SURFACE MODIFIED LIPOSOMES BY COATING WITH CHARGED

CELLULAR & MOLECULAR BIOLOGY LETTERS

Volume 5, (2000) pp 19 –33 Received 12 June 1999 Accepted 7 December 1999

SURFACE MODIFIED LIPOSOMES BY COATING WITH CHARGED HYDROPHILIC MOLECULES M. LUISA SAGRISTÁ, MARGARITA MORA and M. AFRICA DE MADARIAGA ∗ Department of Biochemistry and Molecular Biology, Faculty of Chemistry, University of Barcelona, Martí i Franquès 1, 08028-Barcelona, Spain Abstract: The design of liposomes with a hydrophilic/steric barrier at their bilayer surface allows the modification of their pharmacokinetics and reduces the uptake by the RES. Liposomes can be coated by hydrophilic molecules such as polysaccharides, which disguise the vesicle surface by creating a threedimensional matrix near them and prevent the binding of plasma proteins and their recognition by some cellular receptors. All these considerations, and previous results obtained in our laboratory showing the formation of stable GAGliposome complexes, have lead us to think about the use of the negatively charged glycosaminoglycans (GAGs), alternately to other molecules such as the monosialoganglioside GM1, more expensive, or polyethylene glycol (PEG-PE) that can disturb the structural organization of the bilayer. The present paper describes the effect of the incorporation of GAGs to phospholipid vesicles, in relation to their electrical and permeability properties. The results obtained show that there is an effective coating of the bilayer surface when glycosaminoglycans are added to liposome suspensions. The shielding of the negative surface charge by the neutral hyaluronic acid, in the absence of calcium, and the increase in the negative charge when the negative polyelectrolytes chondroitin sulfate, heparin or dextran sulfate are added to calcium-containing liposome suspensions account for the formation of stable liposome-GAG complexes. Moreover, the reduced permeability of the GAG-coated liposomes points out on their ability to hold encapsulated drugs and, so, their potential usefulness as drug-sustained release carriers. The hydrophilic coating will give to these liposomal carriers long-circulating properties. Key Words: Liposomes, Zeta Potential, Permeability Properties, Hydrophilic Coating, Glycosaminoglycans ∗

corresponding author

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INTRODUCTION Phospholipid vesicles or liposomes have a potential value as drug delivery systems [1, 2], in the diagnostic imaging of tumors [3, 4], as vaccine adjuvants [5, 6], as vectors for gene transfer [7, 8] and as cosmetic agents for the delivery of moisturisers and anti-inflammatory agents to the skin [9]. In Physicochemical terms, liposomes have many of the characteristics of colloidal particles and their stability is determined in part by the classical surface forces. Independently of the route of administration, the in vivo fate of liposomes and the pharmacokinetics of encapsulated drugs are mainly determined by liposome size, lipid composition and lipid bilayer fluidity [10]. It is possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics. It is generally accepted that liposomes can not escape the vascular system except in organs with fenestrated vascular membranes, such as the liver, the spleen and, to a minor part, the bone marrow or under pathological conditions in inflamed or tumorous tissues, when the vascular membrane has become leaky [11]. In addition, classical liposomes administered by intravenous route are generally cleared rapidly from the circulation by cells of the reticuloendothelial system (RES) in liver, lung and spleen [12]. In addition to size, the surface properties, specially charge and hydrophilic character, play a pivotal role in the fate of systemically injected colloids [13]. The surface characteristics of liposomes will determine their interaction with the molecular species in the blood stream and with cell surfaces and, consequently, their usefulness as drug carriers. The manipulation of the bilayer surface of lipid vesicles may be carried out with relative easiness by the choice of the bilayer lipids and will affect the permeability characteristics of the liposomal membrane and both their stability in the biological medium and their interaction with a potential target site. Hence, the development of sterically stabilized long-circulating liposomes represents a milestone in the search of liposomes able to bypass the RES [14]. This stabilization can be achieved by the incorporation of natural hydrophilic components such as gangliosides like G M1 [15-17] or phosphatidylinositol [15], essentially mimicking the outer surface of red blood cells, or of synthetic hydrophilic polymers, specifically polyethylene glycols (PEG) [18, 19]. Moreover, the surface charge of liposomes can be modified by their electrostatic interaction with ions, by coating with natural and synthetic polymers and by the incorporation and covalent linkage of glycoproteins and polymers [20, 21]. Prolonged circulation results in an increased probability of liposomes extravasating in areas where the permeability of the endothelial barrier is increased, specially in tumor tissues [22-24]. Thus, the design of liposomes with a hydrophilic/steric barrier at their bilayer surface allows the modification of their pharmacokinetics and reduces the uptake by the RES. Liposomes can be coated by hydrophilic molecules such as polysaccharides, which disguise the vesicles surface by creating a threedimensional matrix near them and prevent the binding of plasma proteins and their


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recognition by some cellular receptors. All these considerations, and previous results obtained in our laboratory showing the formation of stable GAG-liposome complexes [25], have led us to think about the use of the lineal negatively charged polysaccharides glycosaminoglycans (GAGs) as an alternative to other chemical coatings with the monosialoganglioside GM1, more expensive, or with polyethylene glycol (PEG-PE) that can disturb the estructural organization of the bilayer. Measurement of changes in the Zeta Potential of liposomes under different conditions gives a simple and sensitive experimental method to evaluate the effective coating of their surfaces by charged hydrophilic molecules. The electrical properties of liposomes can be investigated by microelectrophoresis, in which the movement of the liposomes in an electrical field is measured. Among the different techniques to measure the electrophoretic mobilities, the LaserDoppler Anemometry is specially suitable because of its simplicity [26]. The Zeta Potential is calculated directly from the measured mobility by the use of the Henry equation [27]. The present paper describes the effect of the incorporation of different GAGs on the surface charge of liposomes and studies the permeability properties of the GAG-coated liposomes. The influence of hyaluronic acid (HA), chondroitin sulfate (CSA), heparin (HP) and dextran sulfate (DS) in relation to these properties has been evaluated, in the absence and in the presence of calcium. MATERIALS AND METHODS Chemicals Tris was a product from Boehringer Manheim GmbH; HCl, NaCl and CaCl 2.2H2O were from Panreac Química S.A.; chondroitin sulfate (CSA), dextran sulfate (DS) 500000 sodium salt and DS 15000 DEXTRANLIP R 15 sodium salt were obtained by Sigma Chemical Co. and heparin (HP) and hyaluronic acid (HA) by Bioibérica S.A. 5(6)-carboxyfluorescein (CF), purchased from Eastman-Kodak, was purified by Sephadex LH-20 chromatography and acid precipita tion before use [28]. All other reagents were of analytical grade. All aqueous solutions were prepared with milliQ water Millipore systems (Molscheim, France) and all organic solvents were redistilled before use. Preparation of liposomes Multilamellar and unilamellar liposomes were prepared from two bovine brain extracts, which main difference lies in their phosphatidylserine content. BBEI is a complex bovine brain extract, corresponding to the Folch Fraction III from Sigma Chemical Co. (St. Louis, MO), with a high content of phosphatidylserine ( ≈80%), and BBEII is a complex bovine brain extract from Bioibérica S.A., with a more balanced phospholipidic composition and a low percentage of phosphatidylserine (≈10%) [29]. Multilamellar vesicles (MLVs), used in Zeta Potential measurements, were prepared at a concentration of 4 mg lipid/mL in 10 mM Tris-HCl buffer, pH 7.4, alone or with different amounts of NaCl. MLVs were obtained by vortexing a

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dried lipid film with the buffer for 20 minutes, followed by the sonication of the dispersion for 30 minutes in a bath-type sonicator at 45 °C. Liposome preparations were then stored at 4°C until use. For CF leakage studies, intermediate unilamellar vesicles (IUVs) were prepared by extrusion. 6 mg of the corresponding complex bovine brain extract were hydrated with 3 mL of 20 mM Tris-HCl buffer (pH 7.4), containing 0.1 mM EDTA and 50 mM CF. After hydration and in the same way as indicated for MLVs, the lipid dispersion was frozen and thawed five times and sonicated in a bath sonicator for 60 min. Then liposome suspension was passed repeatedly under pressure through 0.8, 0.4, 0.2 and 0.1 µm pore-sized polycarbonate membranes (Poretics products) using an extrusion device from Lipex Biomembranes Inc. (Canadá). Glycosaminoglycans Glycosaminoglycan (GAG) solutions were prepared in 10 mM Tris-HCl buffer the same day to be used and stored at 4°C. HA was first depolymerized by sonication of the corresponding solution for 2 hours (with intervening periods of 30 s every 90 s) in a Braun Labsonic 2000 probe sonifier equipped with a 9.5 mm titanium probe, operating at 48 W. During sonication, the solution was kept in an ice-water bath and under a N 2 stream. In order to remove the titanium particles released from the probe, the HA solution was centrifuged 20 minutes at 6000 x g and 20 °C. Vesicle size analysis The size and size distribution of the phospholipid vesicles (0.1 mg lipid/mL), at two different ionic strengths, were analyzed by photon correlation spectroscopy (PCS) at 25°C. A PCS41 particle size analyzer (Malvern Autosizer IIc) and a 5 mW He-Ne laser (Spectra Physics), at an excitation wavelength of 633 nm, were used to analyze the dynamic light scattered by the liposome suspensions. The water refractive index (1.332) and viscosity (0.886) were used. The data were collected with a Malvern 7032N 72 data channel correlator and the mean hydrodynamic diameter was calculated from a cumulant analysis of the intensity auto-correlation function. Zeta Potential measurements The Zeta Potential of liposomes was measured, using the commercial device ZetaSizer 4 of Malvern Instruments (UK), by laser Doppler Anemometry based on the method of photon correlation spectroscopy [29]. The method uses the autocorrelation function of the light scattered in a colloid solution measured by a photon counting system. The particles move in an electric field of known strength in the interference pattern of two laser beams and produce scattered light which oscillates in time in a way which depends on the speed of the particles. The light scattered by the particles is collected by the photomultiplier and the measured autocorrelation function is first converted, using a Fourier Transform, into a


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frequency spectrum. The frequencies are then converted successively to velocities, electrophoretic mobilities and, finally, zeta potentials. The Malvern ZetaSizer 4 has an optic unit containing a 5 mW Helium-Neon laser, a ZET5104 electrophoresis cell, which uses a 4 mm diameter quartz capillary, with a sample handling unit and a multi-8 bit correlator with 72 data channels and 7 monitor channels with variable time expansion. The measurements of the Zeta Potential were performed, at different electrolyte concentrations and 25 °C, with liposomes prepared from BBEI or BBEII lipids at a concentration of 0.5 mg/mL. In order to check the Malvern device, a carboxy-modified polystyrene latex sample, with a Zeta Potential of -55 mV at 25°C, was used before each set of determinations as a control. Liposome permeability The permeability of the extruded unilamellar liposomes was examined by the fluorescence technique described by Weinstein et al. [30], using 5,6carboxyfluorescein (CF). The dye, obtained from Eastman-Kodak, was purified by Sephadex LH-20 column chromatography and acid precipitation (pH 4.5) as described previously [28]. Extruded unilamellar liposomes were prepared as described above in 20 mM Tris-HCl buffer (pH 7.4), containing 0.1 mM EDTA and 50 mM carboxyfluorescein. The non-encapsulated dye was removed by Sephadex G-50 column chromatography (20 x 1cm). A 400- µL aliquot of the IUVs suspension was applied to the column and 2 mL of liposomes eluted with 20 mM Tris-HCl buffer (pH 7.4), containing 0.1 mM EDTA and 100 mM NaCl, were collected. It was verified, by TLC and phosphorus analysis of the spots, that the lipid composition of the eluted liposomes was the same than that of the initial film. The carboxyfluorescein release from IUVs was monitored on a Kontron SFM25 spectrofluorimeter, using the excitation and the emission wavelengths of 492 and 520 nm, respectively. 100% fluorescence emission was calibrated using a 0.8 µM CF solution. Small aliquots of the eluted vesicle suspension (75 µL) were added to each cuvette, in a total volume of 2 mL, and the fluorescence was measured for 2 h. The total carboxyfluorescein fluorescence was determined by adding 150 µL of a 10% (v/v) Triton X-100 (Merck) solution. The CF latency, L, was calculated according to the equation:

Ft − F0   L = 100 1 −   FT − F0  where Ft is the fluorescence at time t, FT and F0 the fluorescence values obtained after Triton addition and that found at time zero, respectively.

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RESULTS AND DISCUSSION Effect of the ionic strength on the size of complex bovine brain liposomes Up to date, an analytical expression for the evaluation of the Zeta Potential from measured mobility data has only be found for particles which do not interact directly with the solvent molecules [27]. This equation, consequently, should not be valid exactly for lipid vesicles which tend to bind water avidly. The most important is the effect of the conductivity of the liposomal surface and this is specially true for vesicles with radius smaller than 50 nm [31].

Fig. 1. Light scattering particle size analysis of BBEI (A, B, C) and BBEII (D, E, F) liposomes prepared in a 10 mM Tris-HCl buffer media, in the absence (A,D) and in the presence of NaCl 25 mM (B, E) and 50 mM (C, F).

Thus, it is necessary to control the size of the liposomes used to measure the Zeta Potential. In order to obtain vesicles with an appropriate size, multilamellar liposomes were prepared by vortexing a dried lipid film with buffer and sonication in a bath-type sonicator. Figure 1 shows the diameter distribution of liposomes, prepared with the bovine brain lipidic extracts BBEI or BBEII, followed by photon correlation spectroscopy (PCS). The analysis of the data showed two different-sized vesicle populations for BBEI and BBEII liposomes prepared in a 10 mM Tris-HCl buffer medium, having the highest population a mean diameter around 150 nm in both cases. The smaller population, with a mean diameter around 475 nm, was more significative for BBEII liposomes (13%) than for BBEI liposomes (6%). Moreover, it could be observed that the modification of the ionic


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strength of the medium by the addition of 25 and 50 mM NaCl, respectively, did not modify the size distribution of liposomes. Influence of the ionic strength on calcium binding to complex bovine brain liposomes The binding of calcium to complex bovine brain liposomes was determined as the prerequisite to stabilize the interaction between GAGs and phospholipid membranes [32,33]. Zeta Potential measures were carried out to show the cation binding, using 3 mL aliquots of liposome suspensions with a lipidic concentration of 0.5 mg/mL and increasing amounts of calcium ions from 0 to 8 mM. Figure 2 shows the Zeta Potential values obtained at different ionic strength for BBEI (Fig. 2A) and BBEII (Fig. 2B) liposomes.

Fig. 2. Effect of ionic strength on calcium binding to complex bovine brain liposomes. Zeta Potential measures were carried out at 25°C for BBEI (A) and BBEII (B) liposomes in 10 mM Tris-HCl buffer (pH 7.4), in the absence (l) and in the presence of 10 (t), 25 (n) and 50 (u) mM NaCl. [Lipid] = 0.5 mg/mL. The values were the mean of three independent experiments.

The results obtained showed the differences existing between the negative surface charge of BBEI and BBEII liposomes, in accordance with the high phosphatidylserine content of the BBEI extract. Moreover, the effective binding of calcium to the bilayer phospholipids happened at a very low divalent cation concentration and seemed to reach a saturating situation above 2 mM of calcium. When adding different amounts of NaCl to the hydration medium, the extent of the variation of the Zeta Potential values after the addition of calcium to BBEI and BBEII liposome suspensions, at concentrations above 1 mM, was less significant at higher sodium concentrations. The results obtained also showed differences between BBEI and BBEII liposomes with regard to their ability to

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bind calcium and sodium ions, in agreement with the different negative charge at their bilayer surfaces. On the other hand, the competition between sodium and calcium to bind to the bilayer lipids became evident. Coating of complex bovine brain liposomes with glycosaminoglycans The electronegative and polymeric GAG molecules interact with different cations and the extent of the binding is dependent on the nature of both the cation and the GAG [34]. Moreover, it has been reported that the interaction between some liposomes and dextran sulfate needs the presence of calcium in the medium and that PS liposomes do not interact with dextran sulfate even in the presence of calcium concentrations in the milimolar range [32,35]. To show the interaction of BBEI and BBEII multilamellar liposomes with different GAGs, such as hyaluronic acid, chondroitin sulfate, dextran sulfate and heparin, the Zeta Potentials of liposomes were measured before and after their incubation with different amounts of the GAGs (3, 5 and 10 mg/mL) and calcium ions (3, 5, 8 mM). The incubation medium was 10 mM Tris-HCl buffer (pH=7.4). NaCl was not added to the incubation medium due to its competition with calcium, indicated above. Figures 3 and 4 show the changes in Zeta Potentials as a function of GAG and calcium concentrations for BBEI and BBEII liposomes, respectively.

Fig. 3. Effect of calcium and GAG concentrations on BBEI liposome-GAG interactions. The changes in Zeta Potentials are plotted versus GAG concentration in the absence (l) and in the presence of 3 (t), 5 (n) and 8 (u) mM calcium. The measures were carried out at 25°C in a 10 mM Tris-HCl buffer medium. The lipid concentration was 0.5 mg/mL. The values are the mean of three different measures.


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In the absence of calcium, the modification of the surface charge due to the incubation of liposomes with CSA and HP was small. The increases in Zeta Potential were in the range of 5 and 15 mV, depending on the liposome composition and on the nature and concentration of the GAGs. The interaction of HA with both BBEI and BBEII liposomes, in the absence of calcium, was more significant since at the highest GAG concentration used (10 mg/mL), the increase in Zeta Potential was near 30 mV. However, when DS was added to the incubation medium in the absence of the cation, the negative Zeta Potential of BBEI liposomes became 3 mV more negative whereas the maximun increase of the Zeta Potential of BBEII liposomes was of 5 mV. When calcium was added to the incubation medium, in the absence of GAGs, the significant increase in the Zeta Potential values observed (near 50 mV) depended on the calcium concentration. Addition of GAGs to liposome preparations containing calcium led to a new decrease in the Zeta Potential values, which depended on the lipid and GAG nature and on the amount of calcium and the different GAGs in the incubates. Nevertheless, it can be noted that the Zeta Potential values measured in the presence of calcium and GAGs were always less negative than those measured when calcium was not added to the incubation medium. GAG-liposome interactions depended on the polysacharide nature and Zeta Potential modifications could be related with the GAG concentration. GAGs with a higher negative charge (DS and HP) led to greater changes in the surface charge when calcium was present in the incubation medium. However, the small ability of

Fig. 4. Effect of calcium and GAG concentrations on BBEII liposome-GAG interactions. The changes in Zeta Potentials are plotted versus GAG concentration in the absence (l) and in the presence of 3 (t), 5 (n) and 8 (u) mM calcium. The measures were carried out at 25°C in a 10 mM Tris-HCl buffer medium. The lipid concentration was 0.5 mg/mL. The values are the mean of three different measures.

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HA to interact with liposomes in the presence of calcium could be justified by its neutral charge. Thus, there is a direct relationship between the net charge of the GAGs and their ability to interact with BBEI and BBEII liposomes in the absence of calcium with Zeta Potential values of -77 and –59 mV, respectively, or in the presence of different amounts of the cation with Zeta Potential values in the range of -30 and –10 mV. The results obtained in this study suggest that there is an effective coating of the bilayer surface when glycosaminoglycans are added to liposome suspensions. The shielding of the negative surface charge by the neutral hyaluronic acid, in the absence of calcium, and the increase in the negative charge when the negative polyelectrolytes chondroitin sulfate, heparin or dextran sulfate are added to calcium-containing liposome suspensions account for the formation of stable liposome-GAG complexes. It has also been shown that there is an interacion between the negative GAGs and both BEEI and BBEII liposomes in the absence of calcium ions, although this interaction is higher in the presence of the cation, in agreement with the calcium bridge mechanism suggested in the literature [32]. Moreover, the high content in PS of BBEI liposomes did not cause dificulty in their ability to bind DS towards calcium bridges as it has been suggested previously [32].

Fig. 5. Effect of DS molecular weight on liposome-GAG interactions. Zeta Potentials obtained for BBEI (A and B) and BBEII (C and D) liposomes are plotted versus calcium concentration (A and C) in the absence (n) and in the presence of 3 mg DS/mL (l, t) or versus DS concentration (B and D) at a fixed calcium concentration (5 mM). DS polymers of 15,000 (l) and 500,000 (t) g/mol were used. The measurements were carried out at 25°C in a 10 mM Tris-HCl buffer medium. The lipid concentration was 0.5 mg/mL. The values are the mean of three different measures.


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The effect of the GAGs molecular weight in the change of the surface charge of liposomal bilayers was assayed using DS of 15,000 and 500,000 g/mol (Figure 5). At a fixed DS concentration (3 mg/mL), negative Zeta Potentials were obtained for both sized DS molecules, being the absolute values of the Zeta Potentials higher for the polymer of 500,000 g/mol at any calcium concentration assayed. The same effect of the DS molecular weight was observed when the calcium concentration was fixed (5 mM). Therefore, this shows that there is a direct relationship between the DS molecule size and their ability to bind on the liposome surface in presence of calcium. Permeability properties of liposomes coated with glycosaminoglycans Permeability studies were carried out to assess the modification of the bilayer barrier properties as a consequence of the coating of the liposome surface by GAGs. The rate of CF leakage was studied in 20 mM Tris-HCl buffer (pH=7.4), containing 0.1 M EDTA and 0.1 M NaCl, during 120 min at 37°C. The measure of CF efflux was initiated immediately after the free dye was eliminated by molecular sieve chromatography, using BBEI and BBEII liposomes, alone or incubated with calcium or/and CSA or HP. Figure 6 illustrates the time course for CF latency in all the conditions described above. The latency of the dye, calculated as indicated in Materials and methods, shows the percentage of the initial entrapped dye retained into liposomes versus time. The results obtained showed that, in all the experimental conditions assayed, BBEI and BBEII liposomes are not very permeable at physiological conditions and that, except for BBEI liposomes in the absence of calcium, the permeability was lower when liposomes were incubated with the GAGs. The similar permeability of BBEI liposomes, without calcium, in the absence and in the presence of HP and CSA, is in agreement with the very low modification of the Zeta Potential when BBEI liposomes were incubated with CSA and HP in the absence of calcium. Using BBEII liposomes, with a reduced content in PS, the CF latency was lower than that observed for BBEI liposomes in absence of GAGs. Moreover, the higher interaction of BBEII liposomes with CSA and HP in absence of calcium would account for the increased latency of these vesicles when CSA and HP were added to the incubates. The addition of calcium 1mM to the medium, in order to facilitate the interaction between GAGs and the bilayer phospholipids, led to an higher increase of the CF latency when GAGs were added to the incubation media, irrespectively of the use of BBEI or BBEII liposomes. The results obtained in this paper show the possibility of preparing liposomes with a polysaccharide hydrophilic coat, the way of favouring the binding of GAGs to the bilayer surface and the stability of the resultitng liposome-GAG systems. Moreover, the reduced permeability of the GAG-coated liposomes points out their ability to retain encapsulated drugs and, so, their potential usefulness as sustained release drug carriers.

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Fig. 6. Time course of carboxyfluorescein latency from liposomes coated by GAGs. CF fluorescence was measured during 120 min at 37°C in the absence (A and B) and in the presence of calcium 1mM (C and D) for BBEI (A and C) or BBEII (B and D) liposomes. Symbols correspond to IUVs without any GAG (l) or IUVs in the presence of 75 µg/mL of CSA (t) and HP (n). The measures were carried out at 37°C in a 10 mM Tris-HCl buffer medium, containing 0.1 mM EDTA and 0.1 M NaCl. The lipid concentration was 15 µg/mL. The values are the mean of three independent experiments.

The hydrophilic coating provides liposomal carriers with long-circulating properties. Stability in biological media and biodistribution studies to confirm the benefits of GAGs instead of GM 1 or polyethylene glycol, widely reported in the literature, are now in course. Acknowledgements. This work was supported by a grant from DGICYT (PB94-0911-A). REFERENCES 1. Betageri, G. V., Jenkins, S. A. and Parsons, D. L. Liposome Drug Delivery Systems Technomic Publishing Company, Inc., Lancaster, Pennsylvania 1993. 2. Kadir, F., Zuidema, J. and Crommelin, D. J. A. Liposomes as drug delivery systems for intramuscular and subcutaneous injection, in: Drug Carriers in Medical Applications (Rolland, ed.), New York, Marcel Dekker, 1993, 165198.


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31. Cevc, G. Electrostatic characterization of liposomes. Chem. Phys. Lipids 64 (1993) 163-186. 32. Arnold, K., Ohki, S. and Krumbiegel, M. Interaction of dextran sulfate with phospholipid surfaces and liposome aggregation and fusion. Chem. Phys. Lipids 55 (1990) 301-307. 33. Steffan, G., Wulff, S. and Galla, H. J. Divalent cation-dependent interaction of sulfated polysaccharides with phosphatidylcholine and mixed phosphatidylcholine/phosphatidylglycerol liposomes. Chem. Phys. Lipids 74 (1994) 141-150. 34. Casu, B. Structural features and binding properties of chondroitin sulfates, dermatan sulfate and heparan sulfate. Seminars in Thrombosis and Hemostasis 17 (1991) 9-14. 35. Krumbiegel, M. and Arnold, K. Microelectrophoresis studies of the binding of glycosaminoglycans to phosphatidylcholine liposomes. Chem. Phys. Lipids 54 (1990) 1-3.