Protein-polymer conjugates are widely used in therapeutic drug delivery. We report the bindings of trypsin (Try) and trypsin inhibitor (Tryi) with polyamidoamine (PAMAM-G4) dendrimer at physiological conditions, using thermodynamic analysis, UV-Visible and Fourier transform infrared (FTIR) spectroscopic methods. Thermodynamic parameters Delta S, Delta H and Delta G showed protein-PAMAM bindings occur via H-bonding and van der Waals contacts with trypsin inhibitor forming more stable conjugate than trypsin. PAMAM complexation induces more perturbations of trypsin inhibitor structure than trypsin with reduction of protein alpha-helix and major changes of beta-structures. The negative value of Delta G indicates spontaneous protein-polymer conjugation at room temperature. (C) 2015 Elsevier Inc. All rights reserved.