Two-dimensional qNMR of anthraquinones in Frangula alnus (Rhamnus frangula) using surrogate standards and delay time adaption

• Published in: Analytica chimica acta Vol. 1081; pp. 131 - 137
• Main Authors: Çiçek, Serhat S., Ugolini, Tommaso, Girreser, Ulrich
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 12.11.2019
• Subjects: Anthraquinones - analysis; Benzoquinones - standards; Duroquinone; Glycosides - analysis; Magnetic Resonance Spectroscopy - methods; Magnetic Resonance Spectroscopy - standards; Natural product; Quality control; Quantitative NMR; Reference Standards; Rhamnus - chemistry; Rutin; Rutin - standards; Surrogate analytes; Natural product; Duroquinone; Quantitative NMR; Surrogate analytes; Rutin; Quality control
• ISSN: 0003-2670; 1873-4324
• DOI: 10.1016/j.aca.2019.06.046

In the absence of adequate reference material, a problem often encountered in natural product chemistry, we investigated the use of surrogate standards in two-dimensional qNMR for the quantification of anthraquinones in the bark of alder buckthorn (Frangula alnus). Using the integrals of cross signals in the HSQC spectrum obtained from commercial standards rutin and duroquinone and adapting the delays for the 1JCH coupling, we quantified the total amount of anthraquinones and anthraquinone glucosides, as well as the content of the value-determining glucofrangulins and frangulins. Thereby, duroquinone was used as an external standard to establish the calibration curve for the methylated anthraquinone scaffold, whereas calibration curves for the glycosides were obtained using the anomeric proton signals of the rutinose disaccharide. The method was validated for accuracy, precision, specificity, linearity and limit of quantitation and shows clear advantages over the method of the European Pharmacopeia, especially in terms of specificity and meaningfulness of the results. Apart from being a useful alternative in the quality control of alder buckthorn, the presented approach demonstrates, moreover, the versatility of sophisticated 2D measurements in quantitative NMR. [Display omitted] •Quantification of anthraquinones and anthraquinone glycosides in buckthorn bark.•First NMR method for the quantification of anthraquinone glycosides.•Application of surrogate standards in two-dimensional qNMR.•Optimization of the experiment by adapting delays for the 1JCH coupling.