The betacoronavirus, SARS-CoV-2, that is responsible for COVID-19 disease and that was first described in Wuhan, China, in late 2019, has swiftly made its way around our world, resulting in excess of 30,000 deaths to date (1). Efforts to recognize SARS-CoV-2 infection have focused on respiratory symptoms such as cough and shortness of breath (2,3). Currently, the Centers for Disease Control and Prevention (CDC) criteria for identifying persons under investigation for SARS-CoV-2 infection in the United States comprise respiratory symptoms and/or fever only (4). Recent reports from China have described concomitant digestive symptoms, such as nausea, vomiting, diarrhea, and abdominal pain, in patients with confirmed SARS-CoV-2 pulmonary infection (5-8)

Tagging the terminus: N→S acyl transfer in native peptides and proteins can be reliably intercepted with hydrazine. The method allows selective labeling and ligation, without recourse to the use of protein‐splicing elements. NCL=native chemical ligation.

Native chemical ligation is widely used for the convergent synthesis of proteins. The peptide thioesters required for this process can be challenging to produce, particularly when using Fmoc-based solid-phase peptide synthesis. We have previously reported a route to peptide thioesters, following Fmoc solid-phase peptide synthesis, via an N→S acyl shift that is initiated by the presence of a C-terminal cysteine residue, under mildly acidic conditions. Under typical reaction conditions, we occasionally observed significant thioester hydrolysis as a consequence of long reaction times (~48 h) and sought to accelerate the reaction. Here, we present a faster route to peptide thioesters, by replacing the C-terminal cysteine residue with selenocysteine and initiating thioester formation via an N→Se acyl shift. This modification allows thioester formation to take place at lower temperatures and on shorter time scales. We also demonstrate how application of this strategy also accelerates peptide cyclization, when a linear precursor is furnished with an N-terminal cysteine and C-terminal selenocysteine.

In these simplified calculations, overlap has been neglected. The following parameters were used for the Coulomb integrals.= a; aN = a + 0-5/3; as = 0: + I-lg. These were suggested by Professor C. A. Coulson (personal communication), the latter value being derived from that of a0 used by Orgel et uE. (Trans. Faraduy Soc., 1951, 47, 113) by a comparison of the electronegativities of oxygen and sulphur. A somewhat lower value for as might be more acceptable but would only slightly reduce the calculated charges without altering the relation between them. It is realized that our parameters, and therefore the calculated charges and bond orders, vary slightly from those used by Pullman and Metzger (BUZZ. SOC. chim. France, 1948, 15, 1021) for thiazole calculations, but we believe that they are more soundly derived. The only difference between the two sets of results lies in the relative interchange of charge on the thiazole C( 4) and (&) atoms. The large difference between these charges and that on C(z), and the magnitude of the bond orders, remain virtually unaltered.

Graphical abstractWe describe experiments that give us a sense of the impact of native chemical ligation, on N-glycopeptide thioester synthesis via N→S acyl transfer.