2.5. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS)

Mass spectrometry (MS), using the matrix assisted laser desorption time-of-flight (MALDI-TOF) method, was performed to determine the primary structure of LYS and the modified LYS analogues. MALDI-MS was performed by Alphalyse A/S (Odense, Denmark). The proteins were purified using a Millipore C18 ziptip (Merck Millipore, Darmstadt, Germany). Each purified sample was mixed with 2,5-dihydroxyacetonphonone/diammonium hydrogen citrate (DHAP/DAHC) matrix and spotted onto a Big Anchor target from Bruker (Bremen, Germany). A laser beam (Nd: YAG SmartBeam laser: 355 nm, 2,000 Hz repetition rate) was used to desorb the matrix crystals and the molecules were ionized in the gas phase. The mass spectra were acquired using an Autoflex Speed MALDI TOF spectrometer (Bruker).