Chemical Biology Laboratory, National Cancer Institute, 376 Boyles Street, Building 376, Frederick, Maryland 21702, USA.
J Am Chem Soc. 2012 Apr 11;134(14):6316-25. doi: 10.1021/ja212188r. Epub 2012 Mar 27.
Glycopeptides are extremely useful for basic research and clinical applications, but access to structurally defined glycopeptides is limited by the difficulties in synthesizing this class of compounds. In this study, we demonstrate that many common peptide coupling conditions used to prepare O-linked glycopeptides result in substantial amounts of epimerization at the α position. In fact, epimerization resulted in up to 80% of the non-natural epimer, indicating that it can be the major product in some reactions. Through a series of mechanistic studies, we demonstrate that the enhanced epimerization relative to nonglycosylated amino acids is due to a combination of factors, including a faster rate of epimerization, an energetic preference for the unnatural epimer over the natural epimer, and a slower overall rate of peptide coupling. In addition, we demonstrate that use of 2,4,6-trimethylpyridine (TMP) as the base in peptide couplings produces glycopeptides with high efficiency and low epimerization. The information and improved reaction conditions will facilitate the preparation of glycopeptides as therapeutic compounds and vaccine antigens.
糖肽在基础研究和临床应用中非常有用，但由于合成这类化合物的困难，结构明确的糖肽的获取受到限制。在这项研究中，我们证明了许多用于制备 O-连接糖肽的常见肽偶联条件会导致α位的大量差向异构化。事实上，差向异构化导致高达 80%的非天然差向异构体，表明在某些反应中它可能是主要产物。通过一系列的机制研究，我们证明与非糖基化氨基酸相比，增强的差向异构化归因于多种因素的组合，包括更快的差向异构化速率、非天然差向异构体相对于天然差向异构体的能量偏好以及肽偶联的整体速率较慢。此外，我们证明在肽偶联中使用 2,4,6-三甲基吡啶（TMP）作为碱可以高效率、低差向异构化地生成糖肽。这些信息和改进的反应条件将有助于制备糖肽作为治疗化合物和疫苗抗原。
