Introduction

Water has profound lyotropic effects on membrane phospholipids (reviewed in refs 1–3), including depression of the temperature at which they pass from the gel to the liquid crystalline phase. This effect has enormous consequences for the structure and function of phospholipid bilayers both in vitro and in vivo. In fact, studies on natural membranes and pure phospholipids have demonstrated over the past two decades that dramatic alterations in the organization of membrane lipids are induced as a result of dehydration-induced phase transitions (reviewed in refs 10–12). Among these alterations are: phase separations of membrane constituents leading to such phenomena as aggregation of membrane proteins and formation of non-bilayer phases, fusion between adjacent bilayers; and transient permeability changes, resulting in leakage of the contents of vesicles or cells to the surrounding medium (reviewed in refs 18, 19).

The impetus for our own work in this field over the past two decades has been a search for the mechanism by which certain organisms such as seeds of plants, yeast cells, and even some lower animals are capable of surviving more or less complete dehydration (see refs 9, 10, 18, 20 for background). We will not deal extensively with this phenomenon in this review since we have recently done so elsewhere, but it is of such interest that we wish to comment briefly on what is known about the mechanism. According to the available evidence, all such organisms contain large quantities of disaccharides (usually trehalose or sucrose). These sugars have the ability to form hydrogen bonds with the polar headgroups of membrane phospholipids, and in effect replace the water that is hydrogen bonded to the same polar groups in fully hydrated bilayers.