ABSTRACT
Introduction ........................................................................................................ 647
Materials and Methods ..................................................................................... 648
Results and Discussion ..................................................................................... 648
Conclusions......................................................................................................... 654
Acknowledgments ............................................................................................. 654
References ........................................................................................................... 654
Some organisms including bacteria, plants, and animals respond to
drought, salt, or low-temperature stress by increasing the concentration
of compatible solutes (Brown, 1976; Yancy et al., 1982; Booth, 1998).
Compatible solutes, such as sucrose, trehalose, glycinebetaine (GB), and
proline have common characteristics of high solubility in water and
nontoxicity to organisms at high concentrations. The mechanism of
protection against water stress by these compounds, however, is subject
to debate and various hypotheses have been proposed. Compatible solutes
of sugars and polyols are assumed to protect membrane lipids or protein
molecules by direct interaction (Rudolph et al., 1986). As another possibility
for their protective function, high glass-transition temperatures of these
aqueous systems being ready to turn into a glassy state when dehydrated
or freeze-concentrated might contribute to the stabilization of native
structures of membranes or protein molecules (Crowe et al., 1998). In the
case of GB and proline, they are electrically neutral as a whole molecule but
exist as a zwitterion with positive and negative charges at physiological
pH, which is different from the case for sugars and polyols. Then, the
protection mechanism against water stress indicated by them might also be
different from that shown by sugars and polyols (Pollard, 1979). It was
indicated that the protective function of GB against osmotic stress can be
explained by the colligativity of solutions (Carley and Record, 2003).
However, scant information is available about the concentration range
where the aqueous zwitterion solutions hold the colligativity, while the
concentration of GB in the cytoplasm is known to become 1 M or so when it
functions as an osmoprotectant (Booth, 1998). What is the remarkable
difference between aqueous solutions of electrolytes and zwitterions? In
this connection, there are many kinds of zwitterion molecules in biological
systems including amino acids and phospholipids, and they might play a
crucial role in the life process. The reason why they are introduced into the
life process and function variously, for example, as compatible solutes is
unclear and worth further attention. Then we investigated in this study the
characteristics of hydration properties of GB by obtaining the water-
sorption isotherm and freezing behavior of the aqueous solution by the use
of differential scanning calorimetry (DSC).