The adsorption of plasmid DNA onto two different types of ceramic hydroxyapatite beads with a particle diameter of 20  μm, namely Ceramic Hydroxyapatite Type II and the Type III, which is not commercially available, were investigated. Type II and the Type III have a pore diameter of 80 and 240  nm, respectively. Equilibrium and dynamic binding capacity for a 4.9  kbp model plasmid on Ceramic Hydroxyapatite Type II and Type III were enhanced by addition of NaCl to the adsorption buffer. This result indicates that the adsorption mechanism cannot be solely explained by electrostatic interaction. The affinities of plasmid DNA for Ceramic Hydroxyapatite Type II (with a K(D) of ≈0.005  mg/mL) and to Hydroxyapatite Type III (with a K(D) of ≈0.045  mg/mL) were not affected by NaCl, whereas the binding capacity was. This observation corroborates the assumption that a change of the shape of the plasmid molecule is affected and could be the reason for increased binding capacity with salt. The maximal binding capacity shows that at least a part of the CHT II bead must be accessible for the plasmid, whereas CHT III can be saturated with the plasmid. In both cases, an extremely hindered transport takes place.

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