Michler's Ketone: A Novel Negative-Ion Matrix for Enhancing Amino Acid Detection and MALDI Tissue Imaging

Amino acids are essential protein components and key bioactive molecules governing metabolism and signal transduction, playing vital roles in plant growth, physiological homeostasis, immunity, and stress adaptation. Negative-ion matrices for amino acid detection in matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) remain underdeveloped, largely due to low ionization efficiency and strong background interference. To address this limitation, we developed a novel negative-ion MALDI matrix, Michler's ketone (MK), for efficient amino acid detection and imaging. MK exhibits several superior properties as a powerful MALDI matrix, including strong UV-vis absorption at 355 nm, excellent vacuum stability, low matrix-related ion interference, and high detection efficiency for amino acids. Using MK, all 20 proteinogenic amino acids were successfully detected in the mixed standard, whereas conventional matrices, namely, 2-mercaptobenzothiazole (2-MBT) and α-cyano-4-hydroxycinnamic acid (CHCA), detected only 10 and 8 amino acids, respectively. Furthermore, MK enabled high-resolution MALDI-MSI of amino acids in Actinidia arguta tissues, allowing clear visualization of tissue-specific spatial distribution patterns. Overall, MK represents a high-performance negative-ion MALDI matrix that significantly enhances amino acid detection and imaging.