High Critical Temperature above Tg May Contribute to the Stability of Biological Systems

In this study, we characterized the molecular mobility around Tg in sugars, poly-L-lysine and dry desiccation-tolerant biological systems, using ST-EPR, 1H-NMR, and FTIR spectroscopy, to understand the nature and composition of biological glasses. Two distinct changes in the temperature dependence of the rotational correlation time (R) of the spin probe 3-carboxy-proxyl or the second moment (M2) were measured in sugars and poly-L-lysine. With heating, the first change was associated with the melting of the glassy state (Tg). The second change (Tc), at which R abruptly decreased over several orders of magnitude, was found to correspond with the so-called cross-over temperature, where the dynamics changed from solid-like to liquid-like. The temperature interval between Tg and Tc increased in the order of sucrose < trehalose < raffinose staychose < poly-L-lysine < biological tissues, from 17 to >50°C, implying that the stability above Tg improved in the same order. These differences in temperature-dependent mobilities above Tg suggest that proteins rather than sugars play an important role in the intracellular glass formation. The exceptionally high Tc of intracellular glasses is expected to provide excellent long-term stability to dry organisms, maintaining a slow molecular motion in the cytoplasm even at temperatures far above Tg.