*3.3. Dipole Potential in Lipid Bilayers and the Effect of Simple Carbohydrates*

The fluorescence intensity ratio Rex of di-8-ANEPPS dye (Equation (3)) in singlecomponent lipid membranes composed of POPC and SOPC is measured in order to explore possible alterations in the primary hydration shell of the bilayer in the presence of sugars [51]. As discussed above, the measured value of Rex is related to the electrical potential, which is generated transversally at the water-lipid interface and the hydrocarbon interior as a result of the contribution of polarized and polarizable chemical groups of lipid molecules and water surrounding the bilayer [36].

For both types of phospholipids, we obtain an increase in di-8-ANEPPS fluorescence intensity ratio in 1 mmol/L NaCl and 200 mmol/L of the studied sugars compared to its value measured in water. The increase of Rex stands for an increase of membrane dipole potential following from the interaction of solutes in the aqueous phase and the lipid bilayer. Slight alterations in membrane fluorescence intensity ratio are measured for POPC and SOPC bilayers upon the addition of 1 mmol/L NaCl and 200 mmol/L of glucose, fructose or sucrose (Table 3 and Figure 10).

**Table 3.** Di-8-ANEPPS fluorescence intensity ratio and dipole potential of POPC bilayers in aqueous solutions containing 1 mmol/L NaCl and 200 mmol/L glucose, fructose or sucrose; control samples are measured in bidistilled water and in 1 mmol/L NaCl; fluorescence excited at 420 and 520 nm and detected at 670 nm.


The dipole potential measured here for POPC and SOPC membranes with and without sugars is consistent with the values obtained by other authors and methods in the range of 200–1000 mV for biomembranes [52,53]. The dipole potential of PC membranes is reported around 410 ± 150 mV in dependence of their degree of saturation, polar heads and the physicochemical properties of the buffer used. At low pH, as well as in the presence of molecules that increase the molecular order parameter and decrease the rotational order parameter, lipid membranes are susceptible to further increase of the dipole potential, similar to the effect of saturated PC compared to unsaturated PC.

**Figure 10.** Di-8-ANEPPS fluorescence intensity ratio of POPC (circles) and SOPC (triangles) bilayers in 1 mmol/L NaCl sugar-free solutions and 200 mmol/L of glucose, fructose or sucrose; fluorescence excited at 420 and 520 nm and detected at 670 nm. Values are the mean of three independent experiments; the error bars correspond to the standard deviations.
