Freeze-concentration effect

The best explanation of the good results for peptide syntheses in ice-water mixtures are based on the freeze-concentration-model, which just provides for a volume-reducing function for the ice while the liquid aqueous part is still the only relevant phase for the reaction. All observed enhancements of reaction rate would then have to be attributed to an increase in effective concentration. H-NMR relaxation time measurements have been used to determine the amount of unfrozen water in partially frozen systems, thus quantifying the extent of the freeze-concentration effect (Ullmann, 1997). Comparative studies in ice and at room temperature verify the importance of freeze-concentration which, however, is not sufficient for a complete understanding of the observed effects. 

M. Wagner, H. J. Hofmann, and H.-D. Jakubke, Influence of freeze-concentration effect on proteinase-catalysed peptide synthesis in frozen aqueous systems, Biochim. Biophys. Acta 1997, 1338, 253-258. 

As ice crystals grow in the freezing system, the solutes are concentrated. In addition to increased ionic strength effects, the rates of some chemical reactions—particularly second order reactions—may be accelerated by freezing through this freeze-concentration effect. Examples include reduction of potassium ferricyanide by potassium cyanide (2), oxidation of ascorbic acid (3), and polypeptide synthesis (4). Kinetics of reactions in frozen systems has been reviewed by Pincock and Kiovsky (5). 

Figure 7 Scatterplots of (Ca -h Mg ) and HCOa" versus S04 for groundwaters in the proglacial zone of Finsterwalderbreen, a polythermal-based valley glacier on Svalbard. The groundwaters were sampled from three wells, and show high concentrations of the divalent cations and S04 due to the dissolution of secondary sulfate salts and freeze concentration effects. By contrast, HCOs concentrations are depressed because of the high concentrations of Ca, so that there is an inverse association between HC03 and S04. Full details can be found in Cooper et al. (2002) (reproduced by permission of Elsevier from J. Hydrol.

In addition to the freeze-concentration effect, a catalytic role for ice crystals, a favorable orientation of substrate and biocatalyst, the markedly lower dielectric constant of ice compared with water, and the high proton mobility in ice, have been discussed as further factors that possibly influence reactions in frozen systems. In summary, the reverse action of hydrolases provides an attractive alternative to the chemical synthesis of peptides but this approach could also be verified for the synthesis of oligosaccharides and oligonucleotides using glycosidases and ribonu-cleases, respectively11631. 

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

The development of the freeze concentration process for fruit juices has been hampered by the fact that solute concentrate is entrained by the ice crystals. This incomplete separation of the entrained concentrate from the ice results in a considerable increase of the cost of the process. In this investigation sucrose solutions were concentrated by the formation of an ice layer on the externally cooled walls of the crystallizer. The formation of the layer was initiated by secondary nuclei induced by rotating ice seeds, at subcoolings smaller than the critical subcooling needed for spontaneous nucleation. A minimum in the amount of sucrose entrapped in the ice layer was observed at a subcooling smaller than the critical subcooling for spontaneous nucleation. The effect of soluble pectins on the minimum was also studied. 

Hatley RHM, Franks F, Day H, Byth B. Subzero temperature preservation of reactive fluids in the undercooled state. II. The effect on the oxidation of ascorbic acid of freeze concentration and undercooling. Biophys Chem 1986b 24 187-192. 

Kim AI, Knopp S, Akers MJ, Nail SL. The physical state of mannitol after freeze-drying effects of mannitol concentration, freezing rate, and a noncrystallizing cosolute. J Pharm Sci 1998 87 931-935. 

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