3.1 Introduction
55
reaction which is proportional to the acidity constant of amino group and to the
rate constant for the nucleophilic addition by amino group. Therefore, the overall
reaction rate depends on the two factors: basicity and nucleophilicity of the side
chain. Especially, side chain nucleophilicity is not directly amenable to an experimental measurement. Thus, the relation between the side chain structure and the
nucleophilicity for cyclization remains to be explored.
3.1.3 Background—Binding of Cysteine with Dopaquinone
Cysteine forms a covalent bond with dopaquinone at 5-carbon or 2-carbon, but not
at 6-carbon. In contrast, cyclization occurs at 6-carbon. Although cyclization at 2carbon may also be theoretically possible, a previous computational investigation
excluded this possibility [1]. This may be due to the intrinsic energy difference
between before and after the cyclic bond formation, which perturbs the π-conjugated
chain. Therefore, it is straightforward to consider that this preference of 6-carbon
over 2-carbon is found for general nucleophiles. However, in the case of the cysteine
binding, the yield of 6-adduct reported is only 1%, and the major products were
5-adduct (74%) and 2-adduct (14%) [2, 14–16]. Note that dicysteinyldopa, where
5-carbon and 2-carbon are both bound, was also reported with a 5% yield [2, 15, 16].
As a mechanism for the initial step of the cysteine binding, 1,6-Michael addition
mechanism has been proposed [17–19]. In this mechanism, the sulfhydryl group in
cysteine forms a covalent bond at 5-carbon (or 2-carbon), and then 3-oxygen (or 4oxygen) is protonated. The rate for the cysteine binding increases with the cysteine
concentration. Although the binding rate linearly increases at lower concentration
of cysteine, this increase becomes more gradual as the concentration gets higher
[19]. This behavior indicates the presence of a reaction intermediate of the cysteine
binding, and thus supports the 1,6-Michael addition mechanism.
Furthermore, the cysteine binding rate is positively correlated with pH [18, 19].
Therefore, the binding of cysteine must be initiated by sulfhydryl deprotonation.
Jameson et al. proposed a kinetic model based on the 1,6-Michael addition mechanism, and compared the binding of the amino-free cysteine analog thioglycolic acid
with that of cysteine [19]. As a result, the intermediate structure resulting from the
binding of cysteine was unstable as compared to the case of the binding of thioglycolic acid. Therefore, the amino group in cysteine would have an important role in
the reaction with dopaquinone.
3.1.4 Theoretical Approach for o-Quinone Reactions
In this chapter, we describe our theoretical investigations, which were conducted
to understand the two reactions involving dopaquinone and similar o-quinones,
namely cyclization and binding of thiols [20–22]. As a computational method,
55
reaction which is proportional to the acidity constant of amino group and to the
rate constant for the nucleophilic addition by amino group. Therefore, the overall
reaction rate depends on the two factors: basicity and nucleophilicity of the side
chain. Especially, side chain nucleophilicity is not directly amenable to an experimental measurement. Thus, the relation between the side chain structure and the
nucleophilicity for cyclization remains to be explored.
3.1.3 Background—Binding of Cysteine with Dopaquinone
Cysteine forms a covalent bond with dopaquinone at 5-carbon or 2-carbon, but not
at 6-carbon. In contrast, cyclization occurs at 6-carbon. Although cyclization at 2carbon may also be theoretically possible, a previous computational investigation
excluded this possibility [1]. This may be due to the intrinsic energy difference
between before and after the cyclic bond formation, which perturbs the π-conjugated
chain. Therefore, it is straightforward to consider that this preference of 6-carbon
over 2-carbon is found for general nucleophiles. However, in the case of the cysteine
binding, the yield of 6-adduct reported is only 1%, and the major products were
5-adduct (74%) and 2-adduct (14%) [2, 14–16]. Note that dicysteinyldopa, where
5-carbon and 2-carbon are both bound, was also reported with a 5% yield [2, 15, 16].
As a mechanism for the initial step of the cysteine binding, 1,6-Michael addition
mechanism has been proposed [17–19]. In this mechanism, the sulfhydryl group in
cysteine forms a covalent bond at 5-carbon (or 2-carbon), and then 3-oxygen (or 4oxygen) is protonated. The rate for the cysteine binding increases with the cysteine
concentration. Although the binding rate linearly increases at lower concentration
of cysteine, this increase becomes more gradual as the concentration gets higher
[19]. This behavior indicates the presence of a reaction intermediate of the cysteine
binding, and thus supports the 1,6-Michael addition mechanism.
Furthermore, the cysteine binding rate is positively correlated with pH [18, 19].
Therefore, the binding of cysteine must be initiated by sulfhydryl deprotonation.
Jameson et al. proposed a kinetic model based on the 1,6-Michael addition mechanism, and compared the binding of the amino-free cysteine analog thioglycolic acid
with that of cysteine [19]. As a result, the intermediate structure resulting from the
binding of cysteine was unstable as compared to the case of the binding of thioglycolic acid. Therefore, the amino group in cysteine would have an important role in
the reaction with dopaquinone.
3.1.4 Theoretical Approach for o-Quinone Reactions
In this chapter, we describe our theoretical investigations, which were conducted
to understand the two reactions involving dopaquinone and similar o-quinones,
namely cyclization and binding of thiols [20–22]. As a computational method,