Elucidation of the polysaccharide cryoprotection mechanism: kinetic inhibition, thermodynamic Gibbs-Thomson modulation and volumetric confinement

Abstract

The emergence of gel-forming, ice-binding polysaccharides as potential candidates in cryobiology and the discovery of new structure-function relationships has fueled a knowledge convergence effort. Several polysaccharides have shown strong biological post-thaw benefits in cell cryopreservation despite some expressing contradictory ice growth anticipation, the main source of cryoinjury. The bio-based fucose-rich polysaccharide FucoPol, a current model under our scope of expertise, has further demonstrated crystal size reduction, thermal hysteresis, nucleation anticipation, nucleation stochastic narrowing and Gibbs-Thomson growth modulation effects, the latter similar to a type I antifreeze protein, in different thermodynamic settings: bulk vs. directional freezing; isobaric vs. isochoric systems; and sterile vs. biological media. Here, we undergo a critical reiteration of the consortium of findings over the years on cryoprotective polysaccharide research, rationalize several heuristic models to explain a dual nucleation behavior scenario and put forth a unifying theory to explain in which subset of conditions optimal cryoprotection may emerge from bio-based polysaccharides. We argue that gel-forming, ice-binding polysaccharides that show cryoprotective traits by anticipating nucleation and ice growth act by a combination of kinetic hampering of molecular diffusion (concentration effect); Gibbs-Thomson specific ice binding (templating effect) that induces growth modulation and size reduction; and indulge in the formation of a gel architecture of defined porosity (mesh size effect), the main initiator of a predominant pro-nucleation setting, increased stochastic determinism and the annihilation of large r* nuclei that elicits a small-nuclei survivorship bias. Classical Nucleation Theory formalisms support this hypothesis under the circumstance that a change in system state, initiated by a sol-gel transition near hypothermia, must exist to drive a meaningful shift in system energetics that explains the ANTI/PRO nucleation duality observed. The molecular fractioning of kinetically hindered bulk water into fractionally partitioned gel pores reduces system scale by a mesh size constant and enables the Gibbs-Thomson ice-binding affinity condition to be satisfied when r = rGT. A decreased nucleation energy barrier and maximal r* constraint thus invokes a predominant pro-nucleating system by enhancing nucleation susceptibility.