Insights into the Thermodynamics of Polymer Nanodot-Human Serum Albumin Association: A Spectroscopic and Calorimetric Approach

With the advent of newer luminescent nanoparticles for bioimaging applications, their complex interactions with individual biomolecules need to be understood in great detail, before their direct application into cellular environments. Here, we have presented a systematic and detailed study on the interaction between luminescent polymer nanodots (PNDs) and human serum albumin (HSA) in its free and ligand-bound state with the help of spectrophotometric and calorimetric techniques. At physiological pH (pH = 7.4), PNDs quench the intrinsic fluorescence of HSA as a consequence of ground-state complex formation. The binding stoichiometry and various thermodynamic parameters have been evaluated by using isothermal titration calorimetry and the vant Hoff equation. It has been found that the association of PNDs with HSA is spontaneous (Delta G(0) = -32.48 +/- 1.24 kJ mol(-1)) and is driven by a favorable negative standard enthalpy change (Delta H-0 = -52.86 +/- 2.12 kJ mol(-1)) and an unfavorable negative standard entropy change (Delta S-0 = -68.38 +/- 2.96 J mol(-1) K-1). These results have been explained by considering hydrogen bonding interactions between amino and hydroxyl groups (-NH2 and -OH) of PNDs and carboxylate groups (-COO-) of glutamate (Glu) and aspartate (Asp) residues of HSA. The binding constant of PNDs with HSA is estimated to be 4.90 +/- 0.19 X 10(5) M-1. Moreover, it has been observed that warfarin-bound HSA (war-HSA) shows a significantly lower binding affinity (K-b = 1.15 +/- 0.19 X 10(5) M-1) toward PNDs, whereas ibuprofen-bound HSA (ibu-HSA) shows a slightly lower affinity (K-b = 3.47 +/- 0.13 X 10(5) M-1) compared with the free HSA. In addition, our results revealed that PNDs displace warfarin from site I (subdomain IIA) of HSA because of the partial unfolding of war-HSA. We hope that the present study will be helpful to understand the fundamental interactions of these biocompatible PNDs with various biological macromolecules.