Total Synthesis of Homogeneous Antifreeze Glycopeptides and Glycoproteins

Glycosylated Polyhydroxyproline Is a Potent Antifreeze Molecule

Molecules that inhibit the growth of ice crystals are highly desirable for applications in building materials, foods, and agriculture. Antifreezes are particularly essential in biomedicine for tissue banking, yet molecules currently in use have known toxic effects. Antifreeze glycoproteins have evolved naturally in polar fish species living in subzero climates, but practical issues with collection and purification have limited their commercial use. Here, we present a synthetic strategy using polymerization of amino acid N-carboxyanhydrides to produce polypeptide mimics of these potent natural antifreeze proteins. We investigated a set of mimics with varied structural properties and identified a glycopolypeptide with potent ice recrystallization inhibition properties. We optimized for molecular weight, characterized their conformations, and verified their cytocompatibility in a human cell line. Overall, we present a material that will have broad applications as a biocompatible antifreeze.

Synthetic Antifreeze Glycoproteins with Potent Ice-Binding Activity

Antifreeze glycoproteins (AFGPs) are produced by extremophiles to defend against tissue damage in freezing climates. Cumbersome isolation from polar fish has limited probing AFGP molecular mechanisms of action and limited development of bioinspired cryoprotectants for application in agriculture, foods, coatings, and biomedicine. Here, we present a rapid, scalable, and tunable route to synthetic AFGPs (sAFGPs) using N-carboxyanhydride polymerization. Our materials are the first mimics to harness the molecular size, chemical motifs, and long-range conformation of native AFGPs. We found that ice-binding activity increases with chain length, Ala is a key residue, and the native protein sequence is not required. The glycan structure had only minor effects, and all glycans examined displayed antifreeze activity. The sAFGPs are biodegradable, nontoxic, internalized into endocytosing cells, and bystanders in cryopreservation of human red blood cells. Overall, our sAFGPs functioned as surrogates for bona fide AFGPs, solving a long-standing challenge in accessing natural antifreeze materials.

Ice Nucleation Promotion Impact on the Ice Recrystallization Inhibition Activity of Polyols

Heterogeneous ice nucleation occurs vis-à-vis nucleating agents already present in solution yet can occur within a rather broad range of temperatures (0 to ca. -38 °C). Controlling this temperature and the subsequent growth of resulting ice crystals is crucial for the survival of biological organisms (certain insects, fish, and plants that endure subzero temperatures), as well as in the context of medical cryopreservation and food science. In these environments, uncontrolled crystal shape and size can rupture the cell membrane causing irreversible and catastrophic damage. Antifreeze (AF) proteins and synthetic AF analogs address this issue to restrict crystal growth and to shape ice crystals. Yet, if the nucleation temperature is not controlled and occurs in a lower temperature range, nascent ice crystals will have grown to a significantly larger size before the AF agents can be active on their surface to halt or slow the Ostwald ripening process during recrystallization. At a higher nucleation temperature, diffusion of AF macromolecules is enhanced, and dynamic crystal shaping can start earlier, producing smaller crystals overall. While antifreeze proteins, the inspiration for these synthetic analogs, are always applied in a salt buffer aqueous environment (most typically phosphate-buffered saline (PBS) buffer), the heterogeneous nucleation events are stochastic and occur within a wide temperature range. Silver iodide (AgI), however, is a highly effective ice nucleation promoter as its crystal lattice structure is a 98% lattice match to the basal plane of hexagonal ice (Ih) crystals acting as a template for water molecule orientation and decreasing the interfacial free energy. Here, we expose the advantage of purposely seeding such nascent ice crystals with AgI at a defined and higher temperature (-7 °C) in ultrapure water (UPW) such that nucleation can only come from AgI (and also in AgI/PBS), resulting in the most potent synthetic IRI observed to date (at concentrations as low as 0.001 mg·mL^-1).

Revealing the Mechanism of Irreversible Binding of Antifreeze Glycoproteins to Ice

Antifreeze glycoproteins (AFGPs) are a special kind of antifreeze proteins with strong flexibility. Whether their antifreeze activity is achieved by reversibly or irreversibly binding to ice is widely debated, and the molecular mechanism of irreversible binding remains unclear. In this work, the antifreeze mechanism of the smallest AFGP isoform, AFGP8, is investigated at the atomic level. The results indicate that AFGP8 can bind to ice both reversibly through its hydrophobic methyl groups (peptide binding) and irreversibly through its hydrophilic disaccharide moieties (saccharide binding). Although peptide binding occurs faster than saccharide binding, free-energy calculations indicate that the latter is energetically more favorable. In saccharide binding, at least one disaccharide moiety is frozen in the grown ice, resulting in irreversible binding, while the other moieties significantly perturb the water hydrogen-bonding network, thus inhibiting ice growth more effectively. The present study reveals the coexistence of reversible and irreversible bindings of AFGP8, both contributing to the inhibition of ice growth and further provides molecular mechanism of irreversible binding.

Synthetic Thiol and Selenol Derived Amino Acids for Expanding the Scope of Chemical Protein Synthesis

Cells employ post-translational modifications (PTMs) as key mechanisms to expand proteome diversity beyond the inherent limitations of a concise genome. The ability to incorporate post-translationally modified amino acids into protein targets via chemical ligation of peptide fragments has enabled the access to homogeneous proteins bearing discrete PTM patterns and empowered functional elucidation of individual modification sites. Native chemical ligation (NCL) represents a powerful and robust means for convergent assembly of two homogeneous, unprotected peptides bearing an N-terminal cysteine residue and a C-terminal thioester, respectively. The subsequent discovery that protein cysteine residues can be chemoselectively desulfurized to alanine has ignited tremendous interest in preparing unnatural thiol-derived variants of proteogenic amino acids for chemical protein synthesis following the ligation-desulfurization logic. Recently, the 21st amino acid selenocysteine, together with other selenyl derivatives of amino acids, have been shown to facilitate ultrafast ligation with peptidyl selenoesters, while the advancement in deselenization chemistry has provided reliable bio-orthogonality to PTMs and other amino acids. The combination of these ligation techniques and desulfurization/deselenization chemistries has led to streamlined synthesis of multiple structurally-complex, post-translationally modified proteins. In this review, we aim to summarize the latest chemical synthesis of thiolated and selenylated amino-acid building blocks and exemplify their important roles in conquering challenging protein targets with distinct PTM patterns.

Diselenide-selenoester ligation in the chemical synthesis of proteins

Peptides and proteins represent an important class of biomolecules responsible for a plethora of structural and functional roles in vivo. Following their translation on the ribosome, the majority of eukaryotic proteins are post-translationally modified, leading to a proteome that is much larger than the number of genes present in a given organism. In order to understand the functional role of a given protein modification, it is necessary to access peptides and proteins bearing homogeneous and site-specific modifications. Accordingly, there has been significant research effort centered on the development of peptide ligation methodologies for the chemical synthesis of modified proteins. In this chapter we outline the discovery and development of a contemporary methodology called the diselenide-selenoester ligation (DSL) that enables the rapid and efficient fusion of peptide fragments to generate synthetic proteins. The practical aspects of using DSL for the preparation of chemically modified peptides and proteins in the laboratory is described. In addition, recent advances in the application of the methodology are outlined, exemplified by the synthesis and biological evaluation of a number of complex protein targets.

Site-Specific Water Dynamics in the First Hydration Layer of an Anti-Freeze Glyco-Protein: A Simulation Study

Antifreeze glycoproteins (AFGPs) inhibit ice recrystallization by a mechanism remaining largely elusive. Dynamics of AFGPs’ hydration water and its involvement in the antifreeze activity, for instance, have not been identified...

Glycosylation Regulates N-Terminal Proteolysis and Activity of the Chemokine CCL14

Chemokines are secreted proteins that regulate leukocyte migration during inflammatory responses by signaling through chemokine receptors. Full length CC chemokine ligand 14, CCL14(1-74), is a weak agonist for the chemokine receptor CCR1, but its activity is substantially enhanced upon proteolytic cleavage to CCL14(9-74). CCL14 is O-glycosylated at Ser7, adjacent to the site of proteolytic activation. To determine whether glycosylation regulates the activity of CCL14, we used native chemical ligation to prepare four homogeneously glycosylated variants of CCL14(1-74). Each protein was assembled from three synthetic peptide fragments in "one-pot" using two sequential ligation reactions. We show that while glycosylation of CCL14(1-74) did not affect CCR1 binding affinity or potency of activation, sialylated variants of CCL14(1-74) exhibited reduced activity after treatment with plasmin compared to nonsialylated forms. These data indicate that glycosylation may influence the biological activity of CCL14 by regulating its conversion from the full-length to the truncated, activated form.

Ice Inhibition for Cryopreservation: Materials, Strategies, and Challenges

Cryopreservation technology has developed into a fundamental and important supporting method for biomedical applications such as cell-based therapeutics, tissue engineering, assisted reproduction, and vaccine storage. The formation, growth, and recrystallization of ice crystals are the major limitations in cell/tissue/organ cryopreservation, and cause fatal cryoinjury to cryopreserved biological samples. Flourishing anti-icing materials and strategies can effectively regulate and suppress ice crystals, thus reducing ice damage and promoting cryopreservation efficiency. This review first describes the basic ice cryodamage mechanisms in the cryopreservation process. The recent development of chemical ice-inhibition molecules, including cryoprotectant, antifreeze protein, synthetic polymer, nanomaterial, and hydrogel, and their applications in cryopreservation are summarized. The advanced engineering strategies, including trehalose delivery, cell encapsulation, and bioinspired structure design for ice inhibition, are further discussed. Furthermore, external physical field technologies used for inhibiting ice crystals in both the cooling and thawing processes are systematically reviewed. Finally, the current challenges and future perspectives in the field of ice inhibition for high-efficiency cryopreservation are proposed.

Peptidic Antifreeze Materials: Prospects and Challenges

Necessitated by the subzero temperatures and seasonal exposure to ice, various organisms have developed a remarkably effective means to survive the harsh climate of their natural habitats. Their ice-binding (glyco)proteins keep the nucleation and growth of ice crystals in check by recognizing and binding to specific ice crystal faces, which arrests further ice growth and inhibits ice recrystallization (IRI). Inspired by the success of this adaptive strategy, various approaches have been proposed over the past decades to engineer materials that harness these cryoprotective features. In this review we discuss the prospects and challenges associated with these advances focusing in particular on peptidic antifreeze materials both identical and akin to natural ice-binding proteins (IBPs). We address the latest advances in their design, synthesis, characterization and application in preservation of biologics and foods. Particular attention is devoted to insights in structure-activity relations culminating in the synthesis of de novo peptide analogues. These are sequences that resemble but are not identical to naturally occurring IBPs. We also draw attention to impactful developments in solid-phase peptide synthesis and 'greener' synthesis routes, which may aid to overcome one of the major bottlenecks in the translation of this technology: unavailability of large quantities of low-cost antifreeze materials with excellent IRI activity at (sub)micromolar concentrations.

Bioinspired Ice Growth Inhibitors Based on Self-Assembling Peptides

Antifreeze proteins (AFPs) are widely found in organisms living in subzero environments. Their strong ability to inhibit ice growth and recrystallization have inspired considerable bioinspired efforts to engineer artificial ice growth inhibitors for cryopreservation. However, it remains challenging to engineer biocompatible and cost-effective synthetic ice growth inhibitors to meet the increasing needs of cryoprotectants in biomedical research and industry. Here we report the design of artificial ice growth inhibitors based on self-assembling peptides. We demonstrate the importance of threonine residues as well as their spatial arrangement for effective ice binding. The engineered self-assembling ice growth inhibiting peptides show moderate ice inhibiting activity including suppression of ice growth rates and retardation of recrystallization of ice crystals. The applications of these peptides in cryopreservation of enzymes and cells were also demonstrated.

Photo-cleavable purification/protection handle assisted synthesis of giant modified proteins with tandem repeats

Proteins with tandem repeats have essential physical or biological roles in cells and have been widely investigated as biomaterials or vaccines. Chemically derived proteins with tandem repeats would be beneficial for research, owing to their accurate structures, possibly with precise modifications, produced by chemical synthesis. Traditional protein synthesis often leads to severe handling loss due to multiple ligations and HPLC purifications. To improve the protein synthesis efficiency, we developed a purification/protection handle consisting of a His6 tag and a photo-labile linker. This handle has great potential to facilitate purification with immobilized metal affinity chromatography techniques and also provides orthogonal protection for N-terminal Cys. The synthesis of the model proteins Muc1 and antifreeze glycoprotein mimics shows that the handle decreases the requirement for HPLC and enables both convergent and sequential assembly of peptide segments.

Na2CO3-promoted thioesterification via N–C bond cleavage of amides to construct thioester derivatives

A mild, efficient, and transition-metal-free catalytic strategy is developed to construct thioesters via selective N–C bond cleavage of Boc2-activated primary amides. This strategy is successfully carried out with stoichiometric Na2CO3 as the base and provides the corresponding products in moderate to excellent yields.

Elucidating the role of key structural motifs in antifreeze glycoproteins

Antifreeze glycoproteins (AFGPs) are distinctively riveting class of bio-macromolecules, which endows the survival of organisms inhabiting polar and subpolar regions. These proteins are believed to hinder microscopic freezing by interacting with embryonic ice crystals and precluding their further growth. The underlying molecular mechanism by which AFGPs bind to ice has remained elusive due to insufficient structural characterization, with conflicting hypotheses on the possible binding mode of AFGPs - either via the hydrophobic peptide backbone or via the hydrophilic carbohydrate side chains - when interacting with ice. Chemical synthesis has allowed researchers to access synthetic variants of natural AFGPs. These studies revealed that AFGPs exhibit huge variations in their thermal hysteresis and ice shaping behavior with only slight structural variations, especially to the carbohydrate side chains. Four key structural motifs were identified as crucial to AFGP activity: the presence of a threonine γ-methyl group, an α-glycosidic carbohydrate-protein linkage, an acetylamide group (-NHCOCH3) at the C2 position of the carbohydrate linked to the protein, and the presence of carbohydrate hydroxyl groups. In this study, we use molecular dynamics (MD) simulations to probe the microscopic properties of water accompanying these structural variations of AFGPs. We find that these variations primarily influence the conformation space of AFGPs and also crucially control their hydration dynamics. Owing to the disordered nature of AFGPs we use Markov-state modeling to identify the conformational preferences of AFGPs. The simulations reveal the importance of steric bulk, intra-molecular carbohydrate-protein H-bonds and conformational preferences (α- vs. β-linkages) in controlling the spatial segregation of the hydrophilic and hydrophobic regions of AFGPs. We hypothesize that the hydrophobic component of AFGPs is crucial to their binding to ice, which determines the ice shaping ability of AFGPs. However, the hydrophilic carbohydrate hydroxyl groups and their ability to form water bridges control the subsequent hydration dynamics, which is key to the antifreeze properties. Investigating the tetrahedral order parameter of water molecules around the carbohydrates revealed competition between solute- and bulk-influenced solvent structures, with maximum restructuring being observed in the interfacial region 2.5-4.5 Å away from the AFGPs.

Trehalose-functional glycopeptide enhances glycerol-free cryopreservation of red blood cells

Arginine- and trehalose-modified ε-polylysine (ε-PL) demonstrated a high synergistic function with trehalose for RBC cryopreservation.

Metal-free thioesterification of amides generating acyl thioesters

A base-initiated thioesterification of amides with various thiols is reported. This reaction can take place efficiently under metal-free and air-atmospheric conditions, and provides a facile and practically useful approach to the synthesis of valuable acyl thioesters.

Ultrasound-enhanced Ag-catalyzed decarboxylative coupling between α-keto acids and disulfides for the synthesis of thioesters

Herein, we described the ultrasound-assisted synthesis of thioesters via the Ag-catalyzed radical oxidative decarboxylation of α-keto acids, in the presence of disulfides. This protocol takes advantage of the sonication to prepare the title compounds in moderate to very good yields, in only 20 min of reaction. The positive effect of ultrasonic irradiation is attributed to both, the high mass transfer efficiency and to the induced radical formation in the reaction medium.

Design and synthesis of galactose-conjugated fluorinated and non-fluorinated proline oligomers: towards antifreeze molecules

Galactose-conjugated fluorinated and non-fluorinated proline oligomers that exhibit an α-helical structure with hydrophilic and lipophilic parts were designed as potential antifreeze molecules. These galactose-proline oligomers were synthesized and their physical properties were evaluated. Interestingly, the non-fluorinated galactose-proline oligomers showed in contrast to the fluorinated analogues weak antifreeze activity. The difference in antifreeze activity should be attributed to the fluorine gauche effect, which should induce a conformation in fluorinated prolines that is different from that of natural proline. The results obtained in this study thus suggest that the 3D conformation of the galactose-conjugated fluorinated and non-fluorinated proline oligomers is very important for their anti-freezing properties.

Facially Amphipathic Glycopolymers Inhibit Ice Recrystallization

Antifreeze glycoproteins (AFGPs) from polar fish are the most potent ice recrystallization (growth) inhibitors known, and synthetic mimics are required for low-temperature applications such as cell cryopreservation. Here we introduce facially amphipathic glycopolymers that mimic the three-dimensional structure of AFGPs. Glycopolymers featuring segregated hydrophilic and hydrophobic faces were prepared by ring-opening metathesis polymerization, and their rigid conformation was confirmed by small-angle neutron scattering. Ice recrystallization inhibition (IRI) activity was reduced when a hydrophilic oxo-ether was installed on the glycan-opposing face, but significant activity was restored by incorporating a hydrophobic dimethylfulvene residue. This biomimetic strategy demonstrates that segregated domains of distinct hydrophilicity/hydrophobicity are a crucial motif to introduce IRI activity, which increases our understanding of the complex ice crystal inhibition processes.

Antifreeze Glycoproteins Bind Reversibly to Ice via Hydrophobic Groups

Antifreeze molecules allow organisms to survive in subzero environments. Antifreeze glycoproteins (AFGPs), produced by polar fish, are the most potent inhibitors of ice recrystallization. To date, the molecular mechanism by which AFGPs bind to ice has not yet been elucidated. Mutation experiments cannot resolve whether the binding occurs through the peptide, the saccharides, or both. Here, we use molecular simulations to determine the mechanism and driving forces for binding of AFGP8 to ice, its selectivity for the primary prismatic plane, and the molecular origin of its exceptional ice recrystallization activity. Consistent with experiments, AFGP8 in simulations preferentially adopts the PPII helix secondary structure in solution. We show that the segregation of hydrophilic and hydrophobic groups in the PPII helix is vital for ice binding. Binding occurs through adsorption of methyl groups of the peptide and disaccharides to ice, driven by the entropy of dehydration of the hydrophobic groups as they nest in the cavities at the ice surface. The selectivity to the primary prismatic plane originates in the deeper cavities it has compared to the basal plane. We estimate the free energy of binding of AFGP8 and the longer AFGPs4-6, and find them to be consistent with the reversible binding demonstrated in experiments. The simulations reveal that AFGP8 binds to ice through a myriad of conformations that it uses to diffuse through the ice surface and find ice steps, to which it strongly adsorbs. We interpret that the existence of multiple, weak binding sites is the key for the exceptional ice recrystallization inhibition activity of AFGPs.

Palladium-catalyzed carbonylation of thioacetates and aryl iodides for the synthesis of S-aryl thioesters

Palladium-catalyzed carbonylation of thioacetates and aryl iodides provided thioesters in good yields.

Polymer mimics of biomacromolecular antifreezes

Antifreeze proteins from polar fish species are remarkable biomacromolecules which prevent the growth of ice crystals. Ice crystal growth is a major problem in cell/tissue cryopreservation for transplantation, transfusion and basic biomedical research, as well as technological applications such as icing of aircraft wings. This review will introduce the rapidly emerging field of synthetic macromolecular (polymer) mimics of antifreeze proteins. Particular focus is placed on designing polymers which have no structural similarities to antifreeze proteins but reproduce the same macroscopic properties, potentially by different molecular-level mechanisms. The application of these polymers to the cryopreservation of donor cells is also introduced.

From ice-binding proteins to bio-inspired antifreeze materials

Ice-binding proteins (IBP) facilitate survival under extreme conditions in diverse life forms. IBPs in polar fishes block further growth of internalized environmental ice and inhibit ice recrystallization of accumulated internal crystals. Algae use IBPs to structure ice, while ice adhesion is critical for the Antarctic bacterium Marinomonas primoryensis. Successful translation of this natural cryoprotective ability into man-made materials holds great promise but is still in its infancy. This review covers recent advances in the field of ice-binding proteins and their synthetic analogues, highlighting fundamental insights into IBP functioning as a foundation for the knowledge-based development of cheap, bio-inspired mimics through scalable production routes. Recent advances in the utilisation of IBPs and their analogues to e.g. improve cryopreservation, ice-templating strategies, gas hydrate inhibition and other technologies are presented.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Antifreeze glycopeptides: from structure and activity studies to current approaches in chemical synthesis

Antifreeze glycopeptides (AFGPs) are a class of biological antifreeze agents found predominantly in Arctic and Antarctic species of fish. They possess the ability to regulate ice nucleation and ice crystal growth, thus creating viable life conditions at temperatures below the freezing point of body fluids. AFGPs usually consist of 4–55 repetitions of the tripeptide unit Ala–Ala–Thr that is O-glycosylated at the threonine side chains with β-d-galactosyl-(1 → 3)-α-N-acetyl-d-galactosamine. Due to their interesting properties and high antifreeze activity, they have many potential applications, e.g., in food industry and medicine. Current research is focused towards understanding the relationship between the structural preferences and the activity of the AFGPs, as well as developing time and cost efficient ways of synthesis of this class of molecules. Recent computational studies in conjunction with experimental results from NMR and THz spectroscopies were a possible breakthrough in understanding the mechanism of action of AFGPs. At the moment, as a result of these findings, the focus of research is shifted towards the analysis of behaviour of the hydration shell around AFGPs and the impact of water-dynamics retardation caused by AFGPs on ice crystal growth. In the field of organic synthesis of AFGP analogues, most of the novel protocols are centered around solid-phase peptide synthesis and multiple efforts are made to optimize this approach. In this review, we present the current state of knowledge regarding the structure and activity of AFGPs, as well as approaches to organic synthesis of these molecules with focus on the most recent developments.

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Enhanced non-vitreous cryopreservation of immortalized and primary cells by ice-growth inhibiting polymers

Cell cryopreservation is an essential tool in modern biotechnology and medicine. The ability to freeze, store and distribute materials underpins basic cell biology and enables storage of donor cells needed for transplantation and regenerative medicine. However, many cell types do not survive freezing and the current state-of-the-art involves the addition of significant amounts of organic solvents as cryoprotectants, which themselves can be cytotoxic, or simply interfere with assays. A key cause of cell death in cryopreservation is ice recrystallization (growth), which primarily occurs during thawing. Here it is demonstrated that the addition of ice recrystalization inhibiting polymers to solutions containing low (non vitrifying) concentrations of DMSO enhance cell recovery rates by up to 75%. Cell functionality is also demonstrated using a placental cell line, and enhanced cryopreservation of primary rat hepatocytes is additionally shown. The crucial role of the polymers architecture (chain length) is shown, with shorter polymers being more effective than longer ones.

Activation of Ice Recrystallization Inhibition Activity of Poly(vinyl alcohol) using a Supramolecular Trigger

Antifreeze (glyco)proteins (AF(G)Ps) have potent ice recrystallisation inhibition (IRI) activity - a desirable phenomenon in applications such as cryopreservation, frozen food and more. In Nature AF(G)P activity is regulated by protein expression levels in response to an environmental stimulus; temperature. However, this level of regulation is not possible in synthetic systems. Here, a synthetic macromolecular mimic is introduced, using supramolecular assembly to regulate activity. Catechol-terminated poly(vinyl alcohol) was synthesised by RAFT polymerization. Upon addition of Fe3+, larger supramolecular star polymers form by assembly with two or three catechols. This increase in molecular weight effectively 'switches on' the IRI activity and is the first example of external control over the function of AFP mimetics. This provides a simple but elegant solution to the challenge of external control of AFP-mimetic function.

Probing the Biomimetic Ice Nucleation Inhibition Activity of Poly(vinyl alcohol) and Comparison to Synthetic and Biological Polymers

Nature has evolved many elegant solutions to enable life to flourish at low temperatures by either allowing (tolerance) or preventing (avoidance) ice formation. These processes are typically controlled by ice nucleating proteins or antifreeze proteins, which act to either promote nucleation, prevent nucleation or inhibit ice growth depending on the specific need, respectively. These proteins can be expensive and their mechanisms of action are not understood, limiting their translation, especially into biomedical cryopreservation applications. Here well-defined poly(vinyl alcohol), synthesized by RAFT/MADIX polymerization, is investigated for its ice nucleation inhibition (INI) activity, in contrast to its established ice growth inhibitory properties and compared to other synthetic polymers. It is shown that ice nucleation inhibition activity of PVA has a strong molecular weight dependence; polymers with a degree of polymerization below 200 being an effective inhibitor at just 1 mg.mL(-1). Other synthetic and natural polymers, both with and without hydroxyl-functional side chains, showed negligible activity, highlighting the unique ice/water interacting properties of PVA. These findings both aid our understanding of ice nucleation but demonstrate the potential of engineering synthetic polymers as new biomimetics to control ice formation/growth processes.

Modulation of antifreeze activity and the effect upon post-thaw HepG2 cell viability after cryopreservation

Most antifreeze proteins (AFPs) exhibit two types of "antifreeze activity" - thermal hysteresis (TH) and ice recrystallization inhibition (IRI) activity. The mechanism of TH activity has been studied in depth and is the result of an adsorption of AFPs to the surface of ice with an ice-binding face (IBF). In contrast, the mechanism of ice recrystallization and its inhibition is considerably less understood. In this paper, we examine several different antifreeze proteins, glycoproteins and mutants of the Lolium perenne AFP (LpAFP) to understand how IRI activity is modulated independently of TH activity. This study also examines the ability of the various AF(G)Ps to protect HepG2 cells from cryoinjury. Post-thaw cell viabilities are correlated to TH, IRI activity as well as dynamic ice shaping ability and single ice crystal growth progressions. While these results demonstrate that AF(G)Ps are ineffective as cryoprotectants, they emphasize how ice crystal habit and most importantly, ice growth progression affect HepG2 cell survival during cryopreservation.

Formation of C(sp2)–S bonds through decarboxylation of α-oxocarboxylic acids with disulfides or thiophenols

Copper-catalyzed decarboxylative coupling between α-oxocarboxylic acids and diphenyl disulfides or thiophenols is presented, which provided an effective and direct approach for the preparation of useful thioesters through C(sp²)–S bond formation.