Adsorption of DNA and electric fields decrease the rigidity of lipid vesicle membranes

Abstract

The adsorption of calf–thymus DNA-fragments of 300 ± 50 base pairs (bp) to the outer membrane monolayer of unilamellar lipid vesicles in the presence of Ca²⁺ ions has been quantified by the standard method of chemical relaxation spectrometry using polarized light. The vesicles of radius a = 150 ± 45 nm are prepared from bovine brain extract type III containing 80–85% phosphatidylserine (PS) and palmitoyl-oleoyl-phosphatidylcholine (POPC) in the molar ratio PS : 2POPC; total lipid concentration [L_t] = 1 mM in 1 mM HEPES buffer, pH 7.4 at T = 293 K (20 °C).The turbidity relaxations of vesicle suspensions, at the wavelength λ = 365 nm at two characteristic electric field strengths are identified as electroelongation of the whole vesicle coupled to smoothing of thermal membrane undulations and membrane stretching, and at higher fields, to membrane electroporation (MEP). The elongation kinetics indicates that the DNA adsorption renders the membrane more flexible and prone to membrane electroporation (MEP). Remarkably, it is found that the Ca-mediated adsorption of DNA (D) decreases both, bending rigidity κ and stretching modulus K, along an unique Langmuir adsorption isotherm for the fraction of bound DNA at the given Ca concentration [Ca_t] = 0.25 mM. The characteristic chemo-mechanical parameter of the isotherm is the apparent dissociation equilibrium constant K(D,Ca) = 100 ± 10 μM (bp) of the ternary complex DCaB of DNA base pairs (bp) and Ca binding to sites B on the outer vesicle surface. Whereas both κ and K decrease in the presence of high electric fields (E), the key parameter K(D,Ca) is independent of E in the range 0 ≤ E/(kV cm⁻¹) ≤ 40.