Comparative study of four Yucca species by 2D-NMR and LC-MS

Abstract The present study was conducted to compare leaf extracts of Y. gloriosa, Y. treculeana, Y. filamentosa and Y. aloifolia. Plant extracts were obtained by sequential extraction with 50% and 100% methanol using the pressurized liquid extraction method. General comparative analysis of Yucca extracts was performed by 2 D-NMR. The presence of various biologically active components in the extracts was shown, among which steroid saponins predominate. The use of LC-MS and LC-HRMS methods with electrospray ionization made it possible to conduct a comparative analysis of Yucca steroidal saponins. The 33 steroidal saponins of various structures were found. The presence of three characteristic saponins in all Yucca species was shown: yuccaloeside C, yuccaloeside E and yuccaloeside B. The results obtained can be used to control the quality of plant materials and products. Graphical abstract


Introduction
Yucca is a genus of woody and shrubby plants.Yucca is a source of valuable biologically active substances: phenolic compounds, polyphenols, fatty acids, saponins and other components (Oleszek et al. 2001;Piacente et al. 2005;El Sayed et al. 2020;Jim enez et al. 2021).
Yucca saponins of various structures are of the greatest value due to their high biological activity.Their anti-inflammatory, cytotoxic and antifungal activity is most pronounced.These properties allow the use of Yucca saponins in the food, feed, pharmaceutical and cosmetic industries (Oleszek et al. 2001;Piacente et al. 2005;H ribov a et al. 2014;El Hawary et al. 2018;Jim enez et al. 2021).
To extract saponins from Yucca, maceration (El Sayed et al. 2020) or Soxhlet extraction (Oleszek et al. 2001) is mainly used.However, extraction of Yucca saponins by pressurized liquid extraction is promising.This method has been successfully used to extract saponins from many other plants (Mihaylova et al. 2014;Kam et al. 2020;Falev et al. 2021).
Thus, it is important to search for characteristic Yucca saponins that could be used to control the quality of plant raw materials and products from it.Moreover, the search of new commercial sources of Yucca saponins is an important task.
Thus, the aim of this work is comparative study of four Yucca species extracts (Y.gloriosa, Y. treculeana, Y. aloifolia, and Y. filamentosa) by 2 D-NMR and LC-MS.

General comparative study of yucca secondary metabolites by 2D NMR
To characterize the studied extracts as a whole, a non-targeted group screening was carried out by 2 D NMR spectroscopy.The complete HSQC spectra of the studied extracts are shown in Figure S1 of SM.It has been established that the main component of secondary metabolites of the genus Yucca plants are steroid glycosides mainly have a spirostanol-type as an aglycone (SM, Figure S2 and S3), with a lower content of organic acids, sucrose (SM, Figure S4), and phenolic compounds.
Despite a similar set of cross-peaks, some spectral differences can be noted.First of all, it should be noted that the cross-peaks of aglycone on the HSQC spectrum of Y. treculeana are rather weakly expressed.Moreover, there are no cross-peaks for -9 and -14, which may be characteristic of structures with a double bond, which also confirmed by cross-peaks from ethylene groups in the range of dC/dH 120-130/4.8-5.3 (SM, Figure S3).A feature of Y. filamentosa and Y. aloifolia is the presence cross-peaks characteristic of structures with a keto group at dC/dH 15.49/0.96,as well as at dC 54.64 and 212.57(SM, Figure S5).
Cross-peaks in the aromatic region of the spectrum presumably related to the structure of p-hydroxyphenyl lactic acid (SM, Figure S6).They are most clearly expressed on the spectrum of Y. filamentosa, most weakly on the spectrum of Y. aloifolia.
In general, we can say that among all the studied samples, the composition of secondary metabolites of Y. treculeana is the scarcest.A detailed analysis of saponins was carried out by liquid chromatography with mass spectrometric detection.

Comparative study of yucca saponins by liquid chromatographymass spectrometry
A comparative study of saponins of four Yucca species was carried out by HPLC-ESI-MS and HPLC-ESI-HRMS methods in positive mode.It was found that in all the studied Yucca there is a large amount (more than 70) of various saponins.At the same time, the major compounds are 33 saponins (SM, Figures S7 and S8).
The mass spectrum of a compound can provide information on the type of saponins (furostanol or spirostanol).It is known that furostanol saponins (due to the presence of a hydroxy group at C-22) lose H 2 O upon ionization (Kemertelidze et al. 2011).In the high resolution mass spectrums (SM, Table S3) of saponins 1-10, the molecular ions are present.Thus, compounds 1-10 are furostanol saponins.Compounds 11-33 are spirostanol saponins, since molecular ions are present in the high resolution mass spectrums.
Yucca saponins predominantly contain the aglycones tigogenin (SM, Figure S2), hecogenin and gitogenin.Also, Y. treculeana saponin (compound 5) contain aglycone diosgenin.This is confirmed by the literature data (Jim enez et al. 2021) and the presence of protonated aglycones in the mass spectrums of saponins (SM, Table S2).The high resolution mass spectrums of saponins show ions with m/z 415.3207S3).In addition, the high resolution mass spectrums of saponins shows the presence of low intensity characteristic product ions of aglycone According to the literature data (Jim enez et al. 2021), the sugar units of Yucca saponins contain of hexoses (glucose or galactose), xylose and rhamnose.Thus, according to the difference in the elemental composition of saponin and aglycone (or the molecular weight of saponin and aglycone), we can say about the only possible composition of the sugar unit.
Thus, the main information of found 33 major Yucca saponins is shown in Table S4 of SM.In general, Y. gloriosa, Y. treculeana, Y. filamentosa and Y. aloifolia predominantly consist the spirostanol saponins.Yucca saponins mainly contain aglycone tigogenin.However, Y. filamentosa and Y. aloifolia are characterized by the presence of saponins containing hecogenin.Saponins of all Yucca species predominantly contain large sugar units (more than five).The composition of Y. treculeana saponins is the scarcest.