Fast and cost-efficient approaches for 17O-isotopic labeling of carboxylic groups in biomolecules: from free amino acids to peptide chains

17O NMR spectroscopy is a powerful analytical technique, which enables to access unique information regarding the structure and reactivity of biomolecules, such as peptides and proteins. However, due to the exceedingly low natural abundance of 17O (0.04 %), it is necessary to work with 17O-enriched samples, which are not easily accessible because of the experimental constraints and high costs associated with the traditional enrichment procedures. Here, we present simple, fast and cost-efficient labeling strategies for 17O-enrichment of amino acids and peptides. First, using mechanochemical saponification, a variety of unprotected amino acids were enriched within 30 min of milling under ambient conditions, consuming only microliter amounts of costly labeled water, and producing pure molecules with high enrichment levels (up to ~ 40 %), and in medium to high yields (~ 60 - 85 %) without the loss of their optical purity (ee > 99%). The labeling efficiency of the mechanochemical protocol was then compared to a re-optimised enrichment strategy based on acid-catalysed oxygen exchange. Subsequently, 17O-enriched Fmoc/tBu-protected amino acids were produced on a 1 g/day scale with high enrichment levels (~ 40 %), and in high synthetic yields (~ 75 - 85 %), by scaling up the mechanochemical enrichment followed by a Fmoc-protection step. Lastly, a direct site-selective 17O-labeling of carboxylic functions in peptide side-chains was developed and applied to the RGD and GRGDS peptides, reaching up to 29% enrichment level. Producing highly enriched molecules enabled to record 17O solid-state NMR spectra at 14.1 T in reasonable analytical times. Overall, this work represents an important step forward in providing easy access to highly 17O-enriched peptides and proteins to be subsequently studied by high-resolution 17O NMR spectroscopy.