The interaction mechanism between lipopeptide (daptomycin) and polyamidoamine (PAMAM) dendrimers

The interaction mechanism of lipopeptide antibiotic daptomycin and polyamidoamine (PAMAM) dendrimers was studied using fluorescence spectroscopy. The fluorescence changes observed are associated with daptomycin-dendrimer interactions. The binding isotherms were constructed by plotting the fluorescence difference at 460 nm from kynurenine (Kyn-13) of daptomycin in the presence and absence of dendrimer. A one-site and two-site binding model were quantitatively generated to estimate binding capacity and affinity constants from the isotherms. The shape of the binding isotherm and the dependence of the estimated capacity constants on dendrimer sizes and solvent pH values provide meaningful insight into the mechanism of interactions. A one-site binding model adequately describes the binding isotherm obtained under a variety of experimental conditions with dendrimers of various sizes in the optimal binding pH region 3.5 to 4.5. Comparing the pH-dependent binding capacity with the ionization profiles of daptomycin and dendrimer, the ionized aspartic acid residue (Asp-9) of daptomycin primarily interact with PAMAM cationic surface amine.