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Journal of Electroanalytical Chemistry 476 (1999) 46 – 53
Electrochemical behavior of a Cypridina luciferin analogue in
acetonitrile solutions in the presence of proton acceptors
Takeyoshi Okajima, Koichi Tokuda, Takeo Ohsaka *
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori -ku, Yokohama 226 -8502, Japan
Received 2 March 1999; received in revised form 22 June 1999; accepted 10 August 1999
Abstract
The electrochemical redox behavior of the Cypridina luciferin analogue, 6-(4-methoxyphenyl)-2-methylimidazo[1, 2-a ]pyrazin-
3(7H)-one (MCLA) has been examined in acetonitrile solutions containing 2,6-lutidine or water using cyclic and hydrodynamic
voltammetry and double potential-step chronoamperometry. The conjugate base of MCLA (MCLA
-
) is oxidized in two
one-electron steps to the radical species (MCLA
) and further to the carbocation species (MCLA
+
). The radical – radical coupling
of MCLA
yields the corresponding dimer ((MCLA)
2
), which is in equilibrium with MCLA
. MCLA
+
undergoes the addition of
a water molecule to yield a hydroxylated form (MCLAOH) of luciferinol type. The whole mechanism of the electrochemical
redox reactions of MCLA
-
coupled with these chemical reactions has been found to be essentially the same as that proposed for
the chemical redox reactions of Cypridina luciferin and its analogues, and in addition the kinetic data concerning the dimerization
of MCLA
, the dissociation of (MCLA)
2
and the hydroxylation of MCLA
+
have been estimated together with the diffusion
coefficient of MCLA
-
. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Cypridina luciferin analogues; Redox reactions; Dimerization; Hydroxylation; Cyclic voltammetry; Double potential-step chronoamper-
ometry; Hydrodynamic voltammetry
1. Introduction
The present paper presents the electrochemical be-
havior of 6-(4-methoxyphenyl)-2-methylimidazo[1,2-
a ]pyrazin-3(7H)-one (MCLA), which is one of the
analogues of Cypridina hilgendorfii luciferin [1], in ace-
tonitrile media.
Cypridina luciferin reacts with molecular oxygen (
3
O
2
),
and consequently emits light in the presence of Cyprid -
ina luciferase [2–4]. This system is of great interest,
because it is the simplest system of the enzymatic
reactions found in bioluminescence. Goto et al. isolated
Cypridina luciferin in a crystalline state and elucidated
its structure completely [2]. They also prepared some
analogues of luciferin and examined their chemilu-
minescence properties. In order to clarify the reaction
mechanisms of autoxidation of luciferin in the absence
of the luciferase, Goto and co-workers investigated the
chemical redox reactions of Cypridina luciferin and its
analogues by use of oxidizing agents ([Fe(CN)
6
]
3 -
,
PbO
2
and DPPH (2,2-diphenyl-1-picryl hydrazyl)) and
reducing agents (Na
2
S
2
O
4
and NaBH
4
) in dimethyl
sulfoxide, water, various alcohols and water +alcohol
mixtures in the absence of Cypridina luciferase [5,6].
According to the mechanism proposed by them
(Scheme 1), the one-electron oxidation of luciferin (L)
gives a radical species (L
), which is in equilibrium with
its dimer species, biluciferyl (LL). L
is reduced re-
versibly with Na
2
S
2
O
4
to L. L
is further oxidized with
DPPH to luciferinol and its derivatives (LOR), which
can be reduced with Na
2
S
2
O
4
(or NaBH
4
) to L. LOR
is also generated by two-electron oxidation of L with
* Corresponding author. Tel.: +81-45-924-5404; fax: +81-45-924-
5489.
E-mail address: ohsaka@echem.titech.ac.jp (T. Ohsaka)
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII:S0022-0728(99)00362-9