Invited for the cover of this issue is the group of Christian P. R. Hackenberger at the Leibniz‐Forschungsinstitut für Molekulare Pharmakologie and the Humboldt‐Universität zu Berlin. The image depicts a phospho‐lysine peptide mimic which reflects a phosphorylated lysine but is not identical. Read the full text of the article at 10.1002/chem.202003947.

The intrinsic lability of the phosphoramidate P−N bond in phosphorylated histidine (pHis), arginine (pHis) and lysine (pLys) residues is a significant challenge for the investigation of these post‐translational modifications (PTMs), which gained attention rather recently. While stable mimics of pHis and pArg have contributed to study protein substrate interactions or to generate antibodies for enrichment as well as detection, no such analogue has been reported yet for pLys. This work reports the synthesis and evaluation of two pLys mimics, a phosphonate and a phosphate derivative, which can easily be incorporated into peptides using standard fluorenyl‐methyloxycarbonyl‐ (Fmoc‐)based solid‐phase peptide synthesis (SPPS). In order to compare the biophysical properties of natural pLys with our synthetic mimics, the pKa values of pLys and analogues were determined in titration experiments applying nuclear magnetic resonance (NMR) spectroscopy in small model peptides. These results were used to compute electrostatic potential (ESP) surfaces obtained after molecular geometry optimization. These findings indicate the potential of the designed non‐hydrolyzable, phosphonate‐based mimic for pLys in various proteomic approaches.

In this review, we highlight groundbreaking discoveries and applications of Staudinger reactions in the molecular life sciences, starting from the engineering of the Staudinger ligation as a bioorthogonal reaction until most recent applications in modern bioconjugation methods to generate next-generation biopharmaceuticals. Bioorthogonal reactions refer to a set of chemoselective transformations in biological environments able to take place in presence of naturally occurring functional groups. The Staudinger ligation set a new paradigm of such transformations, resulting in the development of various labeling and bioconjugation strategies for the modification of (bio-)molecules of interest.

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