N-Terminal Proline Editing for the Synthesis of Peptides with Mercaptoproline and Selenoproline: Mechanistic Insights Lead to Greater Efficiency in Proline Native Chemical Ligation

Native chemical ligation (NCL) at proline has been limited by cost and synthetic access. In addition, prior examples of NCL using mercaptoproline have exhibited stalling of the reaction after thioester exchange, due to inefficient S → N acyl transfer. Herein, we develop methods, using inexpensive Boc-4R-hydroxyproline, for the solid-phase synthesis of peptides containing N-terminal 4R-mercaptoproline and 4R-selenoproline. The synthesis proceeds via proline editing on the N-terminus of fully synthesized peptides on the solid phase, converting an N-terminal Boc-4R-hydroxyproline to the 4S-bromoproline, followed by an S_N2 reaction with potassium thioacetate or selenobenzoic acid. After cleavage from the resin and deprotection, peptides with functionalized N-terminal proline amino acids were obtained. NCL reactions with mercaptoproline proceeded slowly under standard NCL conditions, with the S-acyl transthioesterification intermediate observed as a major species. Computational investigations indicated that the bicyclic intermediates and transition states for S → N acyl transfer are sufficiently low in energy (10-15 kcal mol^-1 above starting material) that ring strain cannot explain the slow S → N acyl transfer. Instead, the bicyclic zwitterionic tetrahedral intermediate has a low barrier for reversion to the S-acyl intermediate, causing reversion to the thioester (reverse reaction) to occur preferentially over elimination to generate the amide (forward reaction). We hypothesized that a buffer capable of general acid and/or general base catalysis could promote S → N acyl transfer and thus achieve greater efficiency in proline NCL. In the presence of 2 M imidazole at pH 6.8, NCL with mercaptoproline proceeded efficiently to generate the peptide with a native amide bond. NCL with selenoproline also proceeded efficiently to generate the desired products when a thiophenol thioester was employed as a ligation partner. After desulfurization or deselenization, the products obtained were identical to those synthesized directly, confirming that the solid-phase proline editing reactions proceeded stereospecifically and without epimerization.