Aquation

About: Aquation is a research topic. Over the lifetime, 1443 publications have been published within this topic receiving 17507 citations.

Equilibrium, Kinetic and Leaving Group Effect Studies on Ligand Substitution Reactions of a Simple Model for Coenzyme B12

Abstract: The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+ and trans-[CoIII(en)2(Me)NH3]2+ were studied for CN−, SCN−, N3−, and NH3 as entering nucleophiles. These nucleophiles displace the coordinated H2O and NH3 molecules trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constant for the displacement of H2O by NH3 was found to be 17.4 ± 1.1 M−1 at 20 °C, and those for the displacement of NH3 by SCN− and N3− were found to be 4.4 ± 1.5 and 3.1 ± 0.4 at 10 °C, respectively. From temperature and pressure dependence studies, activation parameters (ΔH≠, ΔS≠ and ΔV≠) for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with NH3 were found to be 62 ± 1 kJ mol−1, +3 ± 5 J K−1 mol−1 and +5.7 ± 0.3 cm3 mol−1, respectively, compared to 79 ± 2 kJ mol−1, +40 ± 6 J K−1 mol−1 and +9.0 ± 0.4 cm3 mol−1 for the reverse aquation reaction of trans-[CoIII(en)2(Me)NH3]2+. Based on the reported kinetic and activation parameters, the substitution of coordinated H2O by NH3 follows an Id mechanism in which the entering nucleophile participates in the transition state. The substitution of NH3 by SCN−, N3− and CN− proceeds via the intermediate aqua complex such that aquation of the amine complex becomes the rate-determining step at high entering ligand concentration. A detailed comparison with available data in the literature is made.

Role of Solvation and Desolvation in Polymer 'Catalysis'. I. The Influence of High Pressures on the Spontaneous and Ag$^+$-Induced Aquations of Co(NH$_3$)$_5$Br$^{2+}$ Catalysed by Macro-Ions

Abstract: The spontaneous and Ag$^+$-induced aquation reactions of Co(NH$\_3$)$\_5$Br$^{2+}$ were examined in the presence of polyelectrolytes and at high pressures. The aquation reactions were enhanced by addition of anionic polyelectrolytes, i.e. sodium polyethylenesulphonate and sodium polystyrene-sulphonate. The polyelectrolyte-accelerated reactions were retarded at elevated pressures. The volumes of activation, $\Delta$ V$^{\ddagger}$, were negative in the absence of the macro-ions and positive in their presence for both aquations. The results indicate that the activated complex is dehydrated by the macro-ions to a much larger extent than the reactants. Correspondingly, the entropy of activation was increased by addition of the macro-ions, causing a decrease in the free energy of activation. The dehydration of the activated complex is thus considered to be one of the important factors of polyelectrolyte 'catalysis'. The implications of these findings are discussed.

The trans Influence in the Modulation of Platinum Anticancer Agent Biology: The Effect of Nitrite Leaving Group on Aquation, Reactions with S‐Nucleophiles and DNA Binding of Dinuclear and Trinuclear Compounds

Abstract: To examine the effect of leaving group and trans influence on the geenral reactivity of polynuclear platinum antitumor agents we investigated substitution of the chloride leaving groups with nitrite ion, which forms strong bonds to Pt. It was of interest to explore whether nitrite could be used to modulate biological properties of these agents, in particular the deactivating reactions that occur on reaction with S-nucleophiles, involving loss of linking diamine under the trans influence of sulfur. Reported herein is a study of the synthesis, aquation, DNA binding and reactions with glutathione (GSH), methionine (met) and acetylmethione (AcMet) of nitro derivatives of di- and trinuclear platinum antitumor compounds: To examine the effect of leaving group and trans influence on the general reactivity of polynuclear platinum antitumor agents we investigated substitution of the chloride leaving groups with nitrite ion, which forms strong bonds to Pt. It was of interest to explore whether nitrite could be used to modulate biological properties of these agents, in particular the deactivating reactions that occur on reaction with S-nucleophiles, nvolving loss of the linking diamine under the trans influence of sulfur. Reported herein is a study of the synthesis, aquation, DNA binding and reactions with glutathione (GSH), methionine (Met) and acetylmethione (AcMet) of nitrito derivatives of di- and trinuclear platinum antitumor compounds: [{trans-PtNO2-ACHTUNGTRENUNG(NH3)2}2ACHTUNGTRENUNG(m-NH2ACHTUNGTRENUNG(CH2)6NH2)]ACHTUNGTRENUNG(NO3)2 (1- NO2) and [{trans-PtNO2ACHTUNGTRENUNG(NH3)2}2(mtrans- PtACHTUNGTRENUNG(NH3)2ACHTUNGTRENUNG{NH2ACHTUNGTRENUNG(CH2)6NH2}2)]- ACHTUNGTRENUNG(NO3)4 (1'-NO2). {1H,15N}-HSQC NMR studies revealed that 1-NO2 is inert to aquation reactions, even after prolonged incubation at physiological pH. Monitoring of the interaction of 1-NO2 with the duplex 5'-d(ATATGTACATAT) 2 (I) showed only unreacted complex, consistent with activation by aquation being a requirement for covalent DNA binding. The reaction of 1-NO2 with GSH was studied by 1H, 195Pt, 15N and {1H,15N}-HSQC NMR spectroscopy. For the parent dichlorido compounds (1 and 1') substitution of chloride by GS leads to drug degradation involving liberation of the diamine linker. While the same final products trans-[Pt(SG)2ACHTUNGTRENUNG(NH3)2] (5) and trans-[{Pt(SG)ACHTUNGTRENUNG(NH3)2}2-m-SG] (6) are formed, different mechanisms are involved, consistent with the trans influence NO2 > Cl ; the half-life is slightly longer for 1-NO2 (1.8 h) compared with 1 (1.3 h). Identification of the intermediate trans-[PtACHTUNGTRENUNG(NH3)2ACHTUNGTRENUNG(NO2)(SG)] (4) shows that the nitrito group remains coordinated while the linker amine is substituted by coordination of GS , and then trans labilization of the nitrito group occurs leading to 5 and 6. Reaction of the trinuclear 1'-NO2 with GSH follows essentially the same reaction pathway. Reaction of 1-NO2 with Met and AcMet is much slower and only 20% liberated amine was observed after reaction with Met for 24 h at 37 8C. The final product from reaction with AcMet is trans-[PtACHTUNGTRENUNG(NH3)2ACHTUNGTRENUNG(NO2)- ACHTUNGTRENUNG(AcMet)], as in this case coordination of the S-nucleophile does not lead to trans labilization of the nitrito group.