The Glass Transition Behaviors of Hydroxyethyl Starch Solutions

Abstract

Despite the increasing use of hydroxyethyl starch (HES) as a cryo-and lyoprotectant, information about its glass transition behavior is scarce. The problem stems from the difficulty in detecting the glass transition of HES samples due to the polydispersity of HES and low sensitivity of calorimetric methods. Using an isothermal desorption (controlled air-drying) method and differential scanning calorimetry (DSC), the present study reports a complete glass state transition diagram of the HES-phosphate-buffered saline (HES-PBS) solution (average molecular weight, 262,600; substitution ratio, 0.46). This state diagram is described by the Gordon-Taylor equation. The glass transition temperature (T g ) of amorphous anhydrous solutes and the plasticization constant of water (k) as defined in the model are 406.3 K (133.1°C) and 4.75, respectively. T g depression of the HES-PBS system is less sensitive to water plasticization than other carbohydrates and biopolymers. Heat capacity change (DC p ) associated with glass transition is also hydration-dependent. The relationship between water content and DC p is defined by five hydration ranges, demonstrating complex hydration-dependent thermal behaviors. The thermal transition profile of the HES-PBS system depends on solute concentration. At solute concentrations .75% (wt/wt), no crystallization or devitrification was observed upon cooling or warming. Between 60% and 75%, the solution vitrified during slow cooling, but devitrified upon warming. Between 45% and 60%, the solution vitrified only at high cooling rates. Below ,45%, complete vitrification was not achieved, even at a cooling rate about 2,200°C/min. Incomplete vitrification and devitrification resulted in the formation of heterogeneous domains that contribute to a complex transition profile upon warming. Devitrification temperatures of the HES-PBS solutions were observed at 23.3 6 2.9°C (mean 6 standard deviation) above T g . Finally, the amount of unfrozen water in the freeze concentrate was calculated to be 0.328 g of water/g of solute in the HES-PBS system (i.e., 24.7%, wt/wt). The methodology described herein may be used to overcome the difficulty in glass transition determination of other cryo-and lyoprotectant systems. 55

Protection of liposomes against fusion during drying by oligosaccharides is not predicted by the calorimetric glass transition temperatures of the dry sugars

“…In common with other sugar glasses (Buitink and Leprince 2004 ; Crowe et al 1998 ), water acted as a plasticizer and led to a progressive decrease in T g with increasing water content. The degree of reduction in T g , however, was different for the different oligosaccharides, in accordance with findings for other oligo- and polysaccharides (Sun et al 2004 ). It was not possible to obtain a T g value for extremely dry (below 0.13 g water/g dry weight) DP5 manno-oligosaccharide, because the sample decomposed during heating before T g was reached.…”

Study on the Unfrozen Water Quantity of Maximally Freeze-Concentrated Solutions for Multicomponent Lyoprotectants

“…The fitted unfrozen water hydrated by HES (200/0.5) in this article was 0.3388 g water/g, converting to W 0 g value of HES/water solutions was 74.69 wt%, which was close to the value of 75.3 wt% obtained by Sun et al 39 for HES (262.6/0.46)/phosphate-buffered saline/water solutions, where the mass ratio of HES to NaCl was 63.21, whereas the large distinction from fitted a i value for HES (450/0.7-0.8) may be attributed to large differences in molecular weight.…”

“…The differences between calculated and experimental T 0 g were less than 2 C for trehalose/HES/NaCl/water solutions. It should be noticed that the C-K coefficient k of HES was determined by fitting Equation 7 to glass transition temperatures of trehalose/HES/NaCl/ water quaternary solutions measured in this article, the value of 6.5 was much larger than the fitted G-T plasticization value of 4.75 by Sun et al 39 ; while the T 0 g value for HES/water binary solutions calculated by extrapolating W 0 g to C-K glass transition curve was À53.37 C, which was much smaller than the value of À16 C determined by Kresin et al 13 There were 2 possible explanations: first, the measured T 0 g was not the true glass transition temperature but the ante-melting temperature; second, the application of "modified" C-K (Eq. 6) using C-K coefficients determined from binary solutions, to calculate glass transition temperatures for multicomponent solutions was questionable, and more experimental data were needed to testify Equation 6.…”

“…It should be mentioned that all the results concluded previously are only suitable for "miscible" multicomponent solutions, not for the "phase separated" solutions. 39 For the latter, multiple freezeconcentrated domains will form, leading to multiple T 0 g .…”

Water Sorption and Glass Transition Behavior of Polyalditol (PD30), a New Nonreactive Malto-Oligosaccharide Cryo- and Drying-Protectant

“…The setup of humidity chambers was described in details previously. 8,9 In the present study, each chamber had a total volume of ϳ650 cm 3 , and contained 150 cm 3 of saturated salt solution (with excessive salt present to form a paste). To achieve rapid equilibration, solution-to-vapor volume ratio and solution-to-sample mass ratio were made to be Ͼ3 and 150, respectively.…”