Glycosylamines-based reactive matrix designed for imaging acidity in Ponkan fruit using matrix assisted laser desorption/ionization mass spectrometry imaging

Current imaging techniques , such as position emission tomography , magnetic resonance imaging and optical imaging , have been developed and applied to imaging acidity in live tissues for years, but they still have some limitations. In the present study, a reliable and interference-free pH imaging method based on matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was established for the first time. A novel reactive matrix, named glycosyl-3-aminoquinoline (Gly-3AQ), was designed and synthesized here not only to overcome the background interference in the low mass range of MALDI but also provide a new function of acid-responsiveness for the matrix. This matrix was responsive to pH change due to its acid-catalyzed hydrolysis and the hydrolysis product 3-aminoquinoline (3-AQ) has an increase of peak intensity with pH decrease. The Gly-3AQ-based MALDI pH detection method is demonstrated to have good acid-responsiveness in the range of pH 2.0–7.0, and when pH changes 0.5, relative abundance of 3-AQ changes about 1.5 folds. The method showed a very prominent advantage, that is, free-interference when analyzing real samples. Validation for the method revealed pH calibration curve had good linearity (R > 0.9953) with relative standard deviation less than 13.8%. This method was applied to analyze and visualize acidity variation of citrus during development and the changed acidity underwent relative quantitation and statistical analysis. The newly developed reactive matrix has potential application to in-situ MSI of acidity in the biomedical field.