Designed, Flavin-Based Multistate Catalysis

在可见光激发和没有可见光激发的情况下，黄素酶催化从氧氧化和碳水化合物转移到氧化以及还原的高度多样化的转化。然而，这些反应是由黄素腺嘌呤二核苷酸(FAD)辅助因子的相同的异四氧嘧啶反应位点介导的。后者很容易在氧化态和共价底物加成态之间变化，每种加成态都具有不同的反应活性。在酶中，对不同活性和选择性的控制依赖于异四氧嘧啶和底物与周围肽的非共价接触。因此，当分离的辅因子从周围的多肽中去除时，黄素的大部分活性就会丧失。另一方面，人工黄素酶对溶剂的选择和对异四氧嘧啶的非天然反应性的修饰很敏感。在这项研究计划中，设计了黄素分子催化剂，以使天然和非天然黄素反应性的全谱可用。我们的关键策略依赖于通过非共价相互作用中心精确调节异四氧嘧啶反应中心及其三维‘外层’，从而控制催化剂的反应性和选择性。根据酶的活性和合成用途的目的，选择了三个不同的反应区：i)从空气中活化氧气进行羟化和卤化反应，ii)有机催化糖基化和氧化偶联反应，以及iii)黄素可见光激发脱羧肽的偶联和装订以及烯烃偶联。在这些反应性区域之间的切换-使用相同的黄素催化剂-将通过改变外部参数来实现，例如空气与氩气气氛、氧化与还原条件以及黑暗中的照射与反应。这一策略为多状态反应序列奠定了基础，在多状态反应序列中，单个基于黄素的催化剂连续执行各种不同的转化。这些多状态催化剂将应用于日益复杂的天然产物的立体编辑、现场编辑和化学选择性编辑。在这里，概述的不同的反应性允许各种修饰，包括二级结构和极性的调节、标记的附着和结合位点的改变。最近在天然产物抗生素多样化方面的工作表明，多个改变而不是孤立的改变具有协同效应。在这种情况下，用一个黄素催化剂对多个位点进行选择性修饰，或连续编辑一个位点，预计将有利于获得与生物相关的复杂分子。

Flavoenzymes catalyze highly diverse transformations ranging from oxygenations with oxygen and carbohydrate transfer to oxidations as well as reductions both upon and without visible light excitation. However, these reactions are mediated by the same isoalloxazine reactive site of the flavin adenine dinucleotide (FAD) cofactor. The latter easily changes between oxidation states and covalent substrate adduct states, each of which is characterized by distinct reactivity. In enzymes, control over divergent activity as well as selectivity relies on non-covalent contacts of the isoalloxazine and the substrates with the peptide surrounding. Most flavin activity, therefore, is lost when the isolated cofactor is removed from the peptide surrounding. On the other hand, artificial flavoenzymes are sensitive towards solvent choice and modifications of the isoalloxazine for non-natural reactivity. In this research programme, designed molecular flavin catalysts are proposed in order to make the full spectrum of natural as well as non-natural flavin reactivity accessible. Our key strategy relies on precise tuning of the isoalloxazine reactive center and its three-dimensional ‘outer-sphere’ by means of non-covalent interaction sites, thereby controlling catalyst reactivity and selectivity. Three distinct reactivity areas have been selected based on enzyme activity and the aim for synthetic utility: i) Activation of oxygen from air for hydroxylation and halogenation reactions, ii) organocatalytic glycosylation and oxidative coupling reactions, and iii) decarboxylative peptide conjugation and stapling as well as olefin coupling via visible light excitation of the flavin. Switching between these areas of reactivity - with an identical flavin catalyst - will be accomplished through changing external parameters such as air versus argon atmosphere, oxidative versus reductive conditions, and irradiation versus reaction in the dark. This strategy lays the foundation for multistate reaction sequences, in which a single flavin-based catalyst performs various different transformations consecutively. These multistate catalysts will be applied in stereo-, site-, and chemoselective editing of increasingly complex natural products. Here, the outlined diverse reactivity allows for a variety of modifications including modulation of secondary structure and polarity, attachment of labels, and binding site changes. Recent work in diversification of natural product antibiotics revealed synergistic effects of multiple instead of isolated alterations. In this context, the selective modification of multiple sites - or consecutive editing of a single site - with one designed flavin catalyst is anticipated to be beneficial for accessing biologically relevant complex molecules.