We have calculated the charge transfer (CT) between the PO4- group of DNA and the lysine (Lys) and arginine (Arg) positive side chains of histones in presence of water and K+ ions. The calculations were performed at the HF + MP2 level, using the TZVP basis set. The calculations were corrected for basis set superposition error and besides Mulliken's population analysis we have introduced the - for charged systems more reliable - natural population analysis. The results show that the bare PO4- - Lys and the PO4- - Arg interactions become weaker, mainly, due to the presence of the K+ ion. We have found 0.067e CT for Lys and 0.050e for Arg. (c) 2008 Elsevier B. V. All rights reserved.