Physico-chemical studies on the interaction of dendrimers with lipid bilayers. 1. Effect of dendrimer generation and liposome surface charge.

Studies on the interaction of different generation poly (amido amine) (PAMAM) dendrimers (2G, 4G and 6G) and liposomes of different compositions were carried out by a combined turbidity, dynamic light scattering and atomic force microscopic measurements. Liposomes comprising soy lecithin (SLC, negative surface charge), 1, 2-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC, mildly positive surface charge), 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol (DPPG, negatively charged) and a biologically simulated mixture of DPPC + DPPG (7:3, M/M, negatively charged) were used as model bilayers. 30 wt% cholesterol was used in each combination as it is known to control the fluidity of membrane bilayers. Silica was used as a negatively charged hard sphere model with an aim to compare the results. Both the turbidity and hydrodynamic diameter values of all the liposomes, except DPPC, passed through maxima upon the progressive addition of PAMAM; the effect was insignificant in case of DPPC. Formation of dendriosome, a complex formed between dendrimer and liposome, resulted in the charge reversal of the negatively charged liposomes. Interaction between PAMAM and liposome was found to be governed by electrostatic as well as hydrogen bonding. Generation dependent PAMAM activity followed the order: 6G >4G>2G in terms of overall dendrimer concentration. However, interestingly, the order was reverse when PAMAM activity was considered in terms of total end group concentrations. AFM studies reveal the rupture of bilayer structure upon addition of dendrimer.

Naphthalene sulfonate functionalized dendrimers at the solid-liquid interface: influence of core type, ionic strength, and competitive ionic adsorbates.

The adsorption of naphthalene disulfonic acid surface-functionalized dendrimers (generation 4) on to colloidal alumina particles is reported, considering the role of dendrimer core type (ammonia vs benzylhydrylamine-polylysine) and electrolyte addition on the adsorption affinity and interfacial packing and competitive adsorption. Irrespective of the dendrimer core type, the maximum adsorbed amount increased with increasing ionic strength. The adsorption affinity of a benzylhydrylamine-cored SPL-7013 increased with increasing ionic strength, whereas a decrease was observed for the ammonia-cored SPL-2923. At high ionic strengths (>or=10(-1) M NaCl) dendrimers close pack at the interface as an array of equivalent hard spheres, whereas at lower ionic strengths both dendrimers occupy a lower area than theoretically predicted for either cubic or hexagonal close packing, based on double layer repulsion. The additional attraction between dendrimers is attributed to the intercalation of the neighboring dendrons. Adsorption of SPL-2923 is enhanced by the presence of Ca2+ ions and depressed by the presence of HCO3- and HPO4(2-) ions, whereas SPL-7013 adsorption is only depressed by the presence of HPO4(2-) ions, suggesting a dendrimer-specific competitive adsorption process. This work clearly demonstrates the role of dendrimer architecture on adsorption at an interface, a process of fundamental importance to a wide range of dendrimer applications.