Thermodynamic Parameters For Protonation Research Articles

Potentiometric, kinetic, and thermodynamic investigations into Cu2+ ion binding properties of vinyl imidazole containing IMAC adsorbent

ABSTRACTIn this study, the use of the potentiometric method for the determination of the protonation constant of vinyl imidazole (VIM) and the stability constant of the Cu2+ ion complex of VIM used in the immobilized metal ion affinity chromatography (IMAC) was investigated. For this purpose, poly(ethylene glycol dimethacrylate‐n‐vinyl imidazole) [poly(EGDMA–VIM)] microspheres (average diameter 150–200 μm) were prepared. The microspheres were characterized by elemental analysis, N2 adsorption/desorption isotherms, elemental analysis, energy dispersive spectroscopy (EDS). Protonation constants of vinyl imidazole and the metal‐ligand stability constant of vinyl imidazole with Cu2+ ions have been determined potentiometrically in 0.1M NaCl aqueous solution at 298, 318, and 338 K, respectively. The corresponding thermodynamic parameters of protonation and complexation processes (ΔG, ΔH, and ΔS) were derived and discussed. The formation kinetics of Cu2+‐vinyl imidazole complex were also investigated, and the process obeyed the pseudo‐second‐order kinetic model. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39751.

ApA Cleavage Promoted by Oxa-aza Macrocycles and Their Zn(II) Complexes. The Role of pH and Metal Coordination in the Hydrolytic Mechanism

The thermodynamic parameters for protonation and Zn(II) complex formation with ligand 1,4,7,16,19,22-hexaza-10,13,25,28-tetraoxacyclotriacontane (L1) have been determined. L1 forms stable dizinc complexes from neutral to alkaline pH. The hydrolytic ability toward adenylyl(3′-5′)adenosine (ApA) of L1 and its dizinc(II) complexes have been analyzed by means of HPLC chromatography. Only partially protonated species of L promote ApA hydrolysis suggesting that the cleavage process is cooperatively promoted by a general base catalysis by neutral amine groups and a general acid catalysis by protonated ammonium functions. Concerning the Zn(II) complexes, the hydrolysis rates increase in the presence of the hydroxo complexes [Zn2L1(OH)]3+ and [Zn2L1(OH)2]2+. This indicates that Zn-OH functions play a crucial role in the hydrolytic process, assisting the deprotonation of the 2′-OH group of ApA, which may act as nucleophile in the cleavage process. Both binuclear L1 complexes are better catalysts than the mononuclear [ZnL2(OH)]+ complex (L2 = 1,4-Dioxa-7,10,13-triazacyclopentadecane), indicating a cooperative role of the two Zn(II) ions in ApA cleavage by [Zn2L1(OH)]3+ and [Zn2L1(OH)2]2+, probably due to a bridging coordination of the phosphate moiety of ApA to the two metal centers.

Thermodynamic of metal complexation of the macrocyclic antibiotic rifampicin

The acid-base and complexation equilibria of rifampicin with H+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+were studied by means of potentiometry. The stoichiometric equilibrium constants were determined in 50% (v/v) methanol-water medium at different temperatures and constant ionic strength (0.05 M KCl). It was established that rifampicin has two proton-binding sites. The distribution diagram of the corresponding ionic species as a function of pH is given and indicated that rifampicin exists predominantly in the zwitterionic form at pH [Formula: see text] 5. The thermodynamic parameters of protonation and complexation were derived and discussed. The formation of the complexes is spontaneous, more favourable at lower temperatures, entropically unfavourable, and an enthalpy-driven process. The order of the changes in Gibbs energy and enthalpy accompanying the complexation was found to be Mn2+< Co2+< Ni2+< Cu2+> Pb2+> Zn2+> Cd2+> Hg2+in accordance with the well-known sequence of Irving and Williams. The transition series contraction energy (Er(Mn-Zn)), and the ligand field stabilization energy (δH) were calculated from the enthalpy changes.Key words: rifampicin, thermodynamics, potentiometry, and complexation.