Acquiring thin histological sections of acai(Euterpe oleracea Mart.) seeds for molecular mapping by mass spectrometry imaging

RATIONALEMatrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) of tissues became popular in the last decade. Consequently, adapting sample preparation methods for different materials turned to be a pivotal step for a successful analysis, due to the requirement of samples slices of 12-20 m thickness. However, preparing thin sections compatible with MALDI-IMS for unusual samples is challenging, as existing histological protocols may not be suitable, thus requiring new methods. Acai (Euterpe oleracea Mart.) seed is an example of a challenging material due to its toughness and resistance to crack. Therefore, our goal was to develop a methodology to obtain thin (<20 m) and entire longitudinal sections of acai seeds for MALDI-IMS analysis. METHODSDifferent strategies were evaluated for obtaining thin cuts of seeds, being the combination of the following steps the most suitable option: (i) softening of seeds by water immersion for 24 h; (ii) longitudinal cut of seeds to obtain half-seeds using a razor blade and a hammer; (iii) half-seeds imbibition in gelatin; (iv) sectioning using a cryostat at -20 {degrees}C to obtain samples with <20 m thickness; and (v) collection of samples in an indium tin oxide coated glass slide covered by a double-face copper tape to avoid sample wrapping and ensure adhesion after unfreezing. (iv) storage of samples in a -80 {degrees}C freezer, if necessary. RESULTSAs a result, this adapted sample preparation method enabled the analysis of acai seeds by MALDI-IMS providing spatial distribution of carbohydrates in the endosperm. CONCLUSIONSThe adaptations developed for sample preparation will help investigate the metabolic and physiological properties of acai seeds in future studies.