Electrostatics and Charge Regulation Drive Phosphate Accumulation in Cationic Dendrimers

Branched polycations such as poly-(amidoamine) (PAMAM) dendrimers have been extensively investigated for applications including drug delivery and gene transfection, where phosphate-buffered solutions are commonly employed to maintain pH. Experimental and simulation studies have reported phosphate accumulation around PAMAM and attributed it, at least in part, to phosphate-specific interactions. Here, we investigate whether such specific interactions are necessary to explain phosphate association. Combining constant-pH molecular simulations, potentiometric titrations, and Site Binding theory, we study the coupled ionization of G2 PAMAM dendrimers and phosphate ions in aqueous solution. By employing a coarse-grained model that intentionally excludes phosphate-specific short-range interactions, we isolate the effects of electrostatics and charge regulation. The simulations quantitatively reproduce the ionization behavior of PAMAM and phosphate and reveal a strongly asymmetric charge-regulation response: phosphate has only a minor effect on PAMAM ionization, whereas PAMAM significantly enhances phosphate ionization in its vicinity. This coupling drives phosphate accumulation around PAMAM, with maximum adsorption at circumneutral pH where both species are highly charged. At phosphate concentrations comparable to those used in phosphate-buffered saline, the adsorbed phosphates reduce the effective charge of the PAMAM-phosphate complex. The qualitative agreement with previous experimental observations demonstrates that phosphate accumulation does not necessarily require phosphate-specific interactions but can emerge as a generic consequence of electrostatic attraction and coupled ionization equilibria. More broadly, the results highlight that multivalent buffer ions can act as active regulators of macromolecular electrostatics rather than merely as pH-control agents or passive screening species.