Binding of [Cr(phen)3](3+) to transferrin at extracellular and endosomal pHs: potential application in photodynamic therapy.

Transferrin is an iron-binding blood plasma glycoprotein that controls the level of free iron in biological fluids. This protein has been deeply studied in the past few years because of its potential use as a strategy of drug targeting to tumor tissues. Chromium complex, [Cr(phen)3](3+) (phen=1,10-phenanthroline), has been proposed as photosensitizers for photodynamic therapy (PDT). Thus, we analyzed the binding of chromium complex, [Cr(phen)3](3+), to transferrin for a potential delivery of this diimine complex to tumor cells for PDT. The interaction between [Cr(phen)3](3+) and holotransferrin (holoTf) was studied by fluorescence quenching technique, circular dichroism (CD) and ultraviolet (UV)-visible spectroscopy. [Cr(phen)3](3+) binds strongly to holoTf with a binding constant around 10(5)M(-1), that depends on the pH. The thermodynamic parameters indicated that hydrophobic interactions played a major role in the binding processes. The CD studies showed that there are no conformational changes in the secondary and tertiary structures of the protein. These results suggest that the binding process would occur in a site different from the specific iron binding sites of the protein and would be the same in both protein states. As secondary and tertiary structures of transferrin do not show remarkable changes, we propose that the TfR could recognize the holoTf despite having a chromium complex associated. Understanding the interaction between [Cr(phen)3](3+) with transferrin is relevant because this protein could be a delivery agent of Cr(III) complex to tumor cells. This can allow us to understand further the role of Cr(III) complex as sensitizer in PDT.