In(III) pyridinecarboxylate complexes: Composition, solution equilibria estimation, bioevaluation and interactions with HSA

Abstract

Two In(III) – pyridinecarboxylates ([In(Pic)₂(NO₃)(H₂O)] (InPic; HPic = picolinic acid), [In(HDpic)(Dpic)(H₂O)₂]·5H₂O (InDpic; H₂Dpic = dipicolinic acid), have been synthesized by one-step procedure. The complexes composition was confirmed by physicochemical analyses and X-ray diffraction confirmed molecular structure of both complexes. Moreover, complex species speciation was described in both systems by potentiometry and ¹H NMR spectroscopy and mononuclear complex species were determined; [In(Pic)]²⁺ (logβ₀₁₁ = 6.94(4)), [In(Pic)₂]⁺ (logβ₀₂₁ = 11.98(9)), [In(Dpic)]⁺ (logβ₀₁₁ = 10.42(6)), [In(Dpic)₂]⁻ (logβ₀₂₁ = 17.58(7)) and [In(Dpic)₂(OH)]²⁻ (logβ₋₁₂₁ = 10.18(6)). To confirm the complexes stability in 1 % DMSO, ¹H NMR spectra were measured (immediately after dissolution up to 96 h). Antimicrobial and anticancer assays indicate a more significant sensitivity of S. aureus bacteria and MDA-MB-231 cancer cells to the InPic complex (IC₅₀ = 25 and 340.7 μM) than to the InDpic (IC₅₀ = 50 and 975.4 μM). The interaction and binding mechanism of picolinic/dipicolinic acid and their indium(III) complexes with HSA (human serum albumin) were studied using fluorescence and CD spectroscopy. The results confirmed that the studied compounds had bound successfully to HSA, and the binding parameters and constants (K_SV, K_q, K_b) were calculated together with the number of binding sites. The binding forces were identified based on calculated thermodynamic parameters (ΔG, ΔH, ΔS). Synchronous spectra were used to study the microenvironment of Tyr and Trp residues and displacement assays revealed that site I was the preferred binding site. After binding, conformational changes were found to have occurred in the HSA molecule and the % α-helical content had decreased.