An improvement of matrix-assisted laser desorption/ionization mass spectrometry using an infrared tunable free electron laser

Matrix-assisted laser desorption/ionization (MALDI) combined with time-of-flight mass spectrometry (TOFMS) is a powerful yet robust tool for protein identification, due to its high sensitivity and theoretically unlimited detectable mass range. A large part of functional proteins, such as membrane proteins, are insoluble as native forms in a matrix solution without a strong denaturing condition, hence are not amenable to the conventional MALDI–TOFMS analysis. Aiming at overcoming this difficulty, we have developed a novel MALDI technique (UV/FEL-MALDI). An infrared free electron laser (IR-FEL) has a wide tunability in a mid-IR range and is quite attractive as a source of selective vibrational excitation. The FEL wavelength can be tuned to activate a denaturant, which impedes the conventional MALDI process, without an excess heating of analyte molecules. This scheme lets a dense denaturant to be used for the MALDI sample preparation of insoluble proteins. A simultaneous use of the FEL with a nitrogen pulse laser for MALDI achieves spatially and temporally defined desorption, which is essential to TOFMS detection, while specificity and selectivity owing to an FEL wavelength can be conserved. Some attractive features of the protein clustering have been found in the application of UV/FEL-MALDI to hair keratins, which was chosen as a model of insoluble proteins.