Abstract
Flavan-3-ols, which comprise proanthocyanidins and their monomers, are major flavonoids in strawberries, and they have a wide range of biological activities and health benefits. However, their spatial distribution in strawberry fruit remains poorly understood. Therefore, we performed matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI), to visualize flavan-3-ols in ripe strawberry fruit. Peaks matching the m/z values of flavan-3-ols [M − H]− ions were detected in the negative ion mode using 1,5-diaminonaphthalene as matrix. Catechin and/or epicatechin, three B-type procyanidins, and two B-type propelargonidins were identified by MALDI-tandem MS. These flavan-3-ols were mainly distributed in the calyx, in and around the vascular bundles, and in the skin. In-source fragmentation of proanthocyanidins was determined using their standards, suggesting their distribution was mixed ion images of themselves, and fragment ions generated from those had a higher degree of polymerization. B-type procyanidins were predominantly distributed in the vascular bundles than in the skin, whereas B-type propelargonidins were almost equally distributed between the vascular bundles and skin, suggesting that their distribution patterns are different from the type of their flavan-3-ol monomers. Flavan-3-ols, especially B-type procyanidins, may help prevent pathogen infection not only in the skin but also in and around the vascular bundles.
Highlights
Proanthocyanidins, known as condensed tannins, are oligomers and polymers of flavan-3-ol units that have been used as therapeutic compounds since ancient times [1,2]
To detect proanthocyanidins and their monomers, we initially performed matrix screening using the following matrices: α-cyano-4-hydroxycinnamic acid (CHCA), 9-aminoacridine (9AA), and 1,5-diaminonaphthalene (DAN), which are commonly used in matrix-assisted laser desorption/ionization (MALDI)-Mass spectrometry imaging (MSI)
We showed that peaks matching tocatechin monomer or B-type procyanidins were generated from the B-type procyanidins with a larger degree of polymerization by in-source fragmentation (Figure 6A–D)
Summary
Proanthocyanidins, known as condensed tannins, are oligomers and polymers of flavan-3-ol units that have been used as therapeutic compounds since ancient times [1,2]. Flavan-3-ols, namely proanthocyanidins and their monomers, are the second-most abundant polyphenols after anthocyanins in strawberry fruit [11]. In addition to other classes of metabolites, flavan-3-ols are believed to be associated with the health properties of strawberry fruit. In our previous study using MALDI-MSI, anthocyanin species such as pelargonidin- and cyanidin-glycosides were successfully visualized and differences in their distribution patterns in strawberry fruit were revealed [24]. We visualized the spatial distributions of the identified proanthocyanidins and showed their unique distribution by MALDI-MSI analysis. We expect that these results will contribute to further elucidating the biological roles of flavan-3-ol species
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