萤火虫生物发光中加氧反应机理的理论研究

Mechanistic Study of Oxygenation Reaction in Firefly Bioluminescence

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.

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 L3- 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.