(a) Molecular model of the average repeating unit of heparin shown as atom-colored sticks-and-balls (c), gray; O, red; N, blue; S, yellow; H, white. Na+ counterions are portrayed as purple spheres. (b) Molecular model of protamine structure in physiological solution. The small protein backbone is evidenced by a red ribbon, while the residues are shown as red sticks. One of the guanidinio –NH2 groups is highlighted using red spheres. (c) Molecular model of a generation 2 (G2) ethylenediamine (EDA)-core poly(amidoamine) (PAMAM) dendrimer structure in physiological solution. The dendrimer EDA core is shown as light blue sticks, the branching units are depicted as dodger blue sticks, while the branches and the relevant surface amine groups, positively charged at pH 7.4, are highlighted as navy blue spheres. In panels b,c, Na+ and Cl− ions and counterions are represented by small and large white spheres, respectively, while water molecules are not shown for clarity.