Kinetic pinning and biological antifreezes

On the engulfment of antifreeze proteins by ice

Antifreeze proteins (AFPs) are remarkable biomolecules that suppress ice formation at trace concentrations. To inhibit ice growth, AFPs must not only bind to ice crystals, but also resist engulfment by ice. The highest supercooling, [Formula: see text], for which AFPs are able to resist engulfment is widely believed to scale as the inverse of the separation, [Formula: see text], between bound AFPs, whereas its dependence on the molecular characteristics of the AFP remains poorly understood. By using specialized molecular simulations and interfacial thermodynamics, here, we show that in contrast with conventional wisdom, [Formula: see text] scales as [Formula: see text] and not as [Formula: see text]. We further show that [Formula: see text] is proportional to AFP size and that diverse naturally occurring AFPs are optimal at resisting engulfment by ice. By facilitating the development of AFP structure-function relationships, we hope that our findings will pave the way for the rational design of AFPs.

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.

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.

Engulfment Avalanches and Thermal Hysteresis for Antifreeze Proteins on Supercooled Ice

Antifreeze proteins (AFPs) bind to the ice-water surface and prevent ice growth at temperatures below 0 °C through a Gibbs-Thomson effect. Each adsorbed AFP creates a metastable depression on the surface that locally resists ice growth, until ice engulfs the AFP. We recently predicted the susceptibility to engulfment as a function of AFP size, distance between AFPs, and supercooling [ J. Chem. Phys. 2023, 158, 094501]. For an ensemble of AFPs adsorbed on the ice surface, the most isolated AFPs are the most susceptible, and when an isolated AFP gets engulfed, its former neighbors become more isolated and more susceptible to engulfment. Thus, an initial engulfment event can trigger an avalanche of subsequent engulfment events, leading to a sudden surge of unrestrained ice growth. This work develops a model to predict the supercooling at which the first engulfment event will occur for an ensemble of randomly distributed AFP pinning sites on an ice surface. Specifically, we formulate an inhomogeneous survival probability that accounts for the AFP coverage, the distribution of AFP neighbor distances, the resulting ensemble of engulfment rates, the ice surface area, and the cooling rate. We use the model to predict thermal hysteresis trends and compare with experimental data.

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.

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.

Antifreeze Proteins: A Tale of Evolution From Origin to Energy Applications

Icing and formation of ice crystals is a major obstacle against applications ranging from energy systems to transportation and aviation. Icing not only introduces excess thermal resistance, but it also reduces the safety in operating systems. Many organisms living under harsh climate and subzero temperature conditions have developed extraordinary survival strategies to avoid or delay ice crystal formation. There are several types of antifreeze glycoproteins with ice-binding ability to hamper ice growth, ice nucleation, and recrystallization. Scientists adopted similar approaches to utilize a new generation of engineered antifreeze and ice-binding proteins as bio cryoprotective agents for preservation and industrial applications. There are numerous types of antifreeze proteins (AFPs) categorized according to their structures and functions. The main challenge in employing such biomolecules on industrial surfaces is the stabilization/coating with high efficiency. In this review, we discuss various classes of antifreeze proteins. Our particular focus is on the elaboration of potential industrial applications of anti-freeze polypeptides.

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

Ice-binding proteins (IBPs) are unique molecules that bind to and are active on the interface between two phases of water: ice and liquid water. This property allows them to affect ice growth in multiple ways: shaping ice crystals, suppressing the freezing point, inhibiting recrystallization and promoting nucleation. Advances in the protein's production technologies make these proteins promising agents for medical applications among others. Here, we focus on a special class of IBPs that suppress freezing by causing thermal hysteresis (TH): antifreeze proteins (AFPs). The kinetic pinning model describes the dynamics of a growing ice face with proteins binding to it, which eventually slow it down to a halt. We use the kinetic pinning model, with some adjustments made, to study the TH dependence on the solution's concentration of AFPs by fitting the model to published experimental data. We find this model describes the activity of (moderate) type III AFPs well, but is inadequate for the (hyperactive) Tenebrio molitor AFPs. We also find the engulfment resistance to be a key parameter, which depends on the protein's size. Finally, we explain intuitively how TH depends on the seeding time of the ice crystal in the protein solution. Using this insight, we explain the discrepancy in TH measurements between different assays. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

Combined molecular dynamics and neural network method for predicting protein antifreeze activity

Antifreeze proteins (AFPs) are a diverse class of proteins that depress the kinetically observable freezing point of water. AFPs have been of scientific interest for decades, but the lack of an accurate model for predicting AFP activity has hindered the logical design of novel antifreeze systems. To address this, we perform molecular dynamics simulation for a collection of well-studied AFPs. By analyzing both the dynamic behavior of water near the protein surface and the geometric structure of the protein, we introduce a method that automatically detects the ice binding face of AFPs. From these data, we construct a simple neural network that is capable of quantitatively predicting experimentally observed thermal hysteresis from a trio of relevant physical variables. The model's accuracy is tested against data for 17 known AFPs and 5 non-AFP controls.

Growth suppression of ice crystal basal face in the presence of a moderate ice-binding protein does not confer hyperactivity

Ice-binding proteins (IBPs) affect ice crystal growth by attaching to crystal faces. We present the effects on the growth of an ice single crystal caused by an ice-binding protein from the sea ice microalga Fragilariopsis cylindrus (fcIBP) that is characterized by the widespread domain of unknown function 3494 (DUF3494) and known to cause a moderate freezing point depression (below 1 °C). By the application of interferometry, bright-field microscopy, and fluorescence microscopy, we observed that the fcIBP attaches to the basal faces of ice crystals, thereby inhibiting their growth in the c direction and resulting in an increase in the effective supercooling with increasing fcIBP concentration. In addition, we observed that the fcIBP attaches to prism faces and inhibits their growth. In the event that the effective supercooling is small and crystals are faceted, this process causes an emergence of prism faces and suppresses crystal growth in the a direction. When the effective supercooling is large and ice crystals have developed into a dendritic shape, the suppression of prism face growth results in thinner dendrite branches, and growth in the a direction is accelerated due to enhanced latent heat dissipation. Our observations clearly indicate that the fcIBP occupies a separate position in the classification of IBPs due to the fact that it suppresses the growth of basal faces, despite its moderate freezing point depression.

Operation of Kelvin Effect in the Activities of an Antifreeze Protein: A Molecular Dynamics Simulation Study

Ice growth and melting inhibition activities of antifreeze proteins (AFPs) are better explained by the adsorption-inhibition mechanism. Inhibition occurs as a result of the Kelvin effect induced by adsorbed protein molecules onto the surface of seed ice crystal. However, the Kelvin effect has not been explored by the state-of-the-art experimental techniques. In this work, atomistic molecular dynamics simulations have been carried out with Tenebrio molitor antifreeze protein ( TmAFP) placed at ice-water interface to probe the Kelvin effect in the mechanism of AFPs. Simulations show that, below equilibrium melting temperature, ice growth is inhibited through the convex ice-water interface formation toward the water phase and, above equilibrium melting temperature, ice melting is inhibited through the concave ice-water interface formation inward to ice phase. Simulations further reveal that the radius of curvature of the interface formed to stop the ice growth increases with decrease in the degree of supercooling. Our results are in qualitative agreement with the theoretical prediction of the Kelvin effect and thus reveal its operation in the activities of AFPs.

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.

When are antifreeze proteins in solution essential for ice growth inhibition?

Antifreeze proteins (AFPs) are a widespread class of proteins that bind to ice and facilitate the survival of organisms under freezing conditions. AFPs have enormous potential in applications that require control over ice growth. However, the nature of the binding interaction between AFPs and ice remains the subject of debate. Using a microfluidics system developed in-house we previously showed that hyperactive AFP from the Tenebrio molitor beetle, TmAFP, remains bound to an ice crystal surface after exchanging the solution surrounding the ice crystal to an AFP-free solution. Furthermore, these surface-adsorbed TmAFP molecules sufficed to prevent ice growth. These experiments provided compelling evidence for the irreversible binding of hyperactive AFPs to ice. Here, we tested a moderately active type III AFP (AFPIII) from a fish in a similar microfluidics system. We found, in solution exchange experiments that the AFPIIIs were also irreversibly bound to the ice crystals. However, some crystals displayed "burst" growth during the solution exchange. AFPIII, like other moderately active fish AFPs, is unable to bind to the basal plane of an ice crystal. We showed that although moderate AFPs bound to ice irreversibly, moderate AFPs in solution were needed to inhibit ice growth from the bipyramidal crystal tips. Instead of binding to the basal plane, these AFPs minimized the basal face size by stabilizing other crystal planes that converge to form the crystal tips. Furthermore, when access of solution to the basal plane was physically blocked by the microfluidics device walls, we observed enhancement of the antifreeze activity. These findings provide direct evidence that the weak point of ice growth inhibition by fish AFPs is the basal plane, whereas insect AFPs, which can bind to the basal plane, are able to inhibit its growth and thereby increase antifreeze activity.

Unusual structural properties of water within the hydration shell of hyperactive antifreeze protein

Many hypotheses can be encountered explaining the mechanism of action of antifreeze proteins. One widespread theory postulates that the similarity of structural properties of solvation water of antifreeze proteins to ice is crucial to the antifreeze activity of these agents. In order to investigate this problem, the structural properties of solvation water of the hyperactive antifreeze protein from Choristoneura fumiferana were analyzed and compared with the properties of solvation water present at the surface of ice. The most striking observations concerned the temperature dependence of changes in water structure. In the case of solvation water of the ice-binding plane, the difference between the overall structural ordering of solvation water and bulk water diminished with increasing temperature; in the case of solvation water of the rest of the protein, the trend was opposite. In this respect, the solvation water of the ice-binding plane roughly resembled the hydration layer of ice. Simultaneously, the whole solvation shell of the protein displayed some features that are typical for solvation shells of many other proteins and are not encountered in the solvation water of ice. In the first place, this is an increase in density of water around the protein. The opposite is true for the solvation water of ice - it is less dense than bulk water. Therefore, even though the structure of solvation water of ice-binding plane and the structure of solvation water of ice seem to share some similarities, densitywise they differ.

Ice-binding proteins that accumulate on different ice crystal planes produce distinct thermal hysteresis dynamics

Ice-binding proteins that aid the survival of freeze-avoiding, cold-adapted organisms by inhibiting the growth of endogenous ice crystals are called antifreeze proteins (AFPs). The binding of AFPs to ice causes a separation between the melting point and the freezing point of the ice crystal (thermal hysteresis, TH). TH produced by hyperactive AFPs is an order of magnitude higher than that produced by a typical fish AFP. The basis for this difference in activity remains unclear. Here, we have compared the time dependence of TH activity for both hyperactive and moderately active AFPs using a custom-made nanolitre osmometer and a novel microfluidics system. We found that the TH activities of hyperactive AFPs were time-dependent, and that the TH activity of a moderate AFP was almost insensitive to time. Fluorescence microscopy measurement revealed that despite their higher TH activity, hyperactive AFPs from two insects (moth and beetle) took far longer to accumulate on the ice surface than did a moderately active fish AFP. An ice-binding protein from a bacterium that functions as an ice adhesin rather than as an antifreeze had intermediate TH properties. Nevertheless, the accumulation of this ice adhesion protein and the two hyperactive AFPs on the basal plane of ice is distinct and extensive, but not detectable for moderately active AFPs. Basal ice plane binding is the distinguishing feature of antifreeze hyperactivity, which is not strictly needed in fish that require only approximately 1°C of TH. Here, we found a correlation between the accumulation kinetics of the hyperactive AFP at the basal plane and the time sensitivity of the measured TH.

Dynamical mechanism of antifreeze proteins to prevent ice growth

The fascinating ability of algae, insects, and fishes to survive at temperatures below normal freezing is realized by antifreeze proteins (AFPs). These are surface-active molecules and interact with the diffusive water-ice interface thus preventing complete solidification. We propose a dynamical mechanism on how these proteins inhibit the freezing of water. We apply a Ginzburg-Landau-type approach to describe the phase separation in the two-component system (ice, AFP). The free-energy density involves two fields: one for the ice phase with a low AFP concentration and one for liquid water with a high AFP concentration. The time evolution of the ice reveals microstructures resulting from phase separation in the presence of AFPs. We observed a faster clustering of pre-ice structure connected to a locking of grain size by the action of AFP, which is an essentially dynamical process. The adsorption of additional water molecules is inhibited and the further growth of ice grains stopped. The interfacial energy between ice and water is lowered allowing the AFPs to form smaller critical ice nuclei. Similar to a hysteresis in magnetic materials we observe a thermodynamic hysteresis leading to a nonlinear density dependence of the freezing point depression in agreement with the experiments.

Microfluidic experiments reveal that antifreeze proteins bound to ice crystals suffice to prevent their growth

Antifreeze proteins (AFPs) are a subset of ice-binding proteins that control ice crystal growth. They have potential for the cryopreservation of cells, tissues, and organs, as well as for production and storage of food and protection of crops from frost. However, the detailed mechanism of action of AFPs is still unclear. Specifically, there is controversy regarding reversibility of binding of AFPs to crystal surfaces. The experimentally observed dependence of activity of AFPs on their concentration in solution appears to indicate that the binding is reversible. Here, by a series of experiments in temperature-controlled microfluidic devices, where the medium surrounding ice crystals can be exchanged, we show that the binding of hyperactive Tenebrio molitor AFP to ice crystals is practically irreversible and that surface-bound AFPs are sufficient to inhibit ice crystal growth even in solutions depleted of AFPs. These findings rule out theories of AFP activity relying on the presence of unbound protein molecules.

Thermodynamic Analysis of Thermal Hysteresis: Mechanistic Insights into Biological Antifreezes

Antifreeze proteins (AFPs) bind to ice crystal surfaces and thus inhibit the ice growth. The mechanism for how AFPs suppress freezing is commonly modeled as an adsorption-inhibition process by the Gibbs-Thomson effect. Here we develop an improved adsorption-inhibition model for AFP action based on the thermodynamics of impurity adsorption on the crystal surfaces. We demonstrate the derivation of a realistic relationship between surface protein coverage and the protein concentration. We show that the improved model provides a quantitatively better fit to the experimental antifreeze activities of AFPs from distinct structural classes, including fish and insect AFPs, in a wide range of concentrations. Our theoretical results yielded the adsorption coefficients of the AFPs on ice, suggesting that, despite the distinct difference in their antifreeze activities and structures, the affinities of the AFPs to ice are very close and the mechanism of AFP action is a kinetically controlled, reversible process. The applications of the model to more complex systems along with its potential limitations are also discussed.

Superheating of ice crystals in antifreeze protein solutions

It has been argued that for antifreeze proteins (AFPs) to stop ice crystal growth, they must irreversibly bind to the ice surface. Surface-adsorbed AFPs should also prevent ice from melting, but to date this has been demonstrated only in a qualitative manner. Here we present the first quantitative measurements of superheating of ice in AFP solutions. Superheated ice crystals were stable for hours above their equilibrium melting point, and the maximum superheating obtained was 0.44 degrees C. When melting commenced in this superheated regime, rapid melting of the crystals from a point on the surface was observed. This increase in melting temperature was more appreciable for hyperactive AFPs compared to the AFPs with moderate antifreeze activity. For each of the AFP solutions that exhibited superheating, the enhancement of the melting temperature was far smaller than the depression of the freezing temperature. The present findings clearly show that AFPs adsorb to ice surfaces as part of their mechanism of action, and this absorption leads to protection of ice against melting as well as freezing.

How the overlapping timescales for peptide binding and terrace exposure lead to non-linear step dynamics during growth of calcium oxalate monohydrate

Using in situ atomic force microscopy (AFM), we investigate the inhibition of calcium oxalate monohydrate (COM) step growth by aspartic acid-rich peptides and find that the magnitude of the effect depends on terrace lifetime. We then derive a time dependent step-pinning model in which average impurity spacing depends on the terrace lifetime as given by the ratio of step spacing to step speed. We show that the measured variation in step speed is well fit by the model and allows us to extract the characteristic peptide adsorption time. The model also predicts that a crossover in the timescales for impurity adsorption and terrace exposure leads to bistable growth dynamics described mathematically by a catastrophe. We observe this behavior experimentally both through the sudden drop in step speed to zero upon decrease of supersaturation as well as through fluctuations in step speed between the two limiting values at the point where the catastrophe occurs. We discuss the model's general applicability to macromolecular modifiers and biomineral phases.

Direct visualization of spruce budworm antifreeze protein interacting with ice crystals: basal plane affinity confers hyperactivity

Antifreeze proteins (AFPs) protect certain organisms from freezing by adhering to ice crystals, thereby preventing their growth. All AFPs depress the nonequilibrium freezing temperature below the melting point; however AFPs from overwintering insects, such as the spruce budworm (sbw) are 10-100 times more effective than most fish AFPs. It has been proposed that the exceptional activity of these AFPs depends on their ability to prevent ice growth at the basal plane. To test the hypothesis that the hyperactivity of sbwAFP results from direct affinity to the basal plane, we fluorescently tagged sbwAFP and visualized it on the surface of ice crystals using fluorescence microscopy. SbwAFP accumulated at the six prism plane corners and the two basal planes of hexagonal ice crystals. In contrast, fluorescently tagged fish type III AFP did not adhere to the basal planes of a single-crystal ice hemisphere. When ice crystals were grown in the presence of a mixture of type III AFP and sbwAFP, a hybrid crystal shape was produced with sbwAFP bound to the basal planes of truncated bipyramidal crystals. These observations are consistent with the blockage of c-axial growth of ice as a result of direct interaction of sbwAFP with the basal planes.

Antifreeze proteins at the ice/water interface: three calculated discriminating properties for orientation of type I proteins

Antifreeze proteins (AFPs) protect many plants and organisms from freezing in low temperatures. Of the different AFPs, the most studied AFP Type I from winter flounder is used in the current computational studies to gain molecular insight into its adsorption at the ice/water interface. Employing molecular dynamics simulations, we calculate the free energy difference between the hydrophilic and hydrophobic faces of the protein interacting with ice. Furthermore, we identify three properties of Type I "antifreeze" proteins that discriminate among these two orientations of the protein at the ice/water interface. The three properties are: the "surface area" of the protein; a measure of the interaction of the protein with neighboring water molecules as determined by the number of hydrogen bond count, for example; and the side-chain orientation angles of the threonine residues. All three discriminants are consistent with our free energy results, which clearly show that the hydrophilic protein face orientations toward the ice/water interface, as hypothesized from experimental and ice/vacuum simulations, are incorrect and support the hypothesis that the hydrophobic face is oriented toward the ice/water interface. The adsorption free energy is calculated to be 2-3 kJ/mol.

Fluorescence microscopy evidence for quasi-permanent attachment of antifreeze proteins to ice surfaces

Many organisms are protected from freezing by the presence of extracellular antifreeze proteins (AFPs), which bind to ice, modify its morphology, and prevent its further growth. These proteins have a wide range of applications including cryopreservation, frost protection, and as models in biomineralization research. However, understanding their mechanism of action remains an outstanding challenge. While the prevailing adsorption-inhibition hypothesis argues that AFPs must bind irreversibly to ice to arrest its growth, other theories suggest that there is exchange between the bound surface proteins and the free proteins in solution. By conjugating green fluorescence protein (GFP) to a fish AFP (Type III), we observed the binding of the AFP to ice. This was accomplished by monitoring the presence of GFP-AFP on the surface of ice crystals several microns in diameter using fluorescence microscopy. The lack of recovery of fluorescence after photobleaching of the GFP component of the surface-bound GFP-AFP shows that there is no equilibrium surface-solution exchange of GFP-AFP and thus supports the adsorption-inhibition mechanism for this type of AFP. Moreover, our study establishes the utility of fluorescently labeled AFPs as a research tool for investigating the mechanisms underlying the activity of this diverse group of proteins.

A two-dimensional adsorption kinetic model for thermal hysteresis activity in antifreeze proteins

Antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs), collectively abbreviated as AF(G)Ps, are synthesized by various organisms to enable their cells to survive in subzero environments. Although the AF(G)Ps are markedly diverse in structure, they all function by adsorbing to the surface of embryonic ice crystals to inhibit their growth. This adsorption results in a freezing temperature depression without an appreciable change in the melting temperature. The difference between the melting and freezing temperatures, termed thermal hysteresis (TH), is used to detect and quantify the antifreeze activity. Insights from crystallographic structures of a number of AFPs have led to a good understanding of the ice-protein interaction features. Computational studies have focused either on verifying a specific model of AFP-ice interaction or on understanding the protein-induced changes in the ice crystal morphology. In order to explain the origin of TH, we propose a novel two-dimensional adsorption kinetic model between AFPs and ice crystal surfaces. The validity of the model has been demonstrated by reproducing the TH curve on two different beta-helical AFPs upon increasing the protein concentration. In particular, this model is able to accommodate the change in the TH behavior observed experimentally when the size of the AFPs is increased systematically. Our results suggest that in addition to the specificity of the AFPs for the ice, the coverage of the AFPs on the ice surface is an equally necessary condition for their TH activity.