Interactions and conformational changes of human serum albumin at the surface of electrochemically synthesized thin polypyrrole films

Conformational aspects of the adsorbed human serum albumin (HSA) onto the surface of electrochemically synthesized thin polypyrrole films (PPyTS) have been studied using quartz crystal microbalance (QCM) in situ monitoring, contact angle measurements and synchrotron radiation photoemission and photoabsorption spectroscopies. QCM showed that the surface of the polypyrrole was readily covered with a very thin HSA overlayer (80% coverage within 16 min). The interface of the polypyrrole/HSA systems was characterized by water static contact angles (θw) versus the amount of adsorbed HSA. θw could be much lower than those of the bare polypyrrole and the pure albumin therefore emphasizing a preferential orientation of the most hydrophobic sites of the protein towards the polymer substrate and leaving its relatively hydrophilic sites at the air-adsorbed protein interface. These macroscopic measurements were completed by the synchrotron photoemission (XPS) and absorption (XANES) spectroscopies in order to take a “fingerprint” of the molecular structure of the adsorbed HSA. The C1s region excited by a mono-energetic X-ray beam of 520 eV exhibited a peak at 288 eV due to the peptide link in the adsorbed protein the relative intensity of which increased massively with HSA coverage. The measurements of the carbon C K and oxygen O K X-ray absorption near edge structures (XANES) after adsorption showed the appearance of new signals characteristic to the π* and σ* resonances of the different functionalities of the protein thus furnishing a precious information on the possible orientations of the hydrophobic and hydrophilic amino acids by which the protein might interact with the polypyrrole. The results obtained with the oxygen O K edge absorption are in good agreement with those of contact angle measurements. Combining wettability, XPS and XANES results, a tentative simple model was proposed for the HSA adsorption onto polypyrrole.