Effect of trehalose and sucrose on the hydration and dipole potential of lipid bilayers

The water activity in dimyristoylphosphatidylcholine (DMPC) decreases by 60 % when the lipid is dehydrated in the presence of trehalose concentrations higher than 0.02 M. In contrast, sucrose in concentrations 10 times higher produced only a 20% decrease in the water activity in the sample. Titration s of a DMPC solution in chloroform yielded 14 water molecules per lipid whe n pure water was added and seven water molecules per lipid when the titrati on was done with 0.025 M trehalose. The same concentrations of sucrose prod uced a turbid solution, which made it impossible to quantify the number of water molecules per lipid. Lipid monolayers spread on an air/water interfac e showed a decrease from 480 mV in pure water to 425 mV in 0.1 M trehalose. However, the same concentrations of sucrose produced an increase of less t han 100 mV. Results obtained with Fourier transform infrared spectroscopy ( FTIR) under the same conditions denoted that trehalose binds to the carbony l groups, while sucrose showed no specific binding. It is concluded that pe r lipid molecule, 11 of 14 water molecules can be replaced by three trehalo se molecules. About four are displaced by changes in the water activity of the bulk solution, and seven by specific interactions with the phospholipid s. In this last case, at least two of them are linked to the carbonyls, and this appears to be the cause of the decrease in the dipole potential of th e membrane. In contrast, four sucrose molecules displace only three water m olecules per lipid, with no effect on the dipole potential or the carbonyl groups.