Water-Sorption Behavior of Glycinebetaine and the State Diagram of Its Aqueous System

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).