Rapid semi-on-line monitoring of Fmoc solid-phase peptide synthesis by matrix-assisted laser desorption/ionization mass spectrometry

Summary A simple yet highly effective application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the rapid monitoring of Fmoc solid-phase peptide synthesis is described. A few beads of the resin are removed at any desired step during synthesis, the fully protected peptide is cleaved from the resin and an MS spectrum of the analytes present is produced. Some standard side-chain protecting groups may be cleaved off during sample preparation for MS analysis; however, these cleavages are readily identified. Using this approach, incomplete amino acid acylations are readily detected in approximately the same time as by traditional tests such as ninhydrin. The semi-on-line method also lends itself to ready optimization of synthesis protocols and to the examination of resin-bound peptide side reactions which may not be detectable by chemical means.