On-tissue N-terminal peptide derivatizations for enhancing protein identification in MALDI mass spectrometric imaging strategies.

Matrix-assisted laser desorption/ionization (MALDI) is a new tool that can acquire the localization of various compounds, including peptides and proteins, directly from tissue sections. Despite the important developments recently performed in the field of MALDI imaging in tissue, the precise identification of compounds still needs improvement. We have developed N-terminal chemical derivatization strategies to improve tissue identification of proteins, including de novo sequencing performance. We have first focused on sulfonation agents, such as 4-SPITC and 3-SBASE. These two derivatizations were optimized to be performed directly on tissue sections. By adding a negative charge at the N-terminus of a tryptic digest peptide, we were able to generate a complete y fragment series directly from the tissue. Of these derivatizations, 3-SBASE has shown to be more efficient, as loss of the derivative group is one of the major fragmentation pathways for 4-SPITC. 3-SBASE was optimized so that the derivatization reaction could be automatically performed using an automatic microspotting device. It was then included in an automatic process that included automated trypsin digestion and matrix deposition. Derivatizations allowed the acquisition to be easily interpretable by MS(2) spectra, leading to very precise identification as well as easy manual reading of sequences for de novo sequencing. It was observed that only arginine-terminated peptides were observed after derivatization, likely due to the high gas-phase basicity of such peptides compared to those that are lysine-terminated. We also observed a stop in the y fragmentation series for peptides presenting a miscleavage. We have now begun to study a different derivatization using N-succinimidyloxycarbonylmethyl)tris(2,4,6-trimethoxyphenyl)phosphonium bromide (TMPP). This derivatization allows the orientating of a fragmentation toward a series of fragment ions, and thus it is independent of the presence of basic residues in the sequence. This derivatization can be performed at room temperature, which greatly facilitates the automation of the process. The TMPP derivatization therefore yields an advantageous new generation of derivatives suited for use in tissue.