Total synthesis of malacidin A and development of a rapid peptide/protein desulfurization method

Abstract

The pipeline of developing new antibiotics remains narrow for the last few decades, and the development of novel antibiotics is still an urgent need to defend against the increasing multidrug-resistant pathogens. Recently, Malacidin A was discovered as a new member of the calcium-dependent antibiotics (CDAs) family in a culture-independent manner. Not only it exhibits potent activity against antibiotic-resistant pathogens, but also shows a distinct mode of action (MoA) compared with other existing CDAs. However, the exact stereochemistry of malacidin A is still not fully established yet. In the first chapter of this thesis, the total syntheses of malacidin A and its analogues are developed, involving β-hydroxyaspartic acid (HyAsp) ligation as a key step for cyclization. Its absolute configuration was finally confirmed through a detailed comparison of NMR spectra, advanced Marfey’s analysis, and antibacterial activity between synthetic and native malacidin A. This synthetic route may also provide an opportunity for comprehensive studies of Malacidin A’s structure-activity relationship (SAR) in the future. In the second chapter, a novel desulfurization method was developed for the chemical synthesis of peptides/proteins. The development of the ligation chemistry provides facile access to complicated and functional protein targets especially those with highly homogenous post-translational modifications (PTMs) such as glycosylation, phosphorylation, ubiquitylation, etc., for exploring the fundamental chemical biology, developing therapeutic applications and constructing new materials. However, in the case of commonly-used native chemical ligation (NCL), the utilization of thiol group in the N-terminal Cys limits the applications of NCL due to the less abundance of cysteine residues in naturally occurring proteins. The established peptide/protein desulfurization methods greatly extend the substrate scope of NCL to other non-Cys sites, but more convenient, milder and faster desulfurization methods are under investigated. In this study, a new peptide/protein desulfurization method based on the novel tetraethylborate was discovered, which could desulfurize single or multiple cysteine to alanine extremely fast in an add-and-done manner. Also, the tetraethylborate-mediated desulfurization is simple to operate under ambient conditions without the need to employ special apparatus, add external additive, elevate temperature or irradiate with light. By using such practical new method, peptide and protein substrates can be desulfurized faster and cleaner than previous desulfurization methods. Furthermore, the detailed mechanism behind this tetraethylborate-mediated desulfurization was also explored and discussed and other application of such boron species was investigated as well.