Stuhr-Hansen Nicolai, Bork Nicolai, Strømgaard Kristian
Department of Drug Design and Pharmacology, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark.
Org Biomol Chem. 2014 Aug 14;12(30):5745-51. doi: 10.1039/c4ob00073k. Epub 2014 Jun 26.
The influence of arylthiols on cysteine-free ligation, i.e. the reaction between an alkyl thioester and a primary amine forming an amide bond, was studied in a polar aprotic solvent. We reacted the ethylthioester of hippuric acid with cyclohexylamine in the absence or presence of various quantities of thiophenol (PhSH) in a slurry of disodium hydrogen phosphate in dry DMF. Quantitative conversions into the resulting amide were observed within a few hours in the presence of equimolar amounts of thiophenol. Ab initio calculations showed that the reaction mechanism in DMF is similar to the well-known aqueous reaction mechanism. The energy barrier of the catalyzed amidation reaction is approximately 40 kJ mol(-1) lower than the non-catalyzed amidation reaction. At least partially this can be explained by a hydrogen bond from the amine to the π-electrons of the thiophenol, stabilizing the transition state in the aromatic thioester amidation reaction. Under similar conditions, cysteine-free ligation was achieved by coupling a fully side-chain protected 15 amino acid phosphopeptide thioester to the free N-terminal of a side-chain protected 9 amino acid peptide producing the corresponding 24 amino acid phosphopeptide.
在极性非质子溶剂中研究了芳硫醇对无半胱氨酸连接反应（即烷基硫酯与伯胺之间形成酰胺键的反应）的影响。我们在无水二甲基甲酰胺中的磷酸氢二钠浆液中，于不存在或存在不同量的苯硫酚（PhSH）的情况下，使马尿酸乙酯与环己胺发生反应。在等摩尔量苯硫酚存在下，数小时内即可观察到定量转化为所得酰胺的情况。从头算计算表明，在二甲基甲酰胺中的反应机理与众所周知的水相反应机理相似。催化酰胺化反应的能垒比非催化酰胺化反应低约40 kJ·mol⁻¹。这至少部分可以通过胺与苯硫酚的π电子形成的氢键来解释，该氢键稳定了芳香硫酯酰胺化反应中的过渡态。在相似条件下，通过将完全侧链保护的15个氨基酸的磷酸肽硫酯与侧链保护的9个氨基酸的肽的游离N端偶联，实现了无半胱氨酸连接，生成了相应的24个氨基酸的磷酸肽。
