Mechanism of poly(ethylene glycol)-induced lipid transfer between phosphatidylcholine large unilamellar vesicles: a fluorescent probe study

Abstract

Experiments were performed to assess three possible mechanisms of poly(ethylene glycol) (PEG) induced rapid lipid transfer between large unilamellar vesicles composed of dioleoylphosphatidylcholine: (1) transfer between aggregated vesicles, (2) transfer through an aqueous medium of lowered dielectric constant, and (3) transfer via a PEG carrier. The results showed that close contact between vesicles as a result of PEG dehydration was largely responsible for the rapid lipid transfer observed in the presence of PEG. The rate and extent of lipid transfer were also examined at 10 wt % PEG and analyzed in terms of a two-state model especially developed to account for the initial rate of lipid transfer as followed by the fluorescence lifetime of DPHpPC as a fluorescent lipid probe. Analysis revealed that two rate processes were involved in DPHpPC transfer between bilayers, both in the absence and presence of PEG. Since the maximum extent of transfer was 50%, transbilayer diffusion of DPHpPC seemed not to contribute to either process. The fast process in the presence of PEG was identified as due to rapid interbilayer monomer diffusion between closely apposed vesicles, and, in the absence of PEG, as due to monomer diffusion through the aqueous phase. The origin of the slow process, in both cases, remains obscure. The Arrhenius activation energies (and entropies) for the initial rates at temperatures from 10 to 48 degrees C were 15.3 +/- 0.3 kcal/mol (-26.3 +/- 0.2 eu) and 10.6 +/- 0.5 kcal/mol (-16.1 +/- 0.3 eu) in the absence and presence of PEG, respectively. The slow process was invariant with temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for quantitative interpretation of fluorescence detection of poly(ethylene glycol)-mediated 1-palmitoyl-2-[[[2-[4-(phenyl-trans-1,3,5-hexatrienyl) phenyl]ethyl]oxyl]carbonyl]3-sn-phosphatidylcholine (DPHpPC) transfer and fusion between phospholipid vesicles in the dehydrated state

“…That is, species of the sort Vj (which could fuse to form Fj) are considered, but species of the sort Vfj are ignored. This simplifying approximation is consistent with our earlier observation that lipid exchange or fusion occurred on a very limited basis unless PEG was removed and vesicles were dispersed and then reaggregated with PEG [20].…”

“…Calibration curves were generated for different concentrations (wt%) of PEG at a lipid concentration of 0.25 mM by measuring the DPHpPC lifetime at a minimum of three different lipid/probe ratios, as described by Burgess and Lentz [12]. Calibration data were used to determine the parameters of an empirical equation using a Simplex curve fitting routine [19,20]. Once the calibration curves were established for a particular lipid system, probecontaining (donor) vesicles and probe-free (acceptor) vesicles were added to the buffer solution with a certain amount of PEG, resulting in a final phospholipid concentration of 0.025 and 0.25 mM for the donor and acceptor vesicles, respectively.…”

“…Each subspecies of fused j-met, F~(i, j-i), or of unfused aggregate j-met, Vj(i, j-i), has its own lifetime related to the surface concentrations of probe present in thej-mer [20]. In the following sections, we show how to calculate the fluorescence lifetime of these j-mers.…”

Spontaneous lipid transfer between organized lipid assemblies

“…Other manipulations of the aqueous mileu known to affect spontaneous interbilayer lipid transfer involve addition of chaotropic salts [69,72] or poly(ethylene)glycol [130][131]. In such cases, it is difficult to distinquish whether the primary effect is restructuring of the bulk aqueous milieu or the highly polarized hydration zone associated with the surface of organized lipid assemblies.…”

“…Based on their results, Lentz and co-workers [130][131] proposed that poly(ethylene glycol) enhances the rate of intervesicular lipid transfer by dehydrating and aggregating the PC vesicles thereby altering the properties of the aqueous gap between the membrane surfaces.…”