Spectroscopic Investigation on the Binding of a Cyanine Dye with Transferrin

Abstract

Transferrin (Tf) can control the level of free iron as iron-binding blood plasma glycoprotein in biological fluids. Tf has been exploited in the recent years on account of the potential function as a drug carrier targeting to tumor cells. Cyanine dyes have been widely studied as photosensitizers. The binding mechanism of Tf with 3, 3′-di(3-sulfopropyl)-4, 5, 4′, 5′-dibenzo-9-ethyl-thiacarbocyanine triethylammonium salt (ETC) was characterized at varying pHs and temperatures by fluorescence, UV-Vis absorption, circular dichroism (CD), and molecular modeling methods. The results showed that the static fluorescence quenching occurred between Tf and ETC. It was found that ETC bound strongly with Tf with an intrinsic binding constant (Ka), in the order of 107 m-1. The thermodynamic parameters demonstrated that van der Waals force or hydrogen bonds were the major binding force. The binding of ETC-Tf caused the secondary conformational change of Tf with increasing the α-helix content in Tf, which was confirmed by the results of spectroscopic experiments. Molecular modeling revealed that ETC bound residues located in the N-lobe of Tf by van der Waals force and induced local structural changes of Tf. This study may provide the theoretical foundations for ETC as a probe to label Tf, which is further beneficial to the Tf-targeted drugs in vivo.