Quantum Mechanics-Based Structure Analysis of Cyclic Monoterpene Glycosides from Rhodiola rosea

Abstract

NMR- and MS-guided metabolomic mining for new phytoconstituents from a widely used dietary supplement, Rhodiola rosea, yielded two new (+)-myrtenol glycosides, 1 and 2, and two new cuminol glycosides, 3 and 4, along with three known analogues, 5-7. The structures of the new compounds were determined by extensive spectroscopic data analysis. Quantum mechanics-driven ¹H iterative full spin analysis (QM-HiFSA) decoded the spatial arrangement of the methyl groups in 1 and 2, as well as other features not recognizable by conventional methods, including higher order spin-coupling effects. Expanding applied HiFSA methodology to monoterpene glycosides advances the toolbox for stereochemical assignments, facilitates their structural dereplication, and provides a more definitive reference point for future phytochemical and biological studies of R. rosea as a resilience botanical. Application of a new NMR data analysis software package, CT, for QM-based iteration of NMR spectra is also discussed.

Figure 1.

QM-HiFSA spin simulation analysis of the resonances of H-2’–H-5’ of the glucose portions of 1–4 (A–D), as well as the 1D-TOCSY spectra of glucose in 2 (E) and 4 (F). The calculated spectra and experimental spectra (400 MHz, 298 K) are shown in red and blue, respectively (* denotes impurity signals).

Figure 2.

The importance of relative chemical shifts (Δδ) as indirect, but significant, structural evidence demonstrated for H-10a and H-10b in 1, 2, 5, and 6.

Figure 3.

The ¹H NMR fingerprint of compound 1 generated using the PERCH iteration tool (final RMS = 0.030). Comparison of the observed (blue, obtained in methanol-d₄ at 400 MHz, 298 K) and calculated (red) ¹H spectra, including residuals in green (* denotes an impurity signal).

Figure 4.

The Quantum Interaction and Linkage Table (QuILT) summarizes the full J correlation map the aglycone portion of 1 produced by QM-HiFSA based on the 400 MHz 1D ¹H NMR data. The number of bonds separating two coupled nuclei are color-coded: violet = ²J, blue = ³J, yellow = ⁴J, green = ⁵J, and pink = ⁶J. Multiplicities in parentheses are less than ~1 Hz. Couplings less than an absolute value of 0.10 Hz are given as “⌀” rather than being reported as blank cells, which would wrongly imply them being unknown or undetermined.

Figure 5.

The occurrence of well-resolved and near-identical (ΔJ = 30 mHz) ⁵J_HH couplings in 1 and 2 are evidence for the highly congruent zig-zag arrangement of their connecting bonds and, thus, their identical relative stereochemistry in both compounds. This J-coupling relationship was also verified through H,H homodecoupling experiments (see main text). Notably, the different sugar moieties apparently do not affect the geometry of the multicyclic monoterpene moiety and, thus, its zig-zag long-range coupling pathway.

Figure 6.

Line-shape comparison for H-4a in 1 between homodecoupled (top/blue) and non-homodecoupled (red) ¹H NMR spectra. A Lorentzian–Gaussian apodization function of LB = −0.3 Hz and GF = 0.05 was applied to both. The top/blue signals resulted from homodecoupling irradiating Me-9 and exhibit a sharper line-shape in the H-4a resonance, with sub-peaks more observable when compared with that of the corresponding signal in the bottom/red spectrum.

Figure 7.

Differences in the chemical shifts (in ppm, A) and coupling constants (in Hz, B) determined by HiFSA using the CT vs the PERCH software tools. As shown in 7A, the chemical shifts have an excellent agreement between CT and PERCH, for all the resonances 0≤Δδ≤0.00017 ppm. 7B shows that the coupling constants also exhibit a good fit with the largest difference for ΔJ_H-4b,H-10b no more than 0.45 Hz.

Figure 8.

Diagnostic long-range benzylic couplings (^,J_HH) in 3 and 4