作者:Usama El-Ayaan、Erwin Herlinger、Reginald F. Jameson、Wolfgang Linert
DOI:10.1039/a701054k
日期:——
Iron(III) [in the form of Fe(OH)
2+
] reacted
reversibly in acid aqueous solution with dopamine,
2-(3,4-dihydroxyphenyl)ethylamine (H
2
LH
+
, in which
the phenolic protons are written to the left of L) to give the complex
ion Fe(LH)
2+
. This species then decomposed to yield
iron(II) and a semiquinone, which in turn is oxidised further
to a quinone. The latter cyclised to form leucodopaminochrome
(indoline-5,6-diol), which was finally oxidised by iron(III)
to pink dopaminochrome (6-hydroxy-3H-indol-5-one), presumably
via another semiquinone. The rate of appearance and
disappearance of the complex and of the ortho-quinone were
separately followed by stopped-flow photometric methods. Mechanisms are
proposed for the various steps and these are supported by measurements
at varying ionic strengths. Rate constants for the reversible formation
of the iron–dopamine complex have been evaluated
[k
1
= (2.09 ±
0.05) × 10
3
and
k
-1
= 23 ±
2 dm
3
mol
-1
s
-1
].
The rate of decomposition of the protonated complex to yield
iron(II) and the semiquinone was established as
k
2
= 0.23 ± 0.02
s
-1
and
K
M
H
= 33 ±
0.9 dm
3
mol
-1
[for the protonation
of Fe(LH)
2
+
]. The stability constant of the
Fe(LH)
2+
complex has been calculated (log
K
1
M
= 21.14) and
ε
max
is 1260 dm
3
mol
-1
cm
-1
at 700 nm. The effect of chloride on the rate of
complex formation at low pH has been explained by the fact that
FeCl
2+
also reacts with dopamine
(k
Cl
= 148 ± 7
dm
3
mol
-1
s
-1
) to form the
complex but that this is predominantly reversible via the
non-chloride route at low pH values. The stability constant for
FeCl
2+
formation (a constant not readily accessible by
standard methods) was extracted from the data (log
K
1
Cl
= 1.53). The rate of
disappearance of the quinone enabled the ring-closure reaction
(i.e. the formation of the indole) to be followed and the
mechanism established. All measurements were carried out at
25 °C in solutions of ionic strength 0.10 mol
dm
-3
(KNO
3
) except for ionic strength
dependence studies.
铁(III)[以 Fe(OH) 2+ 的形式]在酸性水溶液中与多巴胺、2-(3,4-二羟基苯基)乙胺(H 2 LH + ,其中酚质子写在 L 的左边)发生可逆反应,生成络离子 Fe(LH) 2+ 。然后,该络合物分解生成铁(II)和半醌,半醌又进一步氧化成醌。后者循环生成亮多巴胺色素(吲哚啉-5,6-二醇),最后被铁(Ⅲ)氧化成粉红色的多巴胺色素(6-羟基-3H-吲哚-5-酮),这可能是通过另一种半醌实现的。用停流光度法分别跟踪了络合物和原醌的出现和消失速度。提出了各个步骤的机理,并通过在不同离子强度下的测量得到了支持。评估了铁-多巴胺复合物可逆形成的速率常数[k 1 = (2.09 ± 0.05) × 10 3 和 k -1 = 23 ± 2 dm 3 mol -1 s -1 ]。 确定了质子化复合物分解生成铁(II)和半奎宁酮的速率为 k 2 = 0.23 ± 0.02 s -1 和 K M H = 33 ± 0.9 dm 3 mol -1 [用于 Fe(LH) 2 + 的质子化]。计算得出了 Fe(LH) 2+ 复合物的稳定常数(log K 1 M = 21.14),在 700 纳米波长下,ε max 为 1260 dm 3 mol -1 cm -1 。氯化物对低 pH 值下络合物形成速率的影响可以用以下事实来解释:FeCl 2+ 也会与多巴胺发生反应(k Cl = 148 ± 7 dm 3 mol -1 s -1 )以形成络合物,但在低 pH 值下,这主要是通过非氯途径可逆的。从数据中提取出了形成 FeCl 2+ 的稳定常数(标准方法不易获得该常数)(log K 1 Cl = 1.53)。根据醌的消失速度,可以跟踪阖环反应(即吲哚的形成)并确定其机理。除离子强度依赖性研究外,所有测量都是在 25 °C、离子强度为 0.10 mol dm -3 (KNO 3) 的溶液中进行的。