Temporary Structural Supports for Chemical Protein Synthesis

The range of functional proteins that can be prepared by chemical protein synthesis includes those bearing complex modifications and incorporating D‐amino acids, and exceeds what can be accessed by biological means, but the technique is still limited by the unfavorable solution behavior of many synthetic protein intermediates in buffer, leading to inefficient ligation, purification, and in vitro folding. One approach to address this limitation is the use of temporary structural supports — chemical modifications, usually solubilizing functionalities such as polyamines or carbohydrates — that are installed on either the backbone or side chains of the synthetic intermediates and removed at a later stage of chemical protein synthesis. The basic processes for introducing and removing such temporary structural supports are reminiscent of the canonical protecting groups ubiquitous in organic chemistry. However, unlike the synthesis of small organic molecules, where solubility is rarely an issue, the purpose of temporary structural supports is to modulate the solution behavior of the synthetic intermediates. In this review, we summarize recent advances in the development of temporary structural supports for chemical protein synthesis and organize them into three categories: 1) Supports to improve solubility; 2) Supports to assist chemical ligation; and 3) Supports to promote folding.