Molecular dynamics simulation has been used to study the specific interactions between poly(ethylene glycol) (PEG) and three drug molecules for which PEG is used to aid delivery: paclitaxel and piroxicam, where PEG is a carrier agent, and hematoporphyrin, where PEG is covalently attached to form a "stealth shield". Simulating at physiological salt concentration, we found no evidence of any specific interaction between paclitaxel or piroxicam with PEG, but found a strong interaction for the case of hematoporphyrin. This interaction is lipophilic in nature, between the nonpolar (CH(2))(2) groups of the PEG and the porphin ring of the hematoporphyrin. This interaction was found to be strong enough that the PEG aggregated to the hematoporphyrin, independent of whether or not it was covalently bound. Interestingly, when the simulation was repeated in absence of salt we found evidence of this interaction being weakened. This led us to hypothesize a previously unforeseen mechanism: interaction with salt cations cause the PEG to coil around the salt ions, each ion binding to many PEG oxygens, increasing the exposure of the nonpolar ethylene groups, thus increasing the effective hydrophobicity of PEG. The Hydrophobic ethylene groups of the PEG chains adhere strongly to the hydrophobic porphin ring. Experiments involving absorption spectra measurements were conducted, and these results also indicated that presence of salt at physiological level increases the effective attractive interaction between PEG and hematoporphyrin. Taken together, our results demonstrate that while PEG, due to its solubility in both polar and nonpolar solvents, may act as a dissolution aid for paclitaxel and piroxicam, of the three drug molecules studied it will only have a protective role for the case of the hematoporphyrin.

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