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Fig. 16 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 16. Definition of the term selenoglucosinolate, precursor of an isoselenocyanate, according to the discoverers, Bertelsen et al. (1988).

Fig. 14 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 14. The usefulness of the J-resolved (JRES) spectrum for resolving overlapping signals. (A) The upfield part of the 1 H NMR spectrum of d40R in D O 2 and dioxane as internal standard (i.s.). A minor impurity (imp.) of MeOH from the HPLC isolation is seen. (B) In the JRES 1 H NMR spectrum, the splitting of signals is plotted (almost) perpendicular to the chemical shift x-axis, allowing resolution of signals at nearly identical chemical shift. The known coupling constants of the 6′a double doublet signal can be used for estimating coupling constants in the newly revealed signals. Three obvious triplets with coupling constant (J) of ca. 8 Hz can be seen. In addition, the complex multiplet of the 5′ proton is visible as a line of low intensity signals. Singlets from the i. s. and imp. do not show coupling, and all signals are seen at the same chemical shift coordinate as in the 1D spectrum. Unpublished results from identification of 40R (Agerbirk et al., 2001a).

Fig. 12 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 12. Complex coupling within branched aliphatic side chains in desulfoglucosinolates (dGSLs), and simplifying effect of substitution. (A) Side chain region of the 1 H NMR spectrum of 3-methylpentyl dGSL (d58) in D O 2 (Agerbirk et al., 2008, 2010a; Olsen et al., 2016). (B) Corresponding region of the spectrum of a hydroxyl derivative, 2-hydroxy-3-methylpentyl dGSL (d149) in D2O (Olsen et al., 2016). It is evident that the coupling of the 1a and 1b signals in B is now first order, and that the signals of the position 2 protons have disappeared from the illustrated range of chemical shifts (to be found at 3.95ppm). Occasional minor impurity peaks (imp) are generally recognizable by having peak areas less than unity.

Fig. 10 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 10. Logical sub-divisions in the group of "aromatic glucosinolates", defined as glucosinolates containing one or more aromatic moieties.

Fig. 2 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 2. Definition of the parent glucosinolate ion (A), and its use in naming natural derivatives (B, D), with numbering system indicated, using the originally proposed writing of names in one word (Dateo, 1961). The numbering system is retained in the isothiocyanate product (C). The modern practice by many authors of splitting glucosinolate anion names in two (given below each name) is followed in the rest of the text. Both practices are considered generally accepted.

Fig. 3 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 3. Examples of glucosinolates derived directly from a standard amino acid precursor, and meaning of the abbreviation 'GSL' in condensed structures.

Fig. 7 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 7. Biosynthesis of the rare, unusual glucosinolate (GSL) 3-methoxycarbonylpropyl GSL (Kjaer and Gmelin, 1957a) as evidenced by traditional tracer studies in an Erysimum sp. (Chisholm, 1973). Positions of radiolabeling are indicated with asterisks. Apart for the terminal methyl group derived from Met, the GSL was biosynthesized from homoGlu (probably derived from Glu), making it the only documented GSL biosynthesis from Glu. A more recent paper (Radulović et al., 2011) reported the free carboxylic acid, 3-carboxypropyl isothiocyanate from autolyzed Erysimum diffusum Ehrh. (diffuse wallflower), further supporting GSL biosynthesis from homoGlu in Erysimum spp. The brackets indicate the lack of modern spectroscopic confirmation of this structure, which is, however, based on very solid classical work.

Fig. 1. A in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 1. A simple glucosinolate (GSL) and its enzymatic conversion to an isothiocyanate (ITC). (A) Structure of benzyl GSL and hydrolysis to the aglucone. (B) Spontaneous rearrangement of the aglucone to benzyl ITC. (C) Configuration of the thiohydroximate double bond and some common terms used in discussing GSL structures. (D) The relation of a thiohydroximate ion to thiohydroxamic and –imic acids.

Fig. 5 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 5. Secondary modifications in glucosinolate (GSL) biosynthesis. (A) Biosynthesis of secondary modifications in Trp-derived GSLs ("indole GSLs") in Arabidopsis thaliana (Pfalz et al. 2011, 2016). (B) Biosynthesis of a more complex indole GSL is still unknown. (C) Two possible biosyntheses of p-hydroxybenzyl GSL (23), the relative importance of which in nature is not known. In the current paper, bold and square brackets around GSL numbers indicate incomplete MS and NMRdocumentation for their existence (Section 2).

Fig. 4 in Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

Fig. 4. Biosynthesis of benzyl glucosinolate from Phe (Wittstock and Halkier, 2000; Wittstock and Halkier, 2002; Mikkelsen et al., 2002). An oxime is the first intermediate, followed by a complex transformation to the thiohydroximate, and finally glucosylation and sulfation. By introduction of the sulfate group in the final step, the unstable intermediate that leads to isothiocyanate formation (Fig. 1) is avoided. The donor of the thioglucose S is glutathione (Schläppi et al., 2008; Geu-Flores et al., 2009a). The relevant (E)-oxime is the immediate product of the CYP79 (Clausen et al., 2015). In the further core biosynthesis not shown, the identity of one intermediate is yet uncertain (either an aci-nitro or a nitrile oxide functionality) (Sønderby et al., 2010a).

Fig. 8 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 8. Reaction scheme for the formation of BOH (6) and BCOOH (8) from reactive benzaldehyde through a Cannizzaro type reaction, catalyzed by a benzaldehyde dehydrogenase (based on Wuensch et al., 2013). The proposed scheme would require only one enzymatic activity for both the oxidative and reductive half-reactions.

Fig. 7 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 7. Proposed scheme for reactions taking place during the maca drying. The sequence is based on the main hydrolytic metabolites observed during the drying process. Step A describes reactions generating the initial hydrolysis products and including BITC (2), BCN (4) and BIOC (7). Step B is the formation of BNH2 (3), the main accumulation product, from BITC (2). Step C describes deamination of BNH2 (3) to generate various deaminated benzenoids. Step D corresponds to the disproportionation reactions leading to pools of BCHO (5), BOH (6) and BCOOH (8) through the action of an aldehyde dehydrogenase. Step E is the condensation of BNH2 (3) with free fatty acids to produce macamides (MAC 9–13) and step F corresponds to esterification or glycosylation of BCOOH (8) as a detoxification mechanism. Numbers correspond to those in Fig. 1. Dark lines show favored reactions according to our results. Compounds in light gray are minor transient products.

Fig. 4. Pearson correlation values for glucosinolate hydrolytic products. Panel A in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 4. Pearson correlation values for glucosinolate hydrolytic products. Panel A shows the correlation of metabolites for the early stage of drying, where reactions are caused by direct damage to the tissue by shredding. Panel B shows the correlation between intermediaries as a result of the late stage tissue dehydration. BCOOR- 2, BCHO-2 and BCOOH-2 shown in panel B correspond to data points in Fig. 5 shown as part of the red solid line while BCOOR (8a), BCHO (5) and BCOOH (8) in panel A correspond to the early stage in the figure shown in solid black lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 6. Time course for the concentration of ammonium ion in maca tissues during drying. Deamination of benzyl amine and amino acids by amine oxidases are potential sources of ammonia under our short (72 h) drying conditions. Peak values of NH+ are 25 μmol g 1 dry wt.. Total amount of deaminated benzenoids 4 accounts for only 0.75 μmol g 1 dry wt. indicating other major sources of ammonium in the process. Curve adjusted by nonlinear regression (R2 = 0.9472).

Fig. 3 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 3. Tissue concentrations during oven drying for benzyl glucosinolate (BGL) and its metabolites. Drying maca tissue was extracted in solvent and the levels of benzyl glucosinolate, its primary hydrolytic products (BITC, BCN) and two products that accumulate in the flour, benzyl amine (BNH2) and macamides (MAC) were analyzed. RW represents residual humidity. Hydrolytic and final accumulation product patterns match those previously reported (Esparza et al., 2015), although for this study, a 72 h drying period and 35 ◦ C constant temperature were employed. Values are expressed in molar fraction of initial glucosinolate concentration, where 1 = 36 ± 5 μmol g 1 dry wt. (N = 6). Nonlinear regression coefficients for the compounds were: R2 = 0.941 (BGL, 1), R2 = 0.971 (BITC, 2), R2 = 0.777 (BCN, 4), R2 = 0.955 (BNH, 3), R2 = 0.985 (MAC, 9–13) and R2 = 0.993 (RW).

Fig. 2 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 2. Time course of major VOCs from glucosinolate hydrolysis measured by headspace SPME-GC-MS. Values are expressed as percentage of average peak total ion current (TIC) for each compound. Compounds, from top left are: benzyl isothiocyanate (BITC, 2), benzyl nitrile (BCN, 4), benzaldehyde (BCHO, 5), benzyl alcohol (BOH, 6), benzyl isocyanate (BIOC, 7), and average relative humidity (RH) inside the chamber (N = 9). Peak TIC values (x 107) for the compounds were: 238.42 (2, 100%); 2.88 (4, 1.2%); 34.9 (5,14.6%); 28.17 (6, 11.8%) and 3.75 (7, 1.6%). Values in parenthesis after the compound number denote the relative intensity of the peak in relation to the isothiocyanate peak. RH = relative humidity in the headspace.

Fig. 5 in Glucosinolate catabolism during postharvest drying determines the ratio of bioactive macamides to deaminated benzenoids in Lepidium meyenii (maca) root flour

Fig. 5. Time course profiles for transient and final deaminated benzenoid products in drying maca tissues. Kinetic profiles show two stages, early "fast" kinetics (solid black lines) that take place in the initial 12 h, corresponding to damage to the tissues done by shredding to reduce size for drying and a second "slow" stage (solid red lines) that shows the dehydration of the tissues in the later part of the drying process. Data points corresponding to the initial and the late phases have been subjected to nonlinear regression separately. The nonlinear regression coefficients for the metabolites were for BCHO (5): R2 = 0.9962 (early phase), R2 = 0.9880 (late phase), for BOH (6): R2 = 0.9999 (early), R2 = 0.9522 (late), BCOOH (8): R2 = 0.9999 (early), R2 = 0.9526 (late), BCOOR (8a): R2 = 0.9677. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)