Protein-polymer interactions play a very important role in a number of applications, but details of these interactions are not fully understood. Chemical modification was introduced here to tune protein polymer interactions in a systematic manner, where methemoglobin (Hb) and poly(acrylic acid) (PAA) served as a model system. Under similar conditions of pH and ionic strength, the influence of protein charge on Hb/PAA interaction was studied using chemically modified Hb by isothermal titration calorimetry (ITC). A small fraction of COOH groups of Hb were amidated with triethylenetetramine (TETA) or ammonium chloride to produce the corresponding charge ladders of Hb-TETA and Hb-ammonia derivatives, respectively. All the Hb/PAA complexes produced here are bioactive, entirely soluble in water, and indicated the retention of Hb structure to a significant extent. Binding of Hb to PAA was exothermic (Delta H < 0). The binding of Hb-TETA charge ladder to PAA indicated decrease of Delta H from -8 +/- 0.2 to -89 +/- 4 kcal/mol, at a rate of -3.8 kcal/mol per unit charge introduced via modification. The Hb-ammonia charge ladder, in contrast, showed a decrease of Delta H from -8 +/- 0.2 to -17 +/- 1.5 kcal/mol, at much slower rate of -1.0 kcal/mol per unit charge. Thus, the amine used for the modification played a strong role in tuning Hb/PAA interactions, even after correcting for the charge, synergistically. Charge clustering may be responsible for this synergy, and this interesting observation may be exploited to construct protein/polymer platforms for advanced biomacromolecular applications.