Author
|
Submitted to: Cryobiology
Publication Type: Abstract Only Publication Acceptance Date: 4/19/2007 Publication Date: 8/1/2007 Citation: Walters, C.T. 2007. Glass formation, glass fragility, molecular mobility and longevity of germplasm stored at cryogenic temperatures. Meeting abstract for the 44th Annual Meeting of the Society for Cryobiology. July 28 - August 1, 2007, Lake Louise, Canada. pp. 44. Interpretive Summary: Cryogenic storage of living specimens presupposes that viability can be maintained indefinitely when aqueous glasses are formed and maintained. Indeed, constrained mobility within glassy matrices is a thermodynamic barrier to many chemical and physical reactions. However, impeding molecular mobility through glass formation does not altogether restrict degradative reactions. One example is the faster-than-expected deterioration that we observe in cryogenically stored germplasm [Walters et al., 2004, Cryobiology 48: 229-244]. There are several possible explanations for this disappointing result that relate to the temperature dependency of molecular mobility at T < Tg. Molecular mobility may not decrease to the extent predicted at cryogenic temperatures. “Strong” glasses (sensu Angell) are detected in seeds, meaning that Arrhenius kinetics, rather than VTF kinetics, describe viscosity changes with temperature [Walters, 2005, Biophysical J 86:1253-1258]. Moreover, measures of viscosity relate to translational motion, and degradative reactions may involve higher orders of mobility, such as rotational or vibrational motion, that do not correlate with glass viscosity. Finally, reduced mobility within the glassy matrix will not deter deteriorative reactions that occur in the lipid domain of cells. Fluidity of lipids can be detected at very low temperatures, especially if fatty acids are unsaturated [e.g., Crane et al., 2006, Planta 223:1081-1089]. We use differential scanning calorimetry to infer molecular mobility in aqueous and lipid domains using concepts of configurational entropy to calculate relaxation in aqueous glasses and crystallization kinetics to detect reorganization of triacyglycerols. In this paper, we will review our methods to measure molecular mobility at cryogenic temperatures and use estimates of mobility to rationalize the slow, but measurable, deterioration of cryogenically stored germplasm. Technical Abstract: Cryogenic storage of living specimens presupposes that viability can be maintained indefinitely when aqueous glasses are formed and maintained. Indeed, constrained mobility within glassy matrices is a thermodynamic barrier to many chemical and physical reactions. However, impeding molecular mobility through glass formation does not altogether restrict degradative reactions. One example is the faster-than-expected deterioration that we observe in cryogenically stored germplasm [Walters et al., 2004, Cryobiology 48: 229-244]. There are several possible explanations for this disappointing result that relate to the temperature dependency of molecular mobility at T < Tg. Molecular mobility may not decrease to the extent predicted at cryogenic temperatures. “Strong” glasses (sensu Angell) are detected in seeds, meaning that Arrhenius kinetics, rather than VTF kinetics, describe viscosity changes with temperature [Walters, 2005, Biophysical J 86:1253-1258]. Moreover, measures of viscosity relate to translational motion, and degradative reactions may involve higher orders of mobility, such as rotational or vibrational motion, that do not correlate with glass viscosity. Finally, reduced mobility within the glassy matrix will not deter deteriorative reactions that occur in the lipid domain of cells. Fluidity of lipids can be detected at very low temperatures, especially if fatty acids are unsaturated [e.g., Crane et al., 2006, Planta 223:1081-1089]. We use differential scanning calorimetry to infer molecular mobility in aqueous and lipid domains using concepts of configurational entropy to calculate relaxation in aqueous glasses and crystallization kinetics to detect reorganization of triacyglycerols. In this paper, we will review our methods to measure molecular mobility at cryogenic temperatures and use estimates of mobility to rationalize the slow, but measurable, deterioration of cryogenically stored germplasm. |
