Conductivity measurement as a convenient technique for determination of liposome capture volume

Abstract

The reduction in conductivity seen between a buffer solution and a liposome preparation in that buffer was evaluated as a means of measuring liposome capture volume. Using DOPC and DOPG lipid to form negatively charged liposomes, conductivity measurements showed that conductivity of the liposome dispersion decreased as lipid concentrations of liposome preparations increased. Independent measurement of capture volumes by gel filtration chromatography showed that conductivity changes correlated with a liposome concentration dependent increase in capture volume. It is proposed that ions from the hydrating/suspending buffer normally contributing to conductivity were trapped within liposomes upon vesicle formation. These internalized and therefore shielded ions were not able to effectively contribute to conductivity of the liposome dispersion. For multilamellar vesicles (MLVs), capture volume was determined by reduction in conductivity over a large lipid concentration range and a broad buffer ionic strength range. Capture volume could also be determined for small unilamellar vesicles (SUVs). However, the greater number of exposed phospholipid head groups in high surface area SUVs contributed to conductivity of the liposome dispersion thereby limiting range of utility. A much higher ionic strength buffer (relative to MLVs) was required before conductivity of phospholipid no longer influenced conductivity of the dispersion. To expand this study, multilamellar vesicles having either neutral (DOPC) or positive (DOPC/stearylamine) charge were evaluated. Similar correlations were found between reduction in conductivity and mannitol entrapment (capture volume). These studies have confirmed that measurement of reduction in conductivity provides an easy and convenient method for determining liposome capture volume.