Correlation of Glucosinolates and Volatile Constituents of Six Brassicaceae Seeds with Their Antioxidant Activities Based on Partial Least Squares Regression

Abstract

Brassicaceae comprises various species representing an economically important source of industrial or pharmaceutical crops. The present study aimed to identify glucosinolates (GSLs) and volatile compounds in six Brassicaceae seeds cultivated in Egypt. An (High Performance Liquid Chromatography-Photodiode Array) HPLC–PDA analysis of GSLs in the alcoholic extracts of Raphanus raphanistrum L. (Rr), Raphanus sativus L. (Rs), Brassica oleracea var. capitata L. (Boc), Brassica oleracea var. botrytis L. (Bob), Brassica rapa L. (Br), and Eruca sativa L. (Es) was carried out using a mixture of 23 standard GSLs. Nineteen GSLs were detected in the studied seeds. Rs had the highest GSL content (135.66 μmol/g Dry weight, DW), while Boc had the lowest GSL content (93.66 μmol/g DW). Glucobrassicin was the major identified compound in Rr, Rs, and Bob. Its highest content was in Rs (28.96 μmol/g DW). Sinigrin was the major identified GSL in Boc (18.02 μmol/g DW), although present with higher content in Bob (22.02 μmol/g DW). Neoglucobrassicin was the major GSL in Br (30.98 μmol/g DW), while glucoerucin was the major GSL in Es (17.84 μmol/g DW). The yields of the steam-distilled oils of the studied seeds ranged between 3.25 ± 0.36 and 9.68 ± 0.25% v/w. A GC–MS analysis of the oils could detect 3, 23, 18, 16, 7, and 9 compounds in Rr, Rs, Boc, Bob, Br, and Es oils, respectively. Sulfur and nitrogenous compounds predominated in all studied oils except Rs, which contained a higher percentage of alkanes. The major identified compound in Rr oil was 4-isothiocyanato-1-(methylthio)-1-butene (94.77 ± 1.25%), while in Br it was 3-butenyl isothiocyanate (69.55 ± 1.02%), thiolane in Rs (15.15 ± 0.22%), and erucin in Es (97.02 ± 1.514%). Both Boc and Bob had the same major compound 4-(methylthio) butanenitrile, which represented 40.35 ± 1.15 and 50.52 ± 1.02% in both oils, respectively. Radical scavenging activity for both GSL extracts and essential oils on DPPH radical ranged between 18.01 ± 0.72 and 114.28 ± 1.15 µg/mL (IC50). The highest antioxidant capacity was for Es oil, while the lowest one was for Rr oil. Generally, it was observed that the GSLs had better antioxidant activity than their corresponding essential oils except for Es oil, which had higher activity. A principal component analysis (PCA) was successfully applied to discriminate among six Brassicaceae seeds based on both HPLC and GC–MS, where complete segregation was achieved among all samples with high correlation between Boc and Bob. Partial Least Squares-Regression (PLS-R) models showed that there is a better correlation between the antioxidant activity and glucosinolate profile when being compared to that of a volatile one. This profiling and variation of GSLs and volatile metabolites of the studied Brassicaceae seeds may be employed in further studies regarding their health-promoting properties.