The molecular chaperone DnaK prevents intracellular protein misfolding and aggregation by transiently binding with newly synthesized polypeptides and protein-folding intermediates. DnaK preferentially binds to peptides with basic residues (Arg/Lys) present on the outside of a hydrophobic core. The electrostatic contribution toward DnaK/peptide binding was determined by measuring the dissociation constant of DnaK complexes with two fluorescein-labeled peptides (f-NRLLLTG and f-NALLLTG) using fluorescence anisotropy. The measured dissociation constants, K-d, differ significantly at low ionic strength: at 20 mM phosphate buffer; K-d for DnaK and f-NRLLLTG is 0.2 muM while that for DnaK and f-NALLLTG is 1 muM. This difference, attributed to stronger Coulombic binding in the case of f-NRLLLTG, vanishes at high ionic strength due to electrostatic screening. For f-NRAAATG, no interaction with DnaK was apparent, showing that hydrophobic interactions are essential in chaperone/peptide binding. The ionic strength dependence of the electrostatic interaction between DnaK and NRLLLTG is interpreted in terms of an approximate analytic model for the potential of mean force between DnaK and dipolar peptide. The calculated electrostatic binding free energy, DeltaG(ele), of about -1 kcal/mol, is in good agreement with the experiment result for low salt concentration, as obtained from the ionic strength dependence of the measured dissociation constants. Our analytic model, here generalized to a pair of particles differing in size, charge, and dipole, provides a useful first estimate of electrostatic effects that can be exploited for control of protein-protein interactions.