Date of Award

Spring 2022

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

First Advisor

John Tsavalas

Second Advisor

Erik Berda

Third Advisor

Nathan Oldenhuis

Abstract

Certain organisms living in cold regions have adapted different strategies to survive in harshly cold temperatures. Some of them use freeze-avoiding strategies in which they can prevent freezing by controlling the concentration of sugars (et. sucrose, trehalose) or polyols (glycerol), regulation of the ice nucleator, and dehydration. Other organisms have adapted to this extremely cold condition by producing antifreeze (gly)proteins (AF(G)Ps) which exhibit ice recrystallization inhibition (IRI), thermal hysteresis activity (THA), and dynamic ice crystal shaping. These proteins discovered in Antarctic fish in 1960 for the first time have been reported in bacteria, fungi, insects, and plants. AF(G)Ps and their synthetic biomimetics have received increasing attention as potential candidates for various industrial and bio-medical applications. Promising results from vitrification and other protocols using antifreeze agents with ice recrystallization inhibition activity have widely been reported in biopreservation. Conversely, understanding of the antifreezing process caused by these macromolecules remains under challenge. This is due to the multifunctional nature of the freezing process and antifreeze macromolecule’s behavior which brings complexity in designing the synthetic antifreeze structures. In addition, the cost, low availability, toxicity at higher concentrations, and instability beside several other drawbacks make their large-scale production challenging. Although several synthetic attempts for the exploitation of AFPs have been studied in the past, challenges remain in the synthetic design of AFP analogs. On the other hand, poly (vinyl alcohol) (PVA) with simple structure has been reported with potent IRI activity as a good candidate for large-scale production and applications. Our group has explored structural variations to polyol-based polymers to contrast with PVA as a control and identified several key structural elements for performance in IRI, THA, as well as in ice nucleation inhibition (INI). These structural features are bioinspired by the typical ice-binding plane of AFPs yet are surprisingly simple to produce with potency approaching that of typical AFPs. Key to the performance is positioning small organic functionalities with known antifreeze properties (glycerol) pendent to a host polymer chain with consideration of their conformational freedom. To build systematic variations into both the backbone and side-chain structures, we used poly (vinyl alcohol), poly (isopropenyl acetate), poly (acrylic acid), and poly(methacrylic acid) parent polymers for such pendent modifications. One structure in particular, glycerol-grafted-PVA (G-g-PVA), shows potency rivaling that of AFPs at similar micromolar concentration. The findings in this study help guide the rational design of synthetic antifreeze polymers useful for applications such as anti-icing coatings through to cryopreservation methods for organ transport and cell preservation. While AFPs are well-known for their ice nucleation and recrystallization inhibition activity along with controlling the ice crystal morphology, the contrasting behavior of ice nucleation promotion by AFPs and its key contribution to the whole antifreezing process also seems necessary to explore in this context. Here, silver iodide (AgI) has been used as an ice nucleator in different polymer solutions in ultra-pure water (UPW) to imitate the ice nucleation process by AFPs. PVA prepared by RAFT polymerization and our glycerol grafted derivative (G-g-PVA), now shown to be the most IRI active polymer to date, was investigated for its ice nucleation and recrystallization activity in AgI dispersion media. The results showed that the ice nucleation rate and temperature was significantly changed by adding the AgI dispersion in PVA and G-g-PVA solutions. The polymer solution in UPW containing AgI dispersion showed significant improvement in IRI activity compared the same polymer in PBS buffer solution. Our results demonstrate the considerable contribution of the ice nucleator in ice nucleation rate and temperature which enhances IRI activity of synthetic antifreeze polymers. These finding both aid our understanding of the ice nucleation promotion impact on synthetic polymers IRI activity along with engineering biomimetics for biomedical and industrial applications. Next, we focused our efforts to transfer these functionalities and performance to the solid-state interface with water. Aqueous dispersions of polymeric colloidal particles served as this substrate and were functionalized with either PVA or G-g-PVA grafted to their surfaces to contrast with performance of the same polymers strictly in the solution state. These functionalized colloids also can be applied as a continuous coating through latex film formation to assess anti-icing and ice-adhesion properties. While these systems also showed encouraging and potent activity, their performance was not enhanced compared to that of the solution state systems. This may have implications for fully solid-state anti-icing coatings, yet our attention then shifted from this scope of work to new funding which required again a solution state approach. In this final application, we explored PVA and G-g-PVA synthesized in our lab for their biopreservation aspects especially for red blood cell (RBC) cryopreservation at -80 °C. Our results again confirmed G-g-PVA to be an excellent candidate for cryopreservation and quite likely for organ cryopreservation. Using this polymer in solution as a cryoprotectant for RBCs showed significant improvement to controls, preventing hemolysis (cell rupture) along with eliminating other drawbacks that have been observed when using small molecule cryoprotective agents like glycerol, dimethyl sulfoxide (DMSO), etc.; especially with regard to the ability to fully remove all traces of the cryoprotectant after cryopreservation storage and thawing. In summary, the studies in this dissertation provide critical insights and approaches for the understanding of the freezing process and ideas that can help understand relevant mechanisms of influencing key freezing steps that have not yet been fully understood. In addition, it provides guidelines to synthesize G-g-PVA, currently the most potent active polymer in terms of IRI and THA shown useful for several high impact applications. In particular, this research provides valuable data and experimental conditions to understand the IRI mechanism to use in engineering next generation highly efficient antifreeze systems.

Download

Share

COinS