Kinetics and mechanism for reduction of oral anticancer platinum(IV) dicarboxylate compounds by L-ascorbate ions †
作者:Kelemu Lemma、Alan M. Sargeson、Lars I. Elding
DOI:10.1039/a909484i
日期:——
Ascorbate (Asc) reductions of the oral anticancer platinum(IV) prodrugs cis,trans,cis-[PtCl2(OAc)2(cha)(NH3)] (JM216) and cis,trans,cis-[PtCl2(OCOC3H7)2(cha)(NH3)] (JM221) and of the isomers of JM216, viz.trans,cis,cis-[PtCl2(OAc)2(cha)(NH3)] (JM394) and trans,trans,trans-[PtCl2(OAc)2(cha)(NH3)] (JM576) (OAc = acetate, cha = cyclohexylamine) have been investigated in a 1.0 M aqueous perchlorate medium using stopped-flow and conventional UV/VIS spectrophotometry as a function of temperature and pH. JM216 and 221 are reduced to cis-[PtCl2(cha)(NH3)] (JM118) and JM394 and 576 to cis- and trans-[Pt(OAc)2(cha)(NH3)], respectively. The redox reactions follow the second-order rate law: −d[Pt(IV)]/dt = k [Pt(IV)] [Asc]tot where k is a pH dependent second-order overall rate constant and [Asc]tot = [Asc2−] + [HAsc−] + [H2Asc]. Reduction of JM216 and JM221 is slow (overall rate constants k298 = 5.08 ± 10−2 and 3.25 × 10−2 mol−1 dm3 s−1 at pH 7.12, respectively) and is suggested to take place via an outer-sphere mechanism. Reductions of JM394 and JM576 are more than three orders of magnitude faster (k298 = 230 ± 6 mol−1 dm3 s−1 at pH 7.0 for JM394). They are suggested to take place by a mechanism involving a reductive attack on one of the mutually trans chloride ligands by Asc2− and less efficiently by HAsc− leading to the formation of a chloride-bridged activated complex. The second-order rate constants for reduction of JM394 by HAsc− and Asc2− at 25 °C are 0.548 ± 0.004 and (4.46 ± 0.01) × 106 mol−1 dm3 s−1, respectively. The rate constants for reduction of JM216 and JM221 by Asc2− at 25 °C are calculated to be 672 ± 15 and 428 ± 10 mol−1 dm3 s−1, respectively and reduction by HAsc− was not observed under these conditions. Thus, Asc2− is up to 7 orders of magnitude more efficient as a reductant than HAsc−. H2Asc is virtually inactive. The activation parameters ΔH‡ and ΔS‡ for reduction of JM216, JM221, JM394, and JM576 by Asc2− are 52 ± 1, 46 ± 1, 56.2 ± 0.5, and 63 ± 2 kJ mol−1 and −97 ± 4, −120 ± 4, −24 ± 2, and −8 ± 5 J K−1 mol−1, respectively. An isokinetic relationship gives further support to the mechanistic assignments.
在 1.0 M 的高氯酸水介质中,使用停流和传统紫外/可见分光光度法研究了反式、顺式、顺式-[PtCl2(OAc)2(cha)(NH3)] (JM394) 和反式、反式、反式-[PtCl2(OAc)2(cha)(NH3)] (JM576) (OAc = 乙酸酯,cha = 环己胺)异构体的温度和 pH 值函数。JM216 和 221 被还原成顺式[PtCl2(cha)(NH3)] (JM118),JM394 和 576 分别被还原成顺式和反式[Pt(OAc)2(cha)(NH3)]。氧化还原反应遵循二阶速率定律:-d[Pt(IV)]/dt=k[Pt(IV)] [Asc]tot,其中 k 是与 pH 值相关的二阶总速率常数,[Asc]tot = [Asc2-] + [HAsc-] + [H2Asc]。JM216 和 JM221 的还原速度很慢(在 pH 值为 7.12 时,总速率常数 k298 分别为 5.08 ± 10-2 和 3.25 × 10-2 mol-1 dm3 s-1),这表明它们是通过外球机制进行还原的。JM394 和 JM576 的还原速度要快三个数量级以上(在 pH 值为 7.0 时,JM394 的 k298 = 230 ± 6 mol-1 dm3 s-1)。据推测,它们的发生机理是 Asc2- 对相互反式的氯配体之一进行还原攻击,HAsc- 的还原攻击效率较低,导致形成氯键活化复合物。在 25 °C 时,HAsc- 和 Asc2- 还原 JM394 的二阶速率常数分别为 0.548 ± 0.004 和 (4.46 ± 0.01) × 106 mol-1 dm3 s-1。根据计算,25 °C时Asc2-还原JM216和JM221的速率常数分别为672 ± 15和428 ± 10 mol-1 dm3 s-1,而在这些条件下没有观察到HAsc-还原JM216和JM221。因此,Asc2- 作为还原剂的效率要比 HAsc- 高出 7 个数量级。H2Asc 几乎没有活性。Asc2-还原 JM216、JM221、JM394 和 JM576 的活化参数 ΔH‡ 和 ΔS‡ 分别为 52 ± 1、46 ± 1、56.2 ± 0.5 和 63 ± 2 kJ mol-1 以及 -97 ± 4、-120 ± 4、-24 ± 2 和 -8 ± 5 J K-1 mol-1。等速运动关系进一步支持了机理分配。