Lipid Exchange Rates of Conventional and Polymer Stabilized Liposomes
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Lipid Exchange Rates of Conventional and Polymer Stabilized Liposomes Awad Ahmed, Nicole Heldt, Gregory Slack, and Yuzhuo Li Department of Chemistry and Center for Advanced Materials Processing, Clarkson University, Potsdam, New York, 13699-5810, USA ABSTRACT Polymer-stabilized liposome systems consisting of polyethylene glycol bound lipids (PEG-lipids) and conventional (nonpolymer stabilized) liposomes were compared in terms of their inter-membrane lipid migration rates. In order to monitor the exchange of lipids between the membranes, 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (PY-PC), a phospholipid with pyrene attached to the hydrophobic tail, was used to label the liposome. Labeled and unlabeled liposome systems were mixed and fluorescence spectroscopy was used to examine the lipid transfer. More specifically, the relative employed to deduce the exchange kinetics. After labeled and unlabeled liposome systems were mixed, the E/M ratio for PY-PC in a polymer stabilized liposome system decreased by 66% over a period of 80 minutes, while the E/M for PY-PC in a conventional liposome system decreased 70% in less than 2 minutes. This suggests that the exchange rate for lipids in polymer stabilized liposome systems is much slower than that of conventional liposome systems. In addition, the exchange rates for both conventional and polymer stabilized liposome systems are accelerated at an elevated temperature. INTRODUCTION Liposome is a very important supermolecular structure because of its similarity to biological membranes and its therapeutic value as delivery agent for enzymes, drugs, genetic manipulation, and diagnostic imaging applications [1]. Recently, liposomes with polyethylene glycol chains attached to the polar head group (polymer stabilized or Stealth liposomes) have been implemented in pharmaceutical applications due to their increased lifetime in the blood stream [1,2]. In the field of liposome studies, fluorescence has emerged as a useful analytical technique, as it provides a large amount of diversified information concerning the biophysics of lipid vesicles and biomembranes [3]. Artificial and biological membranes can be considered as two-dimensional fluids, characterized by a high lateral mobility of lipid components. Almgran was the first to use a fluorescence stopped-flow technique to investigate the migration of pyrene between unilamellar vesicles [4]. The results indicated that the migration of pyrene form one vesicle to another occurs mainly via a desolubilizations-diffusion-resolubilization mechanism [4]. Doody et al. also provided evidence that the transfer of a fluorescent fatty acid probe is through the aqueous phase [5]. Sengupa et al. measured pyrenyl-decanoic acid transfer rates between DPPC vesicles using a stop-flow technique and found that multilamellar vesicles had much slower exchange rates than that of unilamellar vesicles. In addition, charging the surface of the vesicles also reduces the exchange rate [6]. Wolkowicz and collaborators found that an increase in the
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