Browsing by Author "Lin, Chih-Cheng"

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Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A)
(2011)
Lin, Chih-Cheng
;
Chen, Chia-Ling
;
Liu, Kuan-Yu
;
Chang, C. Allen
;
生物科技學系
The [H(+)]-catalyzed dissociation rate constants of several trivalent lanthanide (Ln) complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (LnDO2A(+), Ln = La, Pr, Eu, Er and Lu) have been determined in two pH ranges: 3.73-5.11 and 1.75-2.65 at four different temperatures (19-41.0. degrees C) in aqueous media at a constant ionic strength of 0.1 mol dm(-3) (LiClO(4)). For the study in the higher pH range, i.e. pH 3.73-5.11, copper(II) ion was used as the scavenger for the free ligand DO2A in acetate/acetic acid buffer medium. The rates of Ln(III) complex dissociation have been found to be independent of [Cu(2+)] and all the Ln(III) complexes studied show [H(+)]-dependence at low acid concentrations but become [H(+)]-independent at high acid concentrations. Influence of the acetate ion content in the buffer on the dissociation rate has also been investigated and all the complexes exhibit a first-order dependence on [Acetate]. The dissociation reactions follow the rate law: k(obs) = k(Ac)[Acetate] + K' k(lim)[H(+)]/(1 + K'[H(+)]) where k(AC) is the dissociation rate constant for the [Acetate]-dependent pathway, k(lim) is the limiting rate constant, and K' is the equilibrium constant for the reaction LnDO2A(+) + H(+) double left right arrow LnDO2AH(2+). In the lower pH range, i.e. pH 1.75-2.65, the dye indicator, cresol red, was used to monitor the dissociation rate, and all the Ln(III) complexes also show [H(+)]-dependence dissociation pathways but without the rate saturation observed at higher pH range. The dissociation reactions follow the simple rate law: k(obs) = k(H)[H(+)], where k(H) is the dissociation rate constant for the pathway involving monoprotonated species. The absence of an [H(+)]-independent pathway in both pH ranges indicates that LnDO2A(+) complexes are kinetically rather inert. The obtained k(AC) values follow the order: LaDO2A(+) > PrDO2A(+) > EuDO2A(+) > ErDO2A(+) > LuDO2A(+), whereas the k(lim) and k(H) values follow the order: LaDO2A(+) > PrDO2A(+) > ErDO2A(+) > EuDO2A(+) > LuDO2A(+), mostly consistent with their thermodynamic stability order, i.e. the more thermodynamically stable the more kinetically inert. In both pH ranges, activation parameters, Delta H*, Delta S* and Delta G*, for both acetate-dependent and proton-catalyzed dissociation pathways have been obtained for most of the La(III), Pr(III), Eu(III), Er(III) and Lu(III) complexes, from the temperature dependence measurements of the rate constants in the 19-41 degrees C range. An isokinetic (linear) relationship is found between Lambda H* and Lambda S* values, which supports a common reaction mechanism.
Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid (H(2)ODO2A)
(2014)
Chang, C. Allen
;
Lin, Chih-Cheng
;
Kuo, Chien-Hung
;
交大名義發表
;
生物科技學系
The dissociation rates of LnODO2A(+) complexes (ODO2A(2-) is the deprotonated dianion of the ligand H(2)ODO2A, 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid) are all faster than those of the corresponding LnDO2A(+) complexes (DO2A(2-) is the deprotonated dianion of the ligand H(2)DO2A, 1,4,7,10-tetraazacyclododecane- 1,7-diacetic acid), consistent with the fact that LnODO2A(+) complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A(+) complexes, and H(2)ODO2A is not pre-organized for Ln(3+) ion complexation but H(2)DO2A is. The dissociation kinetics of selected trivalent lanthanide (Ln(3+), Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H(2)ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A(+), were studied in the [H+] range (0.1-2.4) x 10(-4) M in the temperature range 15-45 degrees C. Excess Cu2+ ions were used as the scavenger for the ligand in acetate-acetic acid buffer medium. The dissociation reactions are independent of [Cu2+] and follow the rate law k(obs) =k(d) + k(AC)[Acetate] + K\'k(lim)[H+]/(1 + K\'[H+]), where k(d), k(AC), and k(lim) are the respective dissociation rate constants for the [H+]-independent, acetate-assisted, and the [H+]-dependent limiting pathways; K\' is the equilibrium constant for the protonation reaction LnODO2A(+) + H+ < > LnODO2AH(2+). The dissociation rates of LnODO2A(+) complexes are all faster than those of the corresponding LnDO2A(+) complexes (DO2A(2-) is the fully deprotonated dianion of the ligand H(2)DO2A, 1,4,7,10-tetrazacyclo-dodecane-1,7- diacetic acid), consistent with the notion that LnODO2A(+) complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A(+) complexes, and H(2)ODO2A is not pre-organized for Ln(3+) ion complexation but H(2)DO2A is.