Unfolding of insulin at the surface of ZnO quantum dots

ZnO quantum dots (QDs) have been used in many biomedical applications such as bioimaging, cancer treatments and etc. Crystallinity, particle size, optical absorption and photoluminescence spectra of ZnO QDs were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis absorption spectroscopy and fluorescence spectroscopy respectively. Interaction of ZnO QDs with insulin was investigated by fluorescence quenching, circular dichroism (CD), isothermal titration calorimetry (ITC) and thermal aggregation tests. The fluorescence quenching results showed a static type quenching along with red shift in synchronize fluorescence (a sign of protein unfolding). CD spectroscopy results also confirmed this unfolding and show a reduction in alpha helices content of insulin in contact with ZnO QDs and their conversion to random coils. According to ITC results, the ΔG, ΔH and binding constant of this interaction are −32.35 kJ/mol, −43.21 kJ/mol and 4.69 × 105 M−1, respectively. Thermal aggregation test showed fast aggregation of insulin in the presence of ZnO QDs. Therefore in biological application of ZnO QDs such as bioimaging, presence of such QDs in vicinity of insulin could unfold this protein.