Also, in this case, new materials were manufactured and shown to be endowed with interesting characteristics for the potential development of biodegradable agents to be used in the biomedical field. During three months three-month stay at Wroclaw University, it was also explored the production of γ-PGA from bacterial strains isolated from local honey. Furthermore, a new spectrophotometric ninhydrin-based method, developed in foreign laboratories, enabled the reliable quantification of γ-PGA. In conclusion, it is possible to assume from the research carried out over the last three years that γ-PGA is an anionic polymer with great potential for sustainable and eco-compatible applications. Therefore, the general objective of the present Ph.D. project such as the optimization of the γ-PGA-based bioplastic production to obtain new materials was achieved, since before this study very little was known about the ability of polypeptides made up of the same amino acid unit to give rise to materials with plastic-like properties.

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