Abstract
The red sword bean (Canavalia gladiata) is an underutilized edible bean cultivated in China. It was previously found to have the highest content of antioxidant polyphenols among 42 edible beans, mainly gallic acid, and gallotannins in its red bean coat, an apparently unique characteristic among edible beans. In this study, the main phenolic compounds in red sword bean coats were further separated by Sephadex LH-20 column chromatography, and identified by LC-MS/MS. Furthermore, the FRAP and ABTS antioxidant activities and antibacterial activity (diameter of inhibition zone, DIZ) of main gallotannin-rich fractions were tested. Our results showed that gallotannins of red sword bean coats were mainly comprised of monogalloyl to hexagalloyl hexosides. Interestingly, tetragalloyl, pentagalloyl, and hexagalloyl hexosides were identified as the possible candidates responsible for the red color of the coats. On the other hand, gallotannin-rich fractions exhibited diverse antioxidant and antibacterial activities, and tetragalloyl hexoside overall had the highest free radical scavenging and antibacterial activities. The degree of galloylation did not completely explain the structure-function relationship of gallotannins isolated from red sword bean coats, as there should exist other factors affecting their bioactivities. In conclusion, red sword bean coats are excellent natural sources of gallotannins, and their gallotannin-rich extracts can be utilized as natural antioxidant and antibacterial agents with potential health benefits as well as application in food industry.
Highlights
Gallotannins are important hydrolysable tannins constituting of at least one galloyl moiety and one sugar/cyclitol molecule, such as glucose and quinic acid
The antioxidant activity increased with increased number of galloyl groups within 5–10 galloyl glucoside (GG). This trend was not observed within 1–4 GG, which was overall consistent with our results that we found that digalloyl and trigalloyl hexoside-rich fractions V and VII, respectively, had the highest iron reducing activity, and tetragalloyl hexosiderich fraction VIII had the highest ABTS free radical scavenging activity among 1–6 galloyl hexoside-rich fractions isolated from red sword bean coats
Typhimurium among 1–10 GG separated from Galla chinensis, but 2 GG had the lowest antibacterial activity, which was inconsistent with our results that we found tetragalloyl hexoside-rich fraction VIII overall had the strongest antibacterial activity against four bacteria among 1–6 galloyl hexosides separated from red sword bean coats, and digalloyl hexoside-rich fraction V exhibited the strongest antibacterial activity against B. cereus, and no antibacterial activity was detected for monogalloyl hexoside-rich fraction I
Summary
Gallotannins are important hydrolysable tannins constituting of at least one galloyl moiety and one sugar/cyclitol molecule, such as glucose and quinic acid. Gallotannins and gallotannin-rich extracts have been reported to possess many useful bioactivities, including antioxidant (Tian et al, 2009; Luo et al, 2014; Yang et al, 2014; Zhang et al, 2014), antibacterial (Tian et al, 2009; Engels et al, 2012), antiproliferative (Luo et al, 2014; Li et al, 2015), cardiovascular protective (Larrosa et al, 2010), hepatoprotective (Go et al, 2016), and antidiabetic activities (Chandak et al, 2009). With the current research findings, it is hoping that information generated will add value to this plant and will be useful for future utilization of red sword beans as good natural sources of gallotannins
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