摘要/Abstract

萤火虫生物发光是最常见的生物发光，在生物和医药等领域已得到重要应用.发光过程涉及到其活体内的一系列复杂的化学反应.引起发光的起始反应是单重态荧光素分子与三重态氧气的加成反应.这是一个自旋禁阻的酶催化反应，通常效率很低，但萤火虫是目前将化学能转化为光能最高效的系统.这个自旋禁阻的反应为什么能高效率发生?实验研究认为该反应由单电子转移（SET）诱发而发生，但对反应的详细过程和机理并没有完整的描述.本工作通过理论计算，描述了该反应的完整过程，解释了这个自旋禁阻反应高效发生的原因.
关键词: 荧光素, 氧化, 机理, 单电子转移, 密度泛函理论
As the most common bioluminescence (BL), firefly BL, is of great significance in the fields of biotechnology, biomedicine and so on. The entire BL process involves a series of complicate in vivo chemical reactions. The BL is initiated by the enzymatic oxidation of luciferin. This is a spin-forbidden reaction of low efficiency, because that luciferin is in singlet state and O₂ is in triplet state. However, firefly is till-now the most efficient system of converting chemical energy to light energy. Why this spin-forbidden reaction occurs efficiently? A single electron transfer (SET) mechanism has been confirmed on this reaction by experiments. However, there is lack of a complete and detailed description of the mechanism and reaction process. Via a calculation of density functional theory (DFT), this article described the complete process of this reaction. The oxygenation of luciferin is initiated by a SET from singlet L^3- to triplet O₂ to form RC ³[L^·2-…O₂^·-]. Then the reaction is carried out on the potential energy surface (PES) of triplet state (T₁), on which O₂^·- performs a nucleophilic attack on C4 of L^·2-. There is an intersystem crossing between the ground (S₀) and T₁ PESs nearby the first transition state (TS1). After the ISC (intersystem crossing), the reaction continuously undergoes on the S₀ PES to produce dioxetanone FDO^- via two TSs and two intermediates (Ints). The analysis on electron densities and natural orbitals indicates that there is a quick reaction of biradical annihilation around the ISC. About 11.9 kcal·mol^-1 energy is needed to reach the ISC before the whole reaction occurs on the S₀ PES. The highest barrier of the reactions on the S₀ PES is only 4.2 kcal·mol^-1. The biradical annihilation around the ISC and the very low energy barriers explain the reason of the spin-forbidden reaction with high efficiency. This study is helpful for understanding the initiation of firefly BL and the other oxygen-dependent BL.
Key words: luciferin, oxygenation, mechanism, single electron transfer, density functional theory


PDF全文下载地址:

点我下载PDF