Study of Freezing of Water in Biological Model Systems(The Article of the Society Award Winner)

Abstract

Water in biological model systems was studied by the use of various physical techniques. Studies using differential scanning calorimetry found that water in polymer gels sometimes remains partially unfrozen and turns into a vitreous state during cooling, and freezes during rewarming. Ice crystallization during rewarming, observed with cross-linked polymer gels, depended on density of cross-links as well as water content and cooling rate. The density of cross-links was considered to be interrelated with the flexibility of polymer chains and water diffusivity in gels. From these studies, a concept of compartmentalized water was introduced for water in polymer gels. The concept of compartmentalized water was extended to aqueous solutions by the analysis of their freezing behavior, which provided information of hydration structures of solutes with micro-heterogeneity in aqueous systems. In search of ice nucleation-active substances, spider silk, Agelena opulenta, was found to have an ice nucleation activity. Moreover, for the substances to be strongly ice-nucleation active, it was understood that they are necessary to be hydrophilic, structurally compatible with ice crystal and large-sized as well. With the understanding of physical chemistry of water at low temperatures, cryopreservation of nematode, Caenorhabditis elegans was challenged. High rate of survival after freeze-thawing was attained with the help of dimethylsulfoxide as a cryoprotectant, ice seeding followed by slow cooling and subsequent slow heating after freezing. Finally, the present author recognized that freezing is essentially a kinetic process governed by molecular and thermal transfer.