Electro-optics of membrane electroporation in diphenylhexatriene-doped lipid bilayer vesicles

Kakorin S, Stoylov SP, Neumann E. Electro-optics of membrane electroporation in diphenylhexatriene-doped lipid bilayer vesicles. In: Biophysical Chemistry. BIOPHYSICAL CHEMISTRY. Vol 58. ELSEVIER SCIENCE BV; 1996: 109-116.The electric (linear) dichroisms observed in the membrane electroporation of salt-filled lipid bilayer vesicles (diameter O = 2a = 0.32 mu m; inside [NaCl] = 0.2 M) in isotonic aqueous 0.284 M sucrose-0.2 mM NaCl solution indicate orientation changes of the anisotropic light scattering centers (lipid head groups) and of the optical transition moments of the membrane-inserted probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Both the turbidity dichroism and DPH absorbance dichroism show peculiar features: (1) at external electric fields E greater than or equal to E(sat) the time course of the dichroism shows a maximum value (reversal): E(sat) = 4.0 (+/- 0.2) MV m(-1), T = 293 K (20 degrees C), (2) this reversal value is independent of the field strength for E greater than or equal to E(sat), (3) the dichroism amplitudes exhibit a maximum value E(max) = 3.0 (+/- 0.5) MV m(-1), (4) for the pulse duration of 10 mu s there is one dominant visible normal mode, the relaxation rate increases up to tau(-1) approximate to 0.6 X 10(6) s(-1) at E(sat) and then decreases for E > E(sat). The data can be described in terms of local lipid phase transitions involving clusters L(n) of n lipids in the pore edges according to the three-state scheme C reversible arrow HO = HI, C being the closed bilayer state, HO the hydrophobic pore state and HI the hydrophilic or inverted pore state with rotated lipid and DPH molecules. At E greater than or equal to E(sat) further transitions HO reversible arrow HO* and HI reversible arrow HI* are rapidly coupled to the C reversible arrow HO transition, which is rate-limiting. The vesicle geometry conditions a cos theta dependence of the local membrane field effects relative to the (E) over right arrow direction and the data reflect cos theta averages. The stationary induced transmembrane voltage Delta phi(theta, lambda(m)) = - 1.5aEf(lambda(m))/cos theta/ does not exceed the limiting value Delta phi(sat) = - 0.53 V, corresponding to the field strength E(m),(sat) = - Delta phi(sat)/d = 100 MV m(-1) (10(3) kV cm(-1)), due to increasing membrane conductivity lambda(m). At E = E(sat), f(lambda(m)) = 0.55, lambda(m) = 0.11 mS m(-1). The lipid cluster phase transition model yields an average pore radius of (r) over bar(p) = 0.35 (+/- 0.05) nm of the assumed cylindrical pore of thickness d = 5 nm, suggesting an average cluster size of [n] = 12 (+/-2) lipids per pore edge. For E > E(sat) the total number of DPH molecules in pore states approaches a saturation value; the fraction of DPH molecules in HI pores is 12 (+/-2)% and that in HO pores is 48 (+/-2)%. The percentage of membrane area P approximate to (lambda(m)/lambda(i)) X 100% of conductive openings filled with the intravesicular medium of conductance lambda(i) = 2.2 S m(-1) linearly increases from P approximate to O% (E = 1.8 MV m(-1)) to P = 0.017% (E = 8.5 MV m(-1)). Analogous estimations made by Kinosita et al. (1993) on the basis of fluorescence imaging data for sea urchin eggs give the same order of magnitude for P (0.02 - 0.2%). The increase in P with the field strength is collinear with the increase in concentration of HI and HI* states with the field strength, whereas the HO and HO* states exhibit a sigmoid field dependence. Therefore our data suggest that it is only the HI and HI* pore states which are conductive. It is noted that the various peculiar features of the dichroism data cannot be described by simple whole particle deformation

Optical Properties of DNA in Aqueous Solution

In the study of DNA electric birefringence, it is usual to use theories that consider that molecules in solution are small in relation to the light wavelength. In this work, we study the DNA electric birefringence using a broken-rod macroion (BRM) model composed of two cylindrical arms which does not restrict the size of the molecules. To achieve this, we include the inhomogeneity effect of the light electric field through the molecule and the interaction between its different parts. To analyze the interaction between a molecule and the incident beam of light, we apply the discrete dipole approximation (DDA), according to which each molecule is described as a finite array of electronic coupled oscillators. The electric birefringence is calculated from the oscillator polarizability. This is obtained from experimental data of electric birefringence saturation and from the increment of the solution refraction index in relation to that of the solvent. Furthermore, the oscillator polarizability is also estimated from DNA absorption spectrum using the Kronig–Kramers relations. This allows us to analyze the contributions of the different absorption bands of DNA to the electric birefringence. We analyze the influence of the inhomogeneity of the light electric field and of the intramolecular interactions in the characterization of DNA optical properties using electric birefringence measurements

Mathematical modeling of electro-rotation spectra of small particles in liquid solutions: Application to human erythrocyte aggregates

Electro-rotation can be used to determine the dielectric properties of cells, as well as to observe dynamic changes in both dielectric and morphological properties. Suspended biological cells and particles respond to alternating-field polarization by moving, deforming or rotating. While in linearly polarized alternating fields the particles are oriented along their axis of highest polarizability, in circularly polarized fields the axis of lowest polarizability aligns perpendicular to the plane of field rotation. Ellipsoidal models for cells are frequently applied, which include, beside sphere-shaped cells, also the limiting cases of rods and disks. Human erythrocyte cells, due to their particular shape, hardly resemble an ellipsoid. The additional effect of rouleaux formation with different numbers of aggregations suggests a model of circular cylinders of variable length. In the present study, the induced dipole moment of short cylinders was calculated and applied to rouleaux of human erythrocytes, which move freely in a suspending conductive medium under the effect of a rotating external field. Electro-rotation torque spectra are calculated for such aggregations of different length. Both the maximum rotation speeds and the peak frequencies of the torque are found to depend clearly on the size of the rouleaux. While the rotation speed grows with rouleaux length, the field frequency nup is lowest for the largest cell aggregations where the torque shows a maximum