Robin Rajan

Cryoprotective properties of completely synthetic polyampholytes via reversible addition-fragmentation chain transfer (RAFT) polymerization and the effects of hydrophobicity.

A completely synthetic polyampholyte cryoprotectant was developed with cationic and anionic monomers by reversible addition-fragmentation chain transfer polymerization. The neutralized random polyampholyte, which had an equal composition ratio of monomers, showed high cryoprotective properties in mammalian cells. Introduction of a small amount of hydrophobic monomer enhanced cell viability after cryopreservation, indicating the importance of hydrophobicity. Leakage experiments confirmed that these polyampholytes protected the cell membrane during cryopreservation. Due to low cytotoxicity, this polyampholyte has the potential to replace the convention cryoprotective agent dimethyl sulfoxide. The present study is the first to show that we can design a polymeric cryoprotectant that will protect the cell membrane during freezing using appropriate polymerization techniques.

Toward a Molecular Understanding of the Mechanism of Cryopreservation by Polyampholytes: Cell Membrane Interactions and Hydrophobicity

Cryopreservation enables long-term preservation of cells at ultralow temperatures. Current cryoprotective agents (CPAs) have several limitations, making it imperative to develop CPAs with advanced properties. Previously, we developed a novel synthetic polyampholyte-based CPA, copolymer of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and methacrylic acid(MAA) (poly(MAA-DMAEMA)), which showed excellent efficiency and biocompatibility. Introduction of hydrophobicity increased its efficiency significantly. Herein, we investigated the activity of other polyampholytes. We prepared two zwitterionic polymers, poly(sulfobetaine) (SPB) and poly(carboxymethyl betaine) (CMB), and compared their efficiency with poly(MAA-DMAEMA). Poly-SPB showed only intermediate property and poly-CMB showed no cryoprotective property. These data suggested that the polymer structure strongly influences cryoprotection, providing an impetus to elucidate the molecular mechanism of cryopreservation. We investigated the mechanism by studying the interaction of polymers with cell membrane, which allowed us to identify the interactions responsible for imparting different properties. Results unambiguously demonstrated that polyampholytes cryopreserve cells by strongly interacting with cell membrane, with hydrophobicity increasing the affinity for membrane interaction, which enables it to protect the membrane from various freezing-induced damages. Additionally, cryoprotective polymers, especially their hydrophobic derivatives, inhibit the recrystallization of ice, thus averting cell death. Hence, our results provide an important insight into the complex mechanism of cryopreservation, which might facilitate the rational design of polymeric CPAs with improved efficiency.

A zwitterionic polymer as a novel inhibitor of protein aggregation

We report the novel one-step synthesis of a zwitterionic polymer, polysulfobetaine, via living reversible addition fragmentation chain transfer (RAFT) polymerization. Lysozyme did not aggregate when heated in the presence of this polymer. Amyloid formation, the cause of many diseases, was also suppressed. The zwitterionic polymer was significantly more efficient than previously described inhibitors of protein aggregation. Lysozyme heated in the presence of polysulfobetaine retained its solubility and very high enzymatic efficiency, even after prolonged heating. The secondary structures of lysozyme change with increasing temperature, accompanied by an increase in the β-structure. This change was prevented by mixing the polymer with lysozyme. 1H-NMR before and after aggregation revealed the conformational changes taking place in the lysozyme: during aggregation, lysozyme is transformed into a random coil conformation, thus losing its secondary structure. Presence of the polymer facilitates retention of partial higher order structures and lysozyme solubility at higher temperatures. The high efficiency of the polyampholyte was ascribed to its ability to prevent collisions between aggregating species by acting as a molecular shield.

Inhibition of protein aggregation by zwitterionic polymer-based core-shell nanogels

Protein aggregation is a process by which misfolded proteins polymerizes into aggregates and forms fibrous structures with a β-sheet conformation, known as amyloids. It is an undesired outcome, as it not only causes numerous neurodegenerative diseases, but is also a major deterrent in the development of protein biopharmaceuticals. Here, we report a rational design for the synthesis of novel zwitterionic polymer-based core-shell nanogels via controlled radical polymerization. Nanogels with different sizes and functionalities in the core and shell were prepared. The nanogels exhibit remarkable efficiency in the protection of lysozyme against aggregation. Addition of nanogels suppresses the formation of toxic fibrils and also enables lysozyme to retain its enzymatic activity. Increasing the molecular weight and degree of hydrophobicity markedly increases its overall efficiency. Investigation of higher order structures revealed that lysozyme when heated without any additive loses its secondary structure and transforms into a random coil conformation. In contrast, presence of nanogels facilitates the retention of higher order structures by acting as molecular chaperones, thereby reducing molecular collisions. The present study is the first to show that it is possible to design zwitterionic nanogels using appropriate polymerization techniques that will protect proteins under conditions of extreme stress and inhibit aggregation.

Tunable Dual-Thermoresponsive Core–Shell Nanogels Exhibiting UCST and LCST Behavior

Thermoresponsive polymers change their physical properties as the temperature is changed and have found extensive use in a number of fields, especially in tissue engineering and in the development of drug delivery systems. The synthesis of a novel core-shell nanogel composed of N-isopropylacrylamide and sulfobetaine by reversible addition fragmentation chain transfer polymerization is reported. The core-shell architecture of the nanogels is confirmed using energy dispersive X-ray spectroscopy in scanning transmission electron microscopy. These nanogels exhibit dual thermoresponsive behavior, i.e., the core of the nanogel exhibits lower critical solution temperature, while the shell displays upper critical solution temperature behavior. Transition temperatures can be easily tuned by changing the molecular weight of the constituent polymer. These nanogels can be efficiently used in temperature-triggered delivery of therapeutic proteins and drugs.

Development and Application of Cryoprotectants

Cryopreservation involves the preservation of biological materials, including cells, embryos, tissues, and organs, at ultra-low temperatures (in a state of suspended animation), for a long period of time, and in a way that allows them to be restored whenever required. Freezing of biological samples is generally accompanied by numerous undesirable outcomes such as intra- and extracellular freezing damage and osmotic stress. To prevent these adverse effects, cryoprotective agents (CPAs) are added to biological materials before freezing. Over the years, a number of CPAs have been identified and developed and have been employed successfully for numerous applications. Here, we review the history and development of cryoprotectants and the current understanding of the cryopreservation process. We conclude with a discussion about the application of cryopreservation for various clinical and academic studies.

Zwitterionic Polymer Design that Inhibits Aggregation and Facilitates Insulin Refolding: Mechanistic Insights and Importance of Hydrophobicity

The synthesis of a zwitterionic polymer, poly-sulfobetaine (poly-SPB), which shows remarkable efficiency in the suppression of insulin aggregation is described. Hydrophobic modification of the polymer results in almost complete inhibition at very low polymer concentrations. Further studies reveal that these polymers facilitate the complete retention of insulins secondary structure, which is otherwise lost after incubation. Refolding studies show that addition of polymers to preaggregated insulin sample leads to the refolding of denatured insulin, indicating their potential to facilitate the refolding of the denatured proteins. In addition, 2D NMR studies show that the presence of hydrophobic poly-SPB alters the hydrophobic environment of insulin, which may suppress hydrophobic interactions that lead to aggregation. These results indicate the enormous potential of these polymers for suppressing insulin aggregation and provide significant insight into the complex mechanism of insulin protection by zwitterionic polymers.