pH sensitive functionalized graphene oxide as a carrier for delivering gemcitabine: A computational approach

Atomistic Molecular Dynamics (MD) simulations of both the physisorption process and the hydration behavior of the aggregate consisting of gemcitabine (GEM) - poly(acrylic acid) (PAA) - graphene oxide (GO) are performed in explicit solvent medium at two different pH values. The different ionization of pH sensitive groups characterizing the aggregate influences the energy of physisorption and the hydration shell around the complex. The decrease of negative charge density, occurring at acid pH, facilitates the physisorption process between PAA and GO. On the other hand, at the same pH value, a weaker interactionbetween GEM and PAA is observed. MD simulations show a partial physisorption of the drug from the polymer at pH = 4.0. pH change affects also morphology of GO sheet which is more extended at basicpH value due to intrasheet electrostatic interactions. Radial distribution function (RDF) indicates that carboxylate oxygens of PAA and alkoxide oxygens of GO strongly attract dipolar water molecules, thus affecting the hydration shell around the complex in comparison with unionized GO and bulk water. Finally, an analysis based on H-bond correlation function clearly shows that: (a) hydrogen bond between both carboxylate and alkoxide oxygen atoms are longer lived than those observed in unionized GO and bulk water; (b) RDF decays slower for PAA (f = 0.8) with respect to ionized GO, hence indicating that COO groups bind water molecules stronger than R-O.