Calorimetric and fluorimetric interaction studies of Europium(III) complexes with Bovine Serum Albumin.

Lanthanide (Ln(III)) derivatives, mainly Eu(III) and Tb(III) complexes, have been broadly exploited as sensors in physiological conditions for the detection of relevant clinical biomarkers.[1] This can be done thanks to their peculiar properties such as long Ln(III) luminescence lifetime and the so called antenna effect. The luminescent Ln(III) complex must interact selectively with a target bioanalyte competing with species, such as proteins. Serum albumins, which represent 52% of the protein composition in the circulatory system, play many physiological and pharmacological functions in the delivery of a variety of endogenous and exogenous species. Human serum albumin (HSA) and Bovine serum albumin (BSA) are the most studied serum albumins, and classified as homologous proteins. Changes of the albumin levels in blood could be a sign of several disorders, such as liver disease, neoplasia and more [2]. With the aim to assess the interaction of some Eu(III) luminescent complexes with the BSA and thus the possible competition with other analytes, fluorescence/luminescence titrations with the cited protein were carried out in this work, the ligands employed differ for their antenna moieties. In addition, the thermodynamics of the interactions were studied by isothermal titration calorimetry (ITC). The protonation constants for the ligands alone and their formation constants with Eu(III) have been determined by potentiometric and spectrophotometric techniques [3], the fluorescence/luminescence titrations and ITC titrations of the complexes with BSA have been run at pH = 7.4 (MOPS buffer). The results obtained by ITC measurements, indicated negative enthalpy values for the interaction with BSA. Computational methods provided additional information about the formed adducts: in particular, EuL (L =bpcd and bisoQcd) complexes form adducts with BSA with different stoichiometry, which result from different interaction mechanism. Also, as showed by competitive titrations with site markers (warfarin/ibuprofen/digitoxin) and Molecular Docking/Molecular Dynamics Simulations, the interaction with BSA occurs at different sites for Eu(bpcd) and Eu(bisoQcd) [4]. Bibliography [1] Piccinelli F., De Rosa C., Melchior A., Faura G., Tolazzi M., Bettinelli M. Eu(iii) and Tb(iii) complexes of 6-fold coordinating ligands showing high affinity for the hydrogen carbonate ion: a spectroscopic and thermodynamic study, Dalt. Trans., 2019, 48, 1202–1216. [2] Volkova K.D., Kovalska V.B., Losytskyy M.Y., Bento A., Reis L.V., Santos P.F., Almeida P., Yarmoluk S.M. Studies of benzothiazole and benzoselenazole squaraines as fluorescent probes for albumins detection, J. Fluoresc. 2008, 18, 877–882. [3] Leonzio M., Melchior A., Faura G., Tolazzi M., Bettinelli M., Zinna F., Arrico L., Di Bari L., Piccinelli F. A chiral lactate reporter based on total and circularly polarized Tb(iii) luminescence, New J. Chem. 2018, 42, 7931–7939. [4] De Rosa C., Melchior A., Sanadar M., Tolazzi M., Giorgetti A., Ribeiro R.P., Nardon C., Piccinelli F. Effect of the Heteroaromatic Antenna on the Binding of Chiral Eu(III) Complexes to Bovine Serum Albumin, Inorg. Chem. 2020, 59, 12564–12577.