Graphitized carbon black enrichment and UHPLC-MS/MS allow to meet the challenge of small chain peptidomics in urine

Short peptide sequences represent emerging analytes in a variety of fields, including biomarker discovery, but also a well-known analytical challenge in complex matrices, due to the low abundance, extensive suppression during MS analysis, and lack of workflows, as they cannot be identified by ordinary peptidomics strategies and coverage is extremely limited by metabolomics as well. In this context, in this work, a solid phase extraction method was developed for the cleanup and enrichment of dipeptides, tripeptides, and tetrapeptides in urine using graphitized carbon black Carbograph 4 as the sorbent. The method was first developed on analytical standards spiked in urine, with recoveries in the range of 60-100%. Then the method was applied to urine samples from healthy volunteers. The enriched urine samples were analyzed by ultrahigh performance liquid chromatography (UHPLC) using an orthogonal strategy in which both a reversed phase (RP) C18 column and a zwitterionic hydrophilic interaction liquid chromatography (HILIC) column were used, for better coverage of peptide polarity and improved detection of peptides. High-resolution mass spectra were acquired in data-dependent mode using a suspect screening strategy with inclusion list; peptides were identified by a semiautomated workflow for feature extraction, candidate mass filtering, and MS/MS spectra comparison with in silico mass spectra. The complementarity of the orthogonal separation strategy was confirmed by peptide identification, resulting in 101 peptides identified from the RP runs and 111 peptides from the HILIC runs, with 60 common identifications. The method is applicable to both hydrophobic and hydrophilic peptides. Peptides were stable over 2 h after collection and protease inhibitors were not necessary, as no formation of artifacts was observed.