NMR Spectral Assignments and Hydrolysis Studies of Dulcosides A and B

The complete H and C NMR assignments for the two diterpene glycosides 13-[(2-O-α-L-rhamnopyranosyl --D-glucopyranosyl)oxy]-ent-kaur-16-en-19-oic acid -D-glucopyranosyl ester (Dulcoside A) and 13-[(2-O-α-L-rhamnopyranosyl-3-O--D-glucopyranosyl--D-glucopyranosyl)oxy]-ent-kaur-16-en-19-oic acid β-D-glucopyranosyl ester (Dulcoside B) isolated from Stevia rebaudiana were achieved by the extensive NMR (H and C, COSY, HSQC, HMBC) and mass spectral data. Further, hydrolysis of the two compounds Dulcosides A and B was performed using acid and enzymatic studies to identify their aglycone and sugar residues.


Introduction
Stevia rebaudiana (Bertoni) is a perennial shrub belonging to the family of Asteraceae (Compositae) which is native to Brazil and Paraguay; but now is grown commercially in a number of countries, particularly in Japan, Taiwan, Korea, Thailand and Indonesia (Mosettig et al., 1955;1963).Extracts of the leaves of S. rebaudiana have been used for decades to sweeten food and beverages in Japan, South America and China.The major constituents in the leaves of S. rebaudiana are the potently sweet diterpenoid glycosides namely stevioside, and rebaudioside A, which are glycosides of the diterpene steviol, ent-13-hydroxykaur-16-en-19-oic acid (Brandle et al., 1998); also known as Stevia sweeteners.Stevioside tastes about 150-250 times sweeter than sucrose and rebaudioside A tastes about 200-300 times sweeter than sucrose; both are non-caloric.
In our continuing research to discover natural sweeteners, we have isolated several novel diterpene glycosides (Chaturvedula et al., 2011a-g) from the commercial extracts of S. rebaudiana obtained from various suppliers all over the World.Apart from isolating novel compounds and utilizing them as possible natural sweeteners or sweetness enhancers, we are also engaged in understanding the stability of the steviol glycosides in various systems of interest and characterization of degradation products using various spectroscopic analysis (Chaturvedula et al., 2011h-i), and their synthesis using naturally occurring starting materials (Chaturvedula et al., 2011j).In this article, we are describing the complete 1 H and 13 C NMR spectral assignments for the two diterpene glycosides 13-[(2-O-α-L-rhamnopyranosyl--D-glucopyranosyl)oxy]-ent-kaur-16-en-19-oic acid -D-glucopyranosyl ester (Dulcoside A, 1) and 13-[(2-O-α-L-rhamnopyranosyl-3-O--D-glucopyranosyl--D-glucopyranosyl)oxy] -ent-kaur-16-en-19-oic acid β-D-glucopyranosyl ester (Dulcoside B, 2), also known as rebaudioside C that were identified from S. rebaudiana (Figure 1).The complete NMR assignments were achieved on the basis of 1D ( 1 H and 13 C) and 2D (COSY, HSQC and HMBC) NMR as well as high resolution mass spectroscopic (MS) data.Acid and enzymatic hydrolysis studies were performed on compounds 1 and 2 to identify aglycone and sugar residues in their structures.

General Instrumentation Procedures
Melting points were measured using a SRS Optimelt MPA 100 instrument and are uncorrected.HPLC analysis was performed using an Agilent (Wilmington, DE) 1200 system, including a quaternary pump, a temperature controlled column compartment with additional 6-port switching valve, an autosampler and a UV absorbance detector.The reversed phase (RP) HPLC was employed using a Phenomenex (Torrance, CA) Synergi-Hydro column (250 mm x 4.6 mm, 4 µm) with a Phenomenex Security guard C 18 cartridge and a tertiary solvent mobile phase (A: 0.040% NH 4 OAc/AcOH buffer, B: MeCN and C: 0.040% AcOH).The column was maintained at a temperature of 55 ℃ and the flow rate was 1.0 ml/minute.Charged Aerosol Detector (CAD) was used for the purification of the steviol glycosides 1 and 2 with a total run time of 43 min (Table 1).
Analytical HPLC was carried out with a Waters 600E multisolvent delivery system using a Phenomenex Luna C 18 (150 x 4.6 mm, 5 m) column.IR spectral data was acquired using a Perkin Elmer 400 Fourier Transform Infrared (FT-IR) Spectrometer with Universal attenuated total reflectance (UATR) polarization accessory and NMR spectra were acquired on Varian Unity Plus 600 MHz instruments using standard pulse sequences.The NMR spectra were acquired in CD 3 OD and D 2 O (9:1) mixture; chemical shifts are given in  (ppm), and coupling constants are reported in Hz.HRMS and MS/MS data were generated with a Waters Premier Quadrupole Time-of-Flight (Q-TOF) mass spectrometer equipped with an electrospray ionization source operated in the positive-ion mode and Thermo Fisher Discovery OrbiTrap in the electrospray positive mode.Samples were diluted with water: acetonitrile (1:1) containing 0.1% formic acid and introduced via infusion using the onboard syringe pump.

Plant Material
SG95, the commercial aqueous extract consisting of a mixture of diterpenoid glycosides of the leaves of S. rebaudiana was obtained from the Pure Circle (Kuala Lumpur, Malaysia).The authenticity of the crude extract was confirmed by performing its retention time (t R ) comparison with the internal standard compounds of known steviol glycosides namely rebaudioside A-D, and rubusoside isolated from S. rebaudiana using the preparative HPLC method as reported earlier (Clos et al., 2008).A voucher specimen is deposited at The Coca-Cola Company, No. VSPC-3166-002.

Isolation and Characterization
Compounds 1 and 2 were purified by using an Agilent HPLC 1200 system equipped with a Phenomenex Synergi-Hydro column (250 mm x 4.6 mm, 4 µm) with a Phenomenex Security guard C 18 cartridge.Using the above mentioned HPLC method shown in Table 1, collected the peaks eluting at t R 12.37 and 21.88 min; and dried the corresponding solutions under nitrogen yielded 1 and 2 respectively.

General procedure for acid hydrolysis of 1 and 2
To a solution of each compound of 1 and 2 (250 μg) in MeOH (1 ml) was added 1 ml of 5% H 2 SO 4 and the mixture was refluxed for 8 hours.The reaction mixture was then neutralized with saturated sodium carbonate and extracted with ethyl acetate (EtOAc) (2 x 5 ml) to give an aqueous fraction containing sugars and an EtOAc fraction containing the aglycone part.The aqueous phase was concentrated and compared with standard sugars using the TLC systems EtOAc/n-butanol/water (2:7:1) and CH 2 Cl 2 /MeOH/water (10:6:1) (Bedir et al., 2001;Chaturvedula et al., 2003;Huan et al., 1998); the two sugars were identified as rhamnose and glucose for both the hydrolyzed products of 1 and 2.

Enzymatic hydrolysis of 1 and 2
Each compound 1 and 2 (250 g) was dissolved in 2.5 ml of 0.1 M sodium acetate buffer, pH 4.5 and crude pectinase from Aspergillus niger (50 uL, Sigma-Aldrich, P2736) was added.The mixture was stirred at 50 ℃ for 48 hr.The product precipitated out during the reaction and was filtered and then crystallized.The resulting product obtained from the hydrolysis of 1 and 2 was identified as steviol by comparison of its co-TLC with standard compound and 1 H NMR spectral data (Ohtani et al., 1992).
The identification of sugars present in 1 and their configurations were achieved by preparing their thiocarbamoyl-thiazolidine carboxylate derivatives with L-cysteine methyl ester and O-tolyl isothiocyanate, and in comparison of their retention times with the standard sugars as described in the literature; suggesting the sugar moieties present as β-D-glucopyranosyl and α-L-rhamnopyranosyl units (Tanaka et al., 2007).The 1 H-and 13 C-NMR values for all the protons and carbons were assigned on the basis of COSY, HSQC and HMBC correlations (Table 2).A close comparison of the 1 H and 13 C NMR values of 1 with those of stevioside (3) suggested the presence of three sugar units attached as a 2-O-α-L-rhamnopyranosyl substituted -D-glucopyranosyl substituent at C-13 hydroxyl and an additional β-D-glucosyl moiety at C-19 acid as an ester on steviol (Avent et al., 1990;Ohta et al., 2010), which was supported from the key COSY and HMBC correlations shown as in Figure 2. The large coupling constants observed for the two anomeric protons of the glucose moieties at δ 4.65 (d, J = 8.2 Hz), and 5.34 (d, J = 8.2 Hz), suggested their β-orientation as in steviol glycosides isolated from S. rebaudiana (Chaturvedula et al., 2011a-g).The third anomeric proton which was appeared at δ 5.30 had a coupling constant of 1.2 Hz indicating the -configuration of the L-rhamnosyl unit (Ohta et al., 2010).Thus, the structure of 1 was deduced as 13-[(2-O-α-L-rhamnopyranosyl--D-glucopyranosyl)oxy]-ent-kaur-16-en-19-oic acid -D-glucopyranosyl ester (Dulcoside A).
The molecular formula of compound 2 was deduced as C 44 H 70 O 22 from its HRMS data which showed [M+NH 4 ] + and [M+Na] + adduct ions at m/z 968.4705 and 973.4255 respectively.The 1 H NMR spectrum of 2 also showed the presence of two methyl singlets at δ 0.98, and 1.21, nine methylene and two methine protons, similar to 1 (Table 2).The 1 H NMR of 2 also showed the presence of four anomeric protons as doublets at δ 4.52, 4.62, 5.32, and 5.40, suggesting the presence four sugar residues in its structure; which was further supported by the fragment ions observed in the positive ESI mode MS/MS spectrum of 2 that provided fragment ions corresponding to the sequential loss of three hexose and a deoxyhexose unit respectively at m/z 789, 627, 465, and 319.Acid hydrolysis of 2 afforded sugar units which were identified as glucose and rhamnose and their configurations were assigned as β and α respectively by preparing their corresponding thiocarbamoyl-thiazolidine carboxylate derivatives as described in 1. Enzymatic hydrolysis of 2 furnished steviol similar to 1 suggesting their identical basic skeleton of aglycone in both compounds.
From the hydrolysis experiments and NMR spectral data for 2, it was suggested that this compound is also having steviol backbone with three β-D-glucosyl substituents and an -L-rhamnosyl moiety in its structure.The 1 H and 13 C NMR values for all the protons and carbons were assigned on the basis of COSY, HSQC, and HMBC correlations and were shown in Table 2.A close comparison of the 1 H and 13 C NMR values of 2 with 1 as well as stevioside (3) and rebaudioside-A (4) suggested the presence of additional β-D-glucosyl at C-3′′ position of sugar II (Ohta et al., 2010;Wayne et al., 2009).This was supported by the HMBC correlations as shown in Figure 3