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In the modern protocols of chemical protein syntheses, peptide-α-thioesters have been used as key components for the assembly of full-length polypeptides through chemoselective peptide coupling reactions. A variety of thioester precursors have been developed for the synthesis of the peptide-α-thioesters by Fmoc solid phase peptide synthesis (Fmoc-SPPS). Recently our group found a peptidyl-N-acetylguanidine as a new peptide-α-thioester precursor. This peptide derivative can be converted into a corresponding peptide-α-thioester only by treatment with an excess amount of a thiol in aqueous buffers at around neutral pH. This unique property allowed us to envision the practical use of the peptidyl-N-acetylguanidines for the chemical syntheses of proteins; however, an efficient synthetic method has been lacking. Herein, we report an efficient solid-phase synthesis of peptidyl-N-acetylguanidines. This new synthetic method employing selective activation and cleavage of a peptide bond successfully provided peptidyl-N-acetylguanidines from the on-resin protected peptides prepared by standard Fmoc-SPPS. We also evaluated the reactivity of a peptidyl-N-acetylguanidine in native chemical ligation through the synthesis of glucose-dependent insulinotropic polypeptide analogue. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

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We report the first search for the K L → π 0 γ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of ð7.1 AE 0.3 stat AE 1.6 syst Þ × 10 −8 , no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be 1.7 × 10 −7 at the 90% confidence level.