NMR experiments were conducted on a Varian NMR System spectrometer operating at 600 MHz. Off-target adduct formation was analyzed through the reaction between the respective Michael acceptor (3R or 6R,R) and the corresponding model compound (BME or His-Test) by 1H NMR. His-Test was synthesized from N-acetyl-L-histidine, referred to as Ac-His and used in the LC-MS experiment earlier, to neutralize its carboxyl group: N-acetyl-L-histidine methyl ester. His-Test was much better suited to carry out the 1H NMR experiments because the pH could be kept constant during titration experiments. All reagents were dissolved and the experiments were carried out in PBS (D2O, pH ~7.2) with 75% DMSO. Details of the measurements and the assignment of the spectra are described in Supplementary Note 1.
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