We study the interaction of neutral polyethylene glycol (PEG) molecules of different molecular weights (MVVs) with the charged residues of the alpha-hemolysin channel secreted by Staphylococcus aureus. Previously reported experiments of PEG equilibrium partitioning into this nanopore show that the charge state of the channel changes the ability of PEG entry in an MW-dependent manner. We explain such an effect by parameter-free calculations of the PEG self-energy from the channel 3D atomic structure that include repulsive dielectrophoretic and hydrostatic forces on the polymer. We found that the pH-induced shift in the measured free energy of partitioning Delta Delta G(exp), from single-channel conductance measurements agrees with calculated energy changes Delta Delta E-calc. Our results show that the PEG-sizing technique may need corrections in the case of charged biological pores.