Synthetic polymers are often used for delivery of therapeutic drugs and proteins. We report the binding of milk beta-lactoglobulin (beta-LG) with poly(ethylene glycol) (PEG), methoxypoly(ethylene glycol) polyamidoamine (mPEG-PAMAM-G-3) and polyamidoamine (PAMAM-G4) nanoparticles in aqueous solution at pH 7.4, using Fourier Transform infrared (FTIR), circular dichroism (CD), fluorescence spectroscopic methods, transmission electron microscopy (TEM) and molecular modeling. Structural analysis showed that polymers bind beta-LG via both hydrophilic and hydrophobic contacts with overall binding constants KPEG-8000-beta-LG = 4.8 (+/- 0.4) x 10(4) M-1 and KmPEG-PAMAM-G3-beta-LG = 5.8 (+/- 0.6) x 10(5) M-1 and KPAMAM-G4-beta-LG = 6.7 (+/- 0.9) x 10(4) M-1. The number of binding sites were occupied by polymers on protein (n) was 0.3 for PEG-8000, 0.4 for mPEG-PAMAM-G3 and 0.4 for PAMAM-G4. The order of binding is mPEG-PAMAM-G3 > PAMAM-G4 > PEG-8000. Transmission electron microscopy showed significant changes in protein morphology as polymer-protein complexation progressed with major increase in the diameter of the protein aggregate (180%). Furthermore, modeling showed several H-bonding systems between PEG and different amino acids stabilize polymer-beta-LG complexes. mPEG-PAMAM-G3 is a stronger protein binder than PAMAM-G4 and PEG-8000. (C) 2013 Elsevier Ltd. All rights reserved.