Comparative solution equilibrium studies on pentamethylcyclopentadienyl rhodium complexes of 2,2'-bipyridine and ethylenediamine and their interaction with human serum albumin

Enyedy, Éva Anna and Mészáros, János P. and Dömötör, Orsolya and Hackl, Carmen M. and Roller, Alexander (2015) Comparative solution equilibrium studies on pentamethylcyclopentadienyl rhodium complexes of 2,2'-bipyridine and ethylenediamine and their interaction with human serum albumin. JOURNAL OF INORGANIC BIOCHEMISTRY, 152 (&). pp. 93-103. ISSN 0162-0134

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Abstract

Complex formation equilibri um processes of the (N,N) donor containing 2,2' - bipyridine (bpy) and ethylenediamine (en) with (η 5 - pentamethylcyclopentadienyl) rhodium(III) were investigated in aqueous solution via pH - potentiome try, 1 H NMR spectroscopy, and UV – Vis spectrophotometry in the absence and presence of chloride ions . The structure of [RhCp*(en)Cl]ClO 4 (Cp* , pentamethylcyclopentadienyl ) was also studied b y single - crystal X - ray diffraction. p K a values of 8.56 and 9.58 we re determined for [RhCp*(bpy)(H 2 O)] 2+ and [RhCp*(en)(H 2 O)] 2+ , respectively resulting in the formation of negligible amount of mixed hydroxido complexes at pH 7.4 . Stability and the H 2 O/Cl ‒ co - ligand exchange constants of bpy and en complexes considerably exceed those of the bidentate O - donor deferiprone. The strong affinity of the bpy and en complexes to chloride ions most probably contribute s to their low antiproliferative effect. Interac tion s between human serum albumin (HSA) and [ RhCp* (H 2 O) 3 ] 2+ , its complexes formed with deferiprone, bpy and en were also monitored by 1 H NMR spectroscopy, ultrafiltration/ UV - Vis and spectrofluorometry. Numerous binding sites ( ≥ 8 ) are available for [RhCp* (H 2 O) 3 ] 2+ ; and the interaction takes place most probably via covalent bonds through the imidazole nitrogen of His . According to the various fluorescen ce studies [RhCp* (H 2 O) 3 ] 2+ binds on sites I and II , and coordination of surface side chain donor atoms of th e protein is also feasible. The binding of the bpy and en complex is weaker and slower compared to that of [RhCp* (H 2 O) 3 ] 2+ , and formation of ternary HSA - RhCp* - ligand adducts was proved. I n the case of the deferiprone complex , the RhCp* fragment is cleaved off when HSA is loaded with low equivalents of the comp ound.

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