Submitted to: Biophysical Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2003
Publication Date: 1/20/2004
Citation: Walters, C. 2004. Temperature-dependency of molecular mobility in preserved seeds. Biophysical Journal 86:1253-1258.
Interpretive Summary: We know that germplasm will survive for a long time in cryogenic storage, but we do not know exactly how long. Estimates of the actual time scale for shelf life of cryogenically stored material are critical for efficient genebanking, but are difficult because of instrument limitations or the extended times required for measurements. Methods developed by researchers in the pharmaceutical industry to determine molecular mobility and stability of biological chemicals are applied in this paper to a seed system to indicate molecular mobility at cryogenic temperatures. The data show great changes in molecular mobility as storage temperature is decreased. These changes are of similar magnitude to aging rates of germplasm stored at different temperatures, suggesting that the developed method can be used to predict aging rates for different storage conditions. Molecular mobility is detectable even at ultra-low temperatures, implying that cryogenic storage cannot stop the process of aging (though it will slow it considerably).
Technical Abstract: Though cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual time scale for changes in viability has not been addressed theoretically or empirically. Molecular mobility within preserved biological materials provides a first approximation of the rate of deteriorative reactions that ultimately affect shelflife. Here, temperature effects on molecular mobility in partially dried seeds are calculated from heat capacities, measured using differential scanning calorimetry, and the Vogel-Tamman-Fulcher (VTF) model for relaxation of glasses corrected for configurational entropy. Using this correction, glassy behavior in seeds containing 0.07 g H2O/g dm followed strict VTF behavior at temperatures above and just below the glass transition temperature (Tg) at 28 degrees C. Temperature dependency of relaxation times followed Arrhenius kinetics as temperatures decreased well below Tg. The transition from VTF to Arrhenius kinetics occurred between about 5 and -10 degrees C and is interpreted as a change in glass fragility. Overall, relaxation times calculated for seeds containing 0.07 g H2O/g dm decreased by about 8 orders of magnitude when seeds were cooled from 60 to -60 degrees C, comparable to the magnitude of change in aging kinetics reported for seeds and pollen stored at a similar temperature range. The Kauzmann temperature (TK), often considered the point at which molecular mobility of glasses is practically nil, was calculated as -42 degrees C. Calculated relaxation times, temperature coefficients lower than expected from VTF kinetics, and TK that is 70 degrees C below Tg suggest there is molecular mobility, albeit limited, at cryogenic temperatures.