Abstract
Antifreeze proteins (AFPs) can bind to ice nuclei thereby inhibiting their growth and their hydration shell is believed to play a fundamental role. Here, we use molecular dynamics simulations to characterize the hydration shell of four moderately-active and four hyperactive AFPs. The local water density around the ice-binding-surface (IBS) is found to be lower than that around the non-ice-binding surface (NIBS) and this difference correlates with the higher hydrophobicity of the former. While the water-density increase (with respect to bulk) around the IBS is similar between moderately-active and hyperactive AFPs, it differs around the NIBS, being higher for the hyperactive AFPs. We hypothesize that while the lower water density at the IBS can pave the way to protein binding to ice nuclei, irrespective of the antifreeze activity, the higher density at the NIBS of the hyperactive AFPs contribute to their enhanced ability in inhibiting ice growth around the bound AFPs.
Highlights
Antifreeze proteins (AFPs) can bind to ice nuclei thereby inhibiting their growth and their hydration shell is believed to play a fundamental role
The mechanism proposed for the antifreeze activity is that, by binding to a growing ice nucleus, AFPs cause a microcurvature of the ice surface between the adsorbed AFPs.[1,3]
While it is widely accepted that the lower density at the IBS favors binding to ice, the higher water density at the non-ice-binding surface (NIBS) might discourage ice growth around the bound AFPs, as was suggested many years ago but on the basis of dynamical analysis,[15] contributing to the curvature of the ice surface at the basis of thermal hysteresis (TH)
Summary
AThe hydration shell thickness is taken as 1 nm. Vex and Vshell are reported in nm[3]. In order to understand whether the properties of the local densities and the chemical character of the IBSs and NIBSs can be related to the antifreeze activity of the AFPs, we report ηsurf and Spho/S as a function of ΔT (see Figure 4) For both moderately active and hyperactive AFPs, the hydrophobicity of the IBS is high (with Spho/S in the range of 0.54− 0.70) and similar between the two classes (being on average slightly higher for the moderately active ones). For both classes of proteins the IBS has a high hydrophobic content, and a consequent lower water density, as previously reported by different groups.[15,22,26,43,44] here we show that rather than this, it is the higher hydrophilic content and consequent higher relative density increase of the NIBS that differentiate the hyperactive AFPs from the moderately active ones
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