2.15. Measurement of Membrane Fluidity

The variation of membrane fluidity was assessed using DPH as the fluorescence probe [15]. First, 2 μM of DPH was dissolved in tetrahydrofuran and the mixture was dissolved in PBS. The treated cells were obtained by centrifugation (5000 rpm, 8 min) and rinsing twice in PBS. Subsequently, the cell pellets were incubated in the prepared DPH solution at 35 °C for 40 min. Excessive DPH were removed by centrifugation (9000 rpm, 15 min, 4 °C). The obtained cells were washed and resuspended with PBS. Then, the cells were transferred to a black 96-well plate and measured by a multi-mode microplate reader (Perkin Elmer, Waltham, MA, USA, Ex/Em = 362/432 nm). Fluorescence polarization (P), membrane micro-viscosity (η), and membrane lipid fluidity (MLF) were calculated based on the following equations:

where Ivv and Ivh are the light intensities emitted in the vertical and horizontal directions relative to the beam of excitation. G is the grating factor.