Structure of solvation water around the active and inactive regions of a type III antifreeze protein and its mutants of lowered activity

Interfacial Ice Density Fluctuations Inform Surface Ice-Philicity

The propensity of a surface to nucleate ice or bind to ice is governed by its ice-philicity─its relative preference for ice over liquid water. However, the relationship between the features of a surface and its ice-philicity is not well understood, and for surfaces with chemical or topographical heterogeneity, such as proteins, their ice-philicity is not even well-defined. In the analogous problem of surface hydrophobicity, it has been shown that hydrophobic surfaces display enhanced low water-density (vapor-like) fluctuations in their vicinity. To interrogate whether enhanced ice-like fluctuations are similarly observed near ice-philic surfaces, here we use molecular simulations and enhanced sampling techniques. Using a family of model surfaces for which the wetting coefficient, k, has previously been characterized, we show that the free energy of observing rare interfacial ice-density fluctuations decreases monotonically with increasing k. By utilizing this connection, we investigate a set of fcc systems and find that the (110) surface is more ice-philic than the (111) or (100) surfaces. By additionally analyzing the structure of interfacial ice, we find that all surfaces prefer to bind to the basal plane of ice, and the topographical complementarity of the (110) surface to the basal plane explains its higher ice-philicity. Using enhanced interfacial ice-like fluctuations as a measure of surface ice-philicity, we then characterize the ice-philicity of chemically heterogeneous and topologically complex systems. In particular, we study the spruce budworm antifreeze protein (sbwAFP), which binds to ice using a known ice-binding site (IBS) and resists engulfment using nonbinding sites of the protein (NBSs). We find that the IBS displays enhanced interfacial ice-density fluctuations and is therefore more ice-philic than the two NBSs studied. We also find the two NBSs are similarly ice-phobic. By establishing a connection between interfacial ice-like fluctuations and surface ice-philicity, our findings thus provide a way to characterize the ice-philicity of heterogeneous surfaces.

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.

Revealing the Frank-Evans "Iceberg" Structures within the Solvation Layer around Hydrophobic Solutes

Using computer simulations, the structural properties of solvation water of three model hydrophobic molecules, methane and two fullerenes (C60 and C80), were studied. Systems were simulated at temperatures in the range of 250-298 K. By analyzing both the local ordering of the molecules of water in the solvation layers and the structure of hydrogen bond network, it is shown that in the solvation layer of hydrophobic molecules, ordered aggregates consisting of water molecules are formed. Even though it is difficult to define the exact structure of these aggregates, their existence alone is clearly noticeable. Moreover, these aggregates become more pronounced with the decrease of temperature. The existence of the ordered aggregates around the hydrophobic solutes complies with the concept of "icebergs" proposed by Frank and Evans.

Antifreeze proteins and homogeneous nucleation: On the physical determinants impeding ice crystal growth

Antifreeze proteins (AFPs) are biopolymers capable of interfering with ice growth. Their antifreeze action is commonly understood considering that the AFPs, by pinning the ice surface, force the crystal-liquid interface to bend forming an ice meniscus, causing an increase in the surface free energy and resulting in a decrease in the freezing point ΔT^max. Here, we present an extensive computational study for a model protein adsorbed on a TIP4P/Ice crystal, computing ΔT^max as a function of the average distance d between AFPs, with simulations spanning over 1 µs. First, we show that the lower the d, the larger the ΔT^max. Then, we find that the water-ice-protein contact angle along the line ΔT^max(d) is always larger than 0°, and we provide a theoretical interpretation. We compute the curvature radius of the stable solid-liquid interface at a given supercooling ΔT ≤ ΔT^max, connecting it with the critical ice nucleus at ΔT. Finally, we discuss the antifreeze capability of AFPs in terms of the protein-water and protein-ice interactions. Our findings establish a unified description of the AFPs in the contest of homogeneous ice nucleation, elucidating key aspects of the antifreeze mechanisms and paving the way for the design of novel ice-controlling materials.

Computational Study of Differences between Antifreeze Activity of Type-III Antifreeze Protein from Ocean Pout and Its Mutant

The antifreeze activity of a type-III antifreeze protein (AFP) expressed in ocean pout (Zoarces americanus) is compared with that of a specific mutant (T18N) using all-atom molecular dynamics simulations. The antifreeze activity of the mutant is only 10% of the wild-type AFP. The results from this simulation study revealed the following insights into the mechanism of antifreeze action by type-III AFPs. The AFP gets adsorbed to the advancing ice front due to its hydrophobic nature. A part of the hydrophobicity is caused by the presence of clathrate structure of water molecules near the ice-binding surface (IBS). The mutation in the AFP disrupts this structure and thereby reduces the ability of the mutant to adsorb to the ice-water interface leading to the loss of antifreeze activity. The mutation, however, has no effect on the ability of the adsorbed protein to bind to the growing ice phase. Simulations also revealed that all surfaces of the protein can bind to the ice phase, although the IBS is the preferred surface.

Role of the Solvation Water in Remote Interactions of Hyperactive Antifreeze Proteins with the Surface of Ice

Most protein molecules do not adsorb onto ice, one of the exceptions being so-called antifreeze proteins. In this paper, we describe that there is a force pushing an antifreeze protein molecule away from the ice surface when it is not oriented with its ice-binding plane toward the ice and that this pushing force may be also present even when the protein is oriented with its ice-binding plane toward the ice. This force is absent only when certain specific distance criteria are met, regarding the surface of ice and the protein. It acts at early stages of adsorption, prior to the solidification of water between the ice and the protein molecule nearby. We propose the water-originating mechanism of the generation of this force and also the mechanism of remote attachment of an antifreeze molecule to the ice surface. In liquid water, there exist locally favored structures, ordered and of high specific volume. The presence of a protein molecule usually shifts the equilibrium that exists in liquid water toward increasing the number of high-density, disordered structures and diminishing the number of low-density structures. Creation of the locally favored structures may be hampered not only near the non-ice-binding surfaces but also between the ice surface and the protein surface, if the distance between these surfaces does not allow these structures to develop because the available space is not sufficient for their proper formation. This conclusion is supported by the analysis of the mean geometry of a single hydrogen bond, as well as of the hydrogen bond network in the solvation layer and a structural order parameter that characterizes the separation between the first and second solvation shells of a water molecule.

Hydration Shell of Antifreeze Proteins: Unveiling the Role of Non-Ice-Binding Surfaces

Antifreeze proteins (AFPs) have the ability to inhibit ice growth by binding to ice nuclei. Their ice-binding mechanism is still unclear, yet the hydration layer is thought to play a fundamental role. Here, we use molecular dynamics simulations to characterize the hydration shell of two AFPs and two non-AFPs. The calculated shell thickness and density of the AFPs do not feature any relevant difference with respect to the non-AFPs. Moreover, the hydration shell density is always higher than the bulk density and, thus, no low-density, ice-like layer is detected at the ice-binding surface (IBS) of AFPs. Instead, we observe local water-density differences in AFPs between the IBS (lower density) and the non-IBS (higher density). The lower solvent density at the ice-binding site can pave the way to the protein binding to ice nuclei, while the higher solvent density at the non-ice-binding surfaces might provide protection against ice growth.

Ordered hydration layer mediated ice adsorption of a globular antifreeze protein: mechanistic insight

The ice binding surface of a type III AFP induces water ordering at lower temperature, which mediates its adsorption on the ice surface.

Molecular dynamics study on the role of solvation water in the adsorption of hyperactive AFP to the ice surface

Using computer simulations, the early stages of the adsorption of the CfAFP molecule to the ice surface were analyzed. We found that the ice and the protein interact at least as early as when the protein is about 1 nm away from the ice surface. These interactions are mediated by interfacial solvation water and are possible thanks to the structural ordering of the solvent. This ordering leads to positional preference of the protein relative to the ice crystal before the final attachment to the ice surface takes place, accompanied by the solidification of the interfacial water. It is possible because the solvation water of the ice-binding plane of CfAFP is susceptible to the overlapping with the solvation water of ice and is mostly changeable into ice itself. These remote interactions significantly increase efficacy of the adsorption process by facilitating the geometric adjustment of the active region of the CfAFP molecule to the ice surface. Because of the ordered nature of the water molecules at the ice-binding plane, the energy of their interactions with the ice-binding surface of the protein does not change upon the ongoing solidification of solvation water. However, the structure of the solvation water is not strictly ice-like and the growth of ice in the interfacial water is not initiated at the side of the protein. On the contrary, we find that solvation water of CfAFP solidifies slower than solvation water of ice - the solidification of interfacial water starts at the surface of ice. Moreover, in the presence of the CfAFP molecule, also solvation water of ice solidifies slower compared to the situation when the protein is not present next to the ice surface. Additionally, the presence of the protein molecule shifts the ratio of cubic to hexagonal ice that spontaneously forms at the ice surface, by introducing another layer of ordered water molecules - opposite to the ice lattice, at the other side of the crystallizing layer of water.

The accretion of the new ice layer on the surface of hexagonal ice crystal and the influence of the local electric field on this process

The process of creation of a new layer of ice on the basal plane and on the prism plane of a hexagonal ice crystal is analyzed. It is demonstrated that the ordering of water molecules in the already existing crystal affects the freezing. On the basal plane, when the orientations of water molecules in the ice block are random, the arrangement of the new layer in a cubic manner is observed more frequently-approximately 1.7 times more often than in a hexagonal manner. When the water molecules in the ice block are more ordered, it results in the predominance of the oxygen atoms or the hydrogen atoms on the most outer part of the surface of the ice block. In this case, the hexagonal structure is formed more frequently when the supercooling of water exceeds 10 K. This phenomenon is explained by the influence of the oriented electric field, present as a consequence of the ordering of the dipoles of water molecules in the ice block. This field modifies the structure of solvation water (i.e., the layer of water in the immediate vicinity of the ice surface). We showed that the structure of solvation water predetermines the kind of the newly created layer of ice. This effect is temperature-dependent: when the temperature draws nearer to the melting point, the cubic structure becomes the prevailing form. The temperature at which the cubic and the hexagonal structures are formed with the same probabilities is equal to about 260 K. In the case of the prism plane, the new layer that is formed is always the hexagonal one, which is independent of the arrangement of water molecules in the ice block and is in agreement with previous literature data. For the basal plane, as well as for the prism plane, no evident dependence on the ordering of water molecules that constitute the ice block on the rate of crystallization can be observed.

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.