Quantitative study on the effects of sugars on membrane phase transitions - preliminary investigations

It is well known that sugars and other small solutes can reduce the temperature at which membranes undergo the fluid-gel phase transition at low hydration. The mechanisms for this are now well understood [Bryant et al. Abstract No. 85]. Naively, one might expect that this ability would be a direct function of sugar concentration, and that the effects should increase as the amount of sugar increases. However, the real situation is more complex. Previous work [K.L. Koster, Y.P. Lei, M. Anderson, S. Martin, G. Bryant, Biophys. J. 78 (2000) 1932–1946.] has shown that there are two distinct mechanisms for reduction in the transition temperature: first, if the sugar concentration is too low to form a glass, then the transition temperature can be reduced to (at best) the full hydration value; and second, if a glass forms, the transition temperature can be depressed to a fixed value, largely independent of sugar concentration. However, to the authors’ knowledge there has been no systematic study of the membrane transition temperature as a function of sugar/lipid ratio and level of hydration. In this paper we present the results of such a study. We show that in the absence of a glass, the reduction in the membrane phase transition temperature reaches a maximum value at a limiting sugar:lipid ratio. Beyond that value, the addition of further sugar no longer alters the membrane phase transition temperature. We explain these results in terms of hydration forces between membranes, and comment on the implications of these results for the prevention of damage to membranes during dehydration