Phase transition of dimyristoylphosphatidylglycerol bilayers induced by electric field pulses

The phase transition of dimyristoylphosphatidylglycerol (DMPG) bilayers has been studied by measurements of light scattering under high electric field pulses. Midpoints of phase transitions have been identified by a clear discontinuity of field induced relaxation amplitudes. We show that the phase transition of DMPG suspensions in monovalent salt is virtually independent of the electric field strength up to approx. 35 kV/cm. A shift of the lipid phase by electric field pulses has been observed, however, for DMPG suspensions in the presence of Ca2+ ions. DMPG suspensions exhibit a jump of the phase transition temperature from 17 degrees C at Ca/DMPG molar ratios r less than 1/7 to 32 degrees C at r greater than 1/7. Field pulses of 60 to 100 microseconds applied to DMPG suspensions with Ca2+ at r greater than 1/7 induce discontinuities of relaxation amplitudes in the temperature range 15 to 22 degrees C in addition to the 'standard' one at 32 degrees C, when the electric field strength is above 15 kV/cm. These results indicate that electric field pulses induce a transition from the phase formed at 'high' Ca(2+)- to the one formed at 'low' Ca(2+)-ion concentrations. Our results are consistent with a dissociation field effect on Ca(2+)-lipid complexes which drives the phase transition.