Trehalose interacts with phospholipid polar heads in Langmuir monolayers

Surface pressure-area isotherms, surface potential-area isotherms and fluorescence microscopy were employed to study the behavior of phospholipid monolayers at the air/water interface when trehalose was added to the aqueous subphase. In the presence of this sugar, the critical area corresponding to the onset of surface potential increases, indicating that trehalose is participating in the network of hydrogen bonds between the phospholipid polar heads. In addition, it causes an expansion of the isotherm, hindering the formation of the liquid-condensed phase. The collapse area is significantly increased, indicating that trehalose takes part in the monolayer structure without being expelled even at high surface pressures. A quantitative comparison of the collapse areas and critical areas for surface potential in the presence and in the absence of the sugar shows that an almost fixed number of trehalose molecules interacts with the monolayer independently of the surface packing, thus indicating that the observed effects can be ascribed to a tight binding of trehalose to the polar heads in a defined ratio. No similar effects were observed in the presence of glucose. We rationalize the reported data in light of the water replacement hypothesis, developed to explain the preservation of biomembranes by trehalose; this hypothesis suggests that trehalose forms hydrogen bonds with the membrane polar headgroups, thus replacing the water of hydration at the membrane-fluid interface and maintaining the headgroups at their hydrated position.