Differential Scanning Calorimetry of Unfreezable Water in Water–Protein–Polyol Systems

Abstract

The amount ofunfreezable water in lysozyme and bovine serum albumin in aqueous solutions of xylitol, sorbitol, glucose and sucrose was estimated by a differential scanning calorimeter according to new analytical methods. The antemelting point of aqueous polyol solutions seemed to shift to a higher temperature upon addition of protein, but the incipient melting point was not affected by the coexisting protein. The amount of unfreezable water in both proteins, as well as the heat of fusion of ice, decreased with increasing polyol concentration, regardless of the kind of polyols added. On the basis of these results, the solvation structure of the protein in these threecomponent systems and the mechanism of the polyol-induced stabilization of protein were discussed assuming protein-polyol interactions. 2209Sugars and polyols, such as sucrose and glycerol, have been widely used to protect protein structures or biological cells from damage due to heating or freezing. Recently, these compounds have been·shown to be good humectants in controlling water activity in the intermediate moisture foods. The mechanism of these functions, however, is still obscure, although some explanations have been proposed. 1 "-'9) A basic approach for clarifying the mechanism should be found in the elucidation of the effect of these .compounds on the hydration of proteins. For this reason, we have examined the unfreezable water of proteins ·in three-component systems consisting of waterprotein-polyol by a calorimetric method.Differential scanning calorimetry (DSC), as well as NMR, have been used extensively with two different methods of calculation to estimate the unfreezable water of macromolecules in pure water. The first method consists of estimating the water content at which the integral heat of fusion, measured in a series of samples of decreasing water content, completely disappears. 10 ,11) The second method involves calculating the amount ofunfreezable water from the difference between the determined quantity of ice and the total water content of the respective material, using the assumption that the heat of fusion of freezable water in the solution is equal to that of pure water. 12 "-'15) For three-or multi-component systems, the second method has been used as a matter of convenience, since the first method is technically troublesome. 16 ,17) However, the unfreezable water obtained by this method has often been overestimated, because the heat of fusion of pure ice at O°C was assumed, even though low molecular weight additives caused a large depression of melting temperature. 16 ) Thus, a major fault of this method in its application to multi-component systems is the lack of an accurate value for the heat of fusion of the existing ice. In this work, new analytical treatments based on the above two methods will be reported for the three-component systems of water-protein-polyol, and the mechanism of polyol-induced protein .stabilization will be briefly discussed from the viewpoint of protein-solvent interaction.

Effect of proteolytic squid protein hydrolysate on the state of water and denaturation of lizardfish (Saurida wanieso) myofibrillar protein during freezing

“…The suppressive effects of SPH led to structural alteration of the protein molecules that inhibited the formation of ice-crystal around polar residues of Mf. Thus, in SPH-added Mf, production of amorphous hydrate water by hydrophobic residues was suppressed by the polar side-chains of the peptides as previously shown by Gekko & Satake (1981) and Gekko (1982), Niwa et al, (1986) and Castronuovo, Elia, Niccoli and Velleca (2002). However, as the effect of other nitrogenous compounds in SPH on the protein stabilization was not clarified, therefore the full role of SPH on the stabilization mechanism of protein molecular structure and suppression of freeze-induced denaturation of Mf has not been elucidated in the present report.…”

Calorimetric studies of the state of water in seed tissues

“…Recent determinations of the amount of unfreezable water in galactose/ water solutions (31) point out the sensitivity of the calculations to the moisture range used in the study as well as the assumed AHmcit value, and suggest that AHmelt does indeed change with moisture level. While reductions in AHmeit are expected when the onset temperature is <0°C (25), the value of the apparent AHmcit calculated for water in pea cotyledons at hydration level 3 is smaller than can be accounted for by the low temperature of the transition (according to equations by Gekko and Satake [25], AHmeit of water melting at -350C is -260 J/g H20). An alternative explanation for the low apparent AHmcit calculated for water in proteins or pea cotyledons at hydration level 3 is given by the observation that enthalpies determined in the present way contain a heat of solution factor (30).…”

The Rocky Road of Xylitol to its Clinical Application

“…These include the ability of xylitol to form complexes with certain cations, such as Ca(II), Cu(II), and Fe(II) (Angyal et al, 1974). Another interesting feature of the xylitol molecule is its capacity to displace water molecules from the hydration layer of proteins and also from that of the above-mentioned cations (Lewin, 1974;Gekko and Satake, 1981). In 1969, xylitol was introduced to me as a possible sugar substitute.…”