Flavonoid Mixture Analysis by Matrix-Assisted Diffusion-Ordered Spectroscopy

Abstract

The structural similarity of flavonoids, often present in natural product mixtures, makes their analysis by NMR methods less than straightforward. This similarity is a dual problem for one of the most powerful NMR methods for mixture analysis, diffusion-ordered spectroscopy (DOSY), which relies both on well-resolved peaks and on differences in hydrodynamic radii for separating the signals from different components in a mixture. To overcome these limitations, we use a matrix-assisted DOSY approach that exploits differential chemical interactions with a slow diffusion matrix (here micellar sodium dodecyl sulfate) to resolve flavonoid mixtures in mixed solvents.

Flavonoid glycosides from Persea caerulea. Unraveling their interactions with SDS‐micelles through matrix‐assisted DOSY, PGSE, mass spectrometry, and NOESY

“…Enhanced resolution in DOSY spectra was achieved for the first time by Morris and coworkers by addition of micelles, in the case of hydrophobic interactions . As mentioned previously, Morris and coworkers reported for the first time an efficient methodology for NMR analysis of flavonoids using SDS to separate the NMR signals of compounds of similar size such as flavone, fisetin, (+)‐catechin and quercetin . This method has been also extended to high‐resolution magic angle spinning NMR where the use of silica gel has been used to achieve well‐separated and high‐resolution spectra for the single components …”

“…It is well known that surfactants in water spontaneously self‐assemble to form micelles where the factors influencing their formation in terms of size and shape have been the subject of a plethora of studies in the last decades . The presence of SDS as well defined micelles in binary solvent mixtures such as water–DMSO has been confirmed previously by investigating the dependence of the diffusion coefficients as a function of SDS concentration, showing that the critical micelle concentration (CMC) changed from 7 (pure D 2 O) to 11 mM (20% v/v DMSO‐ d 6 ) or 25 mM (50% DMSO‐ d 6 ), consistent with other literature . Interestingly, Hoffmann et al have studied the interactions of toluene and cyclohexane with SDS as a new tool for the structural elucidation of microemulsions …”

Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible, tuneable matrix

“…This is a particular problem with one of the most successful classes of MAD cosolute, micellar surfactants. 4,9,11,12 These typically have long alkyl chains and hence have wide footprints in the aliphatic region of the 1 H spectrum. Peruorinated microemulsions have been successfully used previously for MAD/NMR chromatography.…”

Matrix‐assisted diffusion‐ordered spectroscopy: choosing a matrix

“…The method is sometimes referred to as chromatographic DOSY, particularly when using a chromatographic stationary phase as the matrix . A change in matrix can be something as simple as a change of solvent, or a change to a mixed solvent, altering the effective hydrodynamic radii of the species of interest . More commonly, the matrix is changed by adding a large species as a co‐solute to modulate the diffusion coefficients of the species that interact with it.…”

“…Matrix‐assisted DOSY holds great potential for the NMR analysis of difficult mixtures such as natural product extracts, chemical reaction mixtures, and foods and beverages, and for metabolite identification. Its efficacy has been demonstrated for a number of classes of problem species, including positional isomers, epimers, enantiomers, and natural product mixtures . However, our understanding of how to choose a matrix for a particular type of sample is just in its infancy, and largely empirical.…”