High critical temperature above Tg may contribute to the stability of biological systems

In this study, we characterized the molecular mobility around T-g in sugars, poly-L-lysine and dry desiccation-tolerant biological systems, using ST-EPR,H-1-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 (tau(R)) of the spin probe 3-carboxy-proxyl or the second moment (M-2) were measured in sugars and poly-L-lysine. With heating, the first change was associated with the melting of the glassy state (T-g). The second change (T-c), at which tau(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 T-g and T-c increased in the order of sucrose < trehalose < raffinose less than or equal to staychose < poly-L-lysine < biological tissues, from 17 to >50 degrees C, implying that the stability above T-g improved in the same order. These differences in temperature-dependent mobilities above T-g suggest that proteins rather than sugars play an important role in the intracellular glass formation. The exceptionally high T-c 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 T-g.