Detecting Molecular Mobility in Cryopreserved Materials using Mechanical Properties: Seeds as a Case Study

Authors: Ballesteros, Daniel; Walters, Christina

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/17/2010
Publication Date: 7/17/2010
Citation: Ballesteros, D., Walters, C.T. 2010. Detecting Molecular Mobility in Cryopreserved Materials using Mechanical Properties: Seeds as a Case Study. Meeting Abstract. Society for Cryobiology, July 17-20, 2010. Bristol UK. pp. P077.

Interpretive Summary: The purpose of this presentation is to examine molecular mobility of materials stored under cryogenic conditions. It is generally assumed that molecular mobility of fluids decrease to almost nil once the temperature decreases to below the glass transition temperature, Tg. Glass transitions have been observed in seeds using a range of thermal and spectroscopic techniques. The Vogel-Fulcher-Tammann (VFT) model predicts nearly infinite viscosity in glasses at cryogenic temperatures and seed banks have assumed a consequent increase in longevity in cryopreserved seeds. Contrary to this prediction, we have observed changes in seed viability during cryogenic storage over a 30 year period. These observations have led us to hypothesize that molecules within the glassy matrix retain motion even at cryogenic temperatures. To test this hypothesis, we have applied concepts of structural mechanics to the study of seeds with diverse compositions to identify and quantify different relaxations occurring in the seed below the Tg. Mechanical properties in seeds ranging in water content were studied using Dynamic Mechanical Analysis (DMA) between -130°C and 90°C. Glass transitions were identified as alpha transitions, and additional beta and gamma relaxations, as well as melting of lipid crystals, were detected as peaks or plateaus in elastic modulus (E’), loss modulus (E”), and damping factor (tan delta) measurements. Most of these relaxations occurred within the glassy state at low temperatures, and are believed to reflect molecular mobility of side chains or molecular restructuring when lipid crystals melt. The temperature and size of alpha and beta transitions were highly affected by seed water content, suggesting that plasticizing effects of water increase molecular mobility and the probability of physical aging in the glass matrix. Temperature effects on the mechanical properties differ in seeds that have been excessively dried, and elastic modulus is constant or increases slightly with warming above the beta relaxation in pea seeds containing less than 0.10 g H2O/g dry mass. Correlations between deterioration rate and molecular mobility detected in seeds under different storage conditions using mechanical properties will address whether seed longevity can be predicted under diverse storage conditions. We will also be studying changes in mechanical properties of seeds as they deteriorate during storage as a potential non-invasive measurement of the progress of aging. Our results show that molecules move in seed glasses even at cryogenic temperatures and this mobility may contribute to changes in seed viability during storage.

Technical Abstract: The purpose of this presentation is to examine molecular mobility of materials stored under cryogenic conditions. It is generally assumed that molecular mobility of fluids decrease to almost nil once the temperature decreases to below the glass transition temperature, Tg. Glass transitions have been observed in seeds using a range of thermal and spectroscopic techniques. The Vogel-Fulcher-Tammann (VFT) model predicts nearly infinite viscosity in glasses at cryogenic temperatures and seed banks have assumed a consequent increase in longevity in cryopreserved seeds. Contrary to this prediction, we have observed changes in seed viability during cryogenic storage over a 30 year period. These observations have led us to hypothesize that molecules within the glassy matrix retain motion even at cryogenic temperatures. To test this hypothesis, we have applied concepts of structural mechanics to the study of seeds with diverse compositions to identify and quantify different relaxations occurring in the seed below the Tg. Mechanical properties in seeds ranging in water content were studied using Dynamic Mechanical Analysis (DMA) between -130°C and 90°C. Glass transitions were identified as alpha transitions, and additional beta and gamma relaxations, as well as melting of lipid crystals, were detected as peaks or plateaus in elastic modulus (E’), loss modulus (E”), and damping factor (tan delta) measurements. Most of these relaxations occurred within the glassy state at low temperatures, and are believed to reflect molecular mobility of side chains or molecular restructuring when lipid crystals melt. The temperature and size of alpha and beta transitions were highly affected by seed water content, suggesting that plasticizing effects of water increase molecular mobility and the probability of physical aging in the glass matrix. Temperature effects on the mechanical properties differ in seeds that have been excessively dried, and elastic modulus is constant or increases slightly with warming above the beta relaxation in pea seeds containing less than 0.10 g H2O/g dry mass. Correlations between deterioration rate and molecular mobility detected in seeds under different storage conditions using mechanical properties will address whether seed longevity can be predicted under diverse storage conditions. We will also be studying changes in mechanical properties of seeds as they deteriorate during storage as a potential non-invasive measurement of the progress of aging. Our results show that molecules move in seed glasses even at cryogenic temperatures and this mobility may contribute to changes in seed viability during storage.