Density Functional Theory-Based Calculation Shed New Light on the Bizarre Addition of Cysteine Thiol to Dopaquinone

• Main Authors: Ryo Kishida, Shosuke Ito, Manickam Sugumaran, Ryan Lacdao Arevalo, Hiroshi Nakanishi, Hideaki Kasai
• Format: Article
• Language: English
• Published: MDPI AG 2021-01-01
• Series: International Journal of Molecular Sciences
• Subjects: dopaquinone; cysteine; melanin; density functional theory; quinone reactions; thiol addition to quinone
• Online Access: https://www.mdpi.com/1422-0067/22/3/1373

Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of l-cysteine, DQ immediately binds to the –SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. l-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of l-cysteine thiolate (Cys–S⁻) to DQ. Interestingly, the C2–S bonded intermediate was less energetically stable than the C6–S bonded case. Furthermore, the most preferred Cys–S⁻-attacked intermediate is at the carbon-carbon bridge between the two carbonyls (C3–C4 bridge site) but not on the C5 site. This structure allows the Cys–S⁻ to migrate onto the adjacent C5 or C2 with small activation energies. Further simulation demonstrated a possible conversion pathway of the C5–S (and C2–S) intermediate into 5-<i>S</i>-cysteinyldopa (and 2-<i>S</i>-cysteinyldopa), which is the experimentally identified major (and minor) product. Based on the results, we propose that the binding of Cys–S⁻ to DQ proceeds via the following path: (i) coordination of Cys–S⁻ to C3–C4 bridge, (ii) migration of Cys–S⁻ to C5 (C2), (iii) proton rearrangement from cysteinyl –NH₃⁺ to O4 (O3), and (iv) proton rearrangement from C5 (C2) to O3 (O4).