Sil Debangsu, Dey Soumyajit, Kumar Amit, Bhowmik Susovan, Rath Sankar Prasad
Department of Chemistry , Indian Institute of Technology Kanpur , Kanpur-208016 , India . ; Email:
Chem Sci. 2016 Feb 1;7(2):1212-1223. doi: 10.1039/c5sc03120f. Epub 2015 Oct 26.
MauG is a diheme enzyme that utilizes two covalently bound c-type hemes to catalyse the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. The two hemes are physically separated by 14.5 Å and a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes undergoes reversible oxidation and reduction to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(iv) MauG. The present work describes the structure and spectroscopic investigation of 2e-oxidations of the synthetic diheme analogs in which two heme centers are covalently connected through a conjugated ethylene bridge that leads to the stabilization of two unusual conformations ( and ' forms) with different and distinct spectroscopic and geometric features. Unlike in MauG, where the two oxidizing equivalents are distributed within the diheme system giving rise to the bis-Fe(iv) redox state, the synthetic analog stabilizes two ferric hemes, each coupled with a porphyrin cation radical, a scenario resembling the binuclear dication diradical complex. Interestingly, charge resonance-transition phenomena are observed here both in 1e and 2e-oxidised species from the same system, which are also clearly distinguishable by their relative position and intensity. Detailed UV-vis-NIR, X-ray, Mössbauer, EPR and H NMR spectroscopic investigations as well as variable temperature magnetic studies have unraveled strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. The extensive π-conjugation also allows antiferromagnetic coupling between iron(iii) centers and porphyrin radical spins of both rings. DFT calculations revealed extended π-conjugation and H-bonding interaction as the major factors in controlling the stability of the conformers.
MauG是一种双血红素酶，它利用两个共价结合的c型血红素来催化蛋白质衍生辅因子色氨酸-色氨酰醌的生物合成。两个血红素在物理上相隔14.5 Å，并提出了一种空穴跳跃机制，其中位于两个血红素之间的色氨酸残基经历可逆氧化和还原，以增加有效的电子耦合元件，并提高双铁（IV）MauG中两个血红素之间可逆电子转移的速率。本工作描述了合成双血红素类似物2e氧化的结构和光谱研究，其中两个血红素中心通过共轭乙烯桥共价连接，导致两种具有不同光谱和几何特征的不寻常构象（α和α'形式）的稳定。与MauG不同，在MauG中两个氧化当量分布在双血红素系统内，产生双铁（IV）氧化还原状态，合成类似物稳定了两个铁血红素，每个都与一个卟啉阳离子自由基耦合，这种情况类似于双核双阳离子双自由基配合物。有趣的是，在同一系统的1e和2e氧化物种中都观察到了电荷共振跃迁现象，它们也可以通过相对位置和强度清楚地区分。详细的紫外-可见-近红外、X射线、穆斯堡尔、电子顺磁共振和核磁共振光谱研究以及变温磁性研究揭示了两个卟啉π-阳离子自由基之间通过桥连乙烯基团的强电子通信。广泛的π共轭还允许铁（III）中心与两个环的卟啉自由基自旋之间发生反铁磁耦合。密度泛函理论计算表明，扩展的π共轭和氢键相互作用是控制构象体稳定性的主要因素。
