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Large-scale analysis of proteins, which can be regarded as functional biomolecule, assumes an important role in the life
science. A MALDI using an ultraviolet laser (UV-MALDI) is one of ionization methods without fragmentation and has
achieved conformation analysis of proteins. Recently, protein analysis has shifted from conformation analysis to
functional and direct one that reserves posttranslational modifications such as the sugar chain addition and
phosphorylation. We have proposed a MALDI using a mid-infrared tunable laser (IR-MALDI) as a new ionization
method. IR-MALDI is promising because most biomolecules have a specific absorption in mid-infrared range, and IR-MALDI
is expected to offer; (1) use of various matrices, (2) use of biomolecules such as water and lipid as the matrix,
and (3) super-soft ionization. First, we evaluated the wavelength dependence of ionization of different matrices using a
difference frequency generation (DFG) laser, which can tune the wavelength within a range from 5.5 to 10.0 &mgr;m. As
results, ionization was specifically occurred at 5.8 &mgr;m which the C=O vibration stretching bond in matrix material and
mass spectrum was observed. Next, protein mass spectrum was observed in the culture cells, MIN6, which secrete
insulin, without the conventional cell-preparation processes. We demonstrate that the IR-MALDI has an advantage over
the conventional method (UV-MALDI) in direct analysis of intracellular proteins.
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