Ice-binding proteins and the applicability and limitations of the kinetic pinning model

Extended Temperature Range of the Ice-Binding Protein Activity

Ice-binding proteins (IBPs) are expressed in various organisms for several functions, such as protecting them from freezing and freeze injuries. Via adsorption on ice surfaces, IBPs depress ice growth and recrystallization and affect nucleation and ice shaping. IBPs have shown promise in mitigating ice growth under moderate supercooling conditions, but their functionality under cryogenic conditions has been less explored. In this study, we investigate the impact of two types of antifreeze proteins (AFPs): type III AFP from fish and a hyperactive AFP from an insect, the Tenebrio molitor AFP, in vitrified dimethylsulfoxide (DMSO) solutions. We report that these AFPs depress devitrification at -80 °C. Furthermore, in cases where devitrification does occur, AFPs depress ice recrystallization during the warming stage. The data directly demonstrate that AFPs are active at temperatures below the regime of homogeneous nucleation. This research paves the way for exploring AFPs as potential enhancers of cryopreservation techniques, minimizing ice-growth-related damage, and promoting advancements in this vital field.

Psychrophiles to control ice-water phase changes in frost-susceptible soils

The phase changes of soil water or porous media have a crucial influence on the performance of natural and man-made infrastructures in cold regions. While various methods have been explored to address the impacts of frost-action arising from these phase changes, conventional approaches often rely on chemicals, mechanical techniques, and the reuse of waste materials, which often exhibit certain limitations and environmental concerns. In contrast, certain organisms produce ice-binding proteins (IBPs) or antifreeze proteins (AFPs) to adapt to low temperatures, which can inhibit ice crystal growth by lowering the freezing point and preventing ice crystallization without the need for external intervention. This study explores the potential of three psychrophilic microbes: Sporosarcina psychrophile, Sporosarcina globispora, and Polaromonas hydrogenivorans, to induce non-equilibrium freezing point depression and thermal hysteresis in order to control ice lens growth in frost-susceptible soils. We hypothesize that the AFPs produced by psychrophiles will alter the phase changes of porous media in frost-susceptible soils. The growth profiles of the microbes, the concentration of released proteins in the extracellular solution, and the thermal properties of the protein-mixed soils are monitored at an interval of three days. The controlled soil showed a freezing point of - 4.59 °C and thermal hysteresis of 4.62 °C, whereas protein-treated soil showed a maximum freezing point depression of - 8.54 °C and thermal hysteresis of 7.71 °C. Interestingly, except for the controlled sample, all the protein-treated soil samples were thawed at a negative temperature (minimum recorded at - 0.85 °C). Further analysis showed that the treated soils compared to porous media mixed soil freeze (1.25 °C vs. 0.51 °C) and thaw (2.75 °C vs. 1.72 °C) at extensive temperature gap. This freezing and thawing temperature gap is the temperature difference between the beginning of ice core formation and completed frozen, and the beginning of ice core thawing and completed thawed for the treated soil samples selected from different incubation days. Overall, this study presents a novel bio-mediated approach using psychrophilic microbes to control ice formation in frost-susceptible soils.

Divergent Mechanisms of Ice Growth Inhibition by Antifreeze Proteins

Antifreeze proteins (AFPs) are biomolecules that can bind to ice and hinder its growth, thus holding significant potential for biotechnological and biomedical applications. AFPs are a subset of ice-binding proteins (IBPs) and are found in various organisms across different life kingdoms. This mini-review investigates the underlying mechanisms by which AFPs impede ice growth, emphasizing the disparities between hyperactive and moderate AFPs. Hyperactive AFPs exhibit heightened thermal hysteresis (TH) activity and can bind to both the basal and prism planes of ice crystals, enabling them to endure extremely cold temperatures. In contrast, moderate AFPs predominantly bind to the prism/pyramidal planes and demonstrate lower TH activity. The structural diversity of AFPs and the presence of ordered water molecules on their ice-binding sites (IBS) have been subjects of debate among researchers. Multiple hypotheses have been proposed concerning the significance of ordered water molecules in ice binding. Gaining insights into the binding dynamics and the factors influencing TH activity in AFPs is crucial for the development of efficient synthetic compounds and the establishment of comprehensive models to elucidate ice growth inhibition. Here we emphasize the necessity for further research to unravel the mechanisms of AFPs and presents a pathway for constructing models capable of comprehensively explaining their inhibitory effects on ice growth.

Corn cob hemicelluloses as stabilizer for ice recrystallization inhibition in ice cream

Food stabilizers, such as guar gum and locust bean gum (LBG), are often added to ice cream to improve its texture and to combat its main shelf-life concern - ice recrystallization. Recently these gums have become increasingly expensive due to the limited supplies. In this study, holocellulose nanocrystals (holoCNCs) and hemicelluloses (hemiCs) were prepared from readily available corn cobs and tested for ice recrystallization inhibition (IRI) activities in the 25.0 % sucrose solution and ice cream mixes (ICMs). In the sucrose solution, holoCNCs were not IRI active at a concentration of 0.5 %, but hemiCs demonstrated a good IRI activity, even at 0.1 %. In the ICMs, the IRI activity of hemiCs was better than those of guar gum and LBG at a concentration of 0.2 %. Adding 0.2-0.5 % hemiCs had no negative influences on the physicochemical properties of ICMs and ice cream, including viscosity profile, particle size distribution, overrun, hardness, and meltdown rate. These research findings demonstrated corn cob hemiCs' potential as a more sustainable ice cream stabilizer.

Free energy barriers for anti-freeze protein engulfment in ice: Effects of supercooling, footprint size, and spatial separation

Anti-freeze proteins (AFPs) protect organisms at freezing conditions by attaching to the ice surface and arresting its growth. Each adsorbed AFP locally pins the ice surface, resulting in a metastable dimple for which the interfacial forces counteract the driving force for growth. As supercooling increases, these metastable dimples become deeper, until metastability is lost in an engulfment event where the ice irreversibly swallows the AFP. Engulfment resembles nucleation in some respects, and this paper develops a model for the "critical profile" and free energy barrier for the engulfment process. Specifically, we variationally optimize the ice-water interface and estimate the free energy barrier as a function of the supercooling, the AFP footprint size, and the distance to neighboring AFPs on the ice surface. Finally, we use symbolic regression to derive a simple closed-form expression for the free energy barrier as a function of two physically interpretable, dimensionless parameters.

Nanoscopy of single antifreeze proteins reveals that reversible ice binding is sufficient for ice recrystallization inhibition but not thermal hysteresis

Antifreeze proteins (AFPs) bind ice to reduce freezing temperatures and arrest ice crystal ripening, making AFPs essential for the survival of many organisms in ice-laden environments and attractive as biocompatible antifreezes in many applications. While their activity was identified over 50 years ago, the physical mechanisms through which they function are still debated because experimental insights at the molecular scale remain elusive. Here, we introduce subzero nanoscopy by the design and incorporation of a freezing stage on a commercial super-resolution setup to resolve the interfacial dynamics of single AFPs with ice crystal surfaces. Using this method, we demonstrate irreversible binding and immobilization (i.e., pinning) of individual proteins to the ice/water interface. Surprisingly, pinning is lost and adsorption becomes reversible when freezing point depression activity, but not ice recrystallization inhibition, is eliminated by a single mutation in the ice-binding site of the AFP. Our results provide direct experimental evidence for the adsorption-inhibition paradigm, pivotal to all theoretical descriptions of freezing point depression activity, but also reveal that reversible binding to ice must be accounted for in an all-inclusive model for AFP activity. These mechanistic insights into the relation between interfacial interactions and activity further our understanding and may serve as leading principles in the future design of highly potent, biocompatible antifreezes with tunable affinity.

Oriented attachment kinetics for rod-like particles at a flat surface: Buffon's needle at the nanoscale

The adsorption of large rod-like molecules or crystallites on a flat crystal face, similar to Buffon's needle, requires the rods to "land," with their binding sites in precise orientational alignment with matching sites on the surface. An example is provided by long, helical antifreeze proteins (AFPs), which bind at specific facets and orientations on the ice surface. The alignment constraint for adsorption, in combination with the loss in orientational freedom as the molecule diffuses toward the surface, results in an entropic barrier that hinders the adsorption. Prior kinetic models do not factor in the complete geometry of the molecule, nor explicitly enforce orientational constraints for adsorption. Here, we develop a diffusion-controlled adsorption theory for AFP molecules binding at specific orientations to flat ice surfaces. We formulate the diffusion equation with relevant boundary conditions and present analytical solutions to the attachment rate constant. The resulting rate constant is a function of the length and aspect ratio of the AFP, the distance threshold associated with binding, and solvent conditions such as temperature and viscosity. These results and methods of calculation may also be useful for predicting the kinetics of crystal growth through oriented attachment.

Potent Time-Dependent Ice Recrystallization Inhibition Activity of Cellulose Nanocrystals in Sucrose Solutions

Exploring novel materials with ice recrystallization inhibition (IRI) activity in several fields often starts with a quantitative analysis of ice crystal size change by a splat assay or sandwich assay on a short time scale from 0.5 to 1 h. This study found that this time scale was insufficient to evaluate the IRI activity of cellulose nanocrystals (CNCs) in a model ice cream system-25.0% sucrose solution. No IRI activity was observed in CNCs incubated with ice crystals on a short time scale of 0.5-2.0 h. However, over longer time scales, the growth of ice crystals was entirely inhibited by 1.0% CNCs (between 2 and 24 h) and 0.5% CNCs (between 24 and 72 h) with corresponding final crystal sizes of 25 and 40 μm, respectively. Additionally, ice shaping was observed on a long exposure time, but not on a short exposure time. The findings presented here can be explained by a time-dependent surface coverage of CNCs on ice crystals. The data here indicate the importance of choosing a suitable exposure time for evaluating the IRI activity of new materials and prompt a better understanding of IRI mechanisms involving CNCs.

Single Ice Crystal Growth with Controlled Orientation during Directional Freezing

Ice growth has attracted great attention for its capability of fabricating hierarchically porous microstructure. However, the formation of tilted lamellar microstructure during freezing needs to be reconsidered due to the limited control of ice orientation with respect to the thermal gradient during in situ observations, which can greatly enrich our insight into architectural control of porous biomaterials. This paper provides an in situ study of the solid/liquid interface morphology evolution of directionally solidified single crystal ice with its C-axis (optical axis) perpendicular to directions of both the thermal gradient and the incident light in poly(vinyl alcohol, PVA) solutions. Multifaceted morphology and V-shaped lamellar morphology were clearly observed in situ for the first time. Quantitative characterizations on lamellar spacing, tilt angle, and tip undercooling of lamellar ice platelets provide a clearer insight into the inherent ice growth habit in polymeric aqueous systems and are suggested to exert significant impact on future design and optimization in porous biomaterials.