Quantitative 1 H nuclear magnetic resonance (qHNMR) methods for accurate purity determination of glucosinolates isolated from Isatis indigotica roots.

INTRODUCTION: Glucosinolates (1-5) are important secondary metabolites found in Isatis indigotica roots. Due to their high hydrophilic and ionic nature, purified glucosinolates often contain salt impurities and moisture. Accurate assessment of their purities is important for glucosinolates being utilised as chemical markers. OBJECTIVE: To develop and validate quantitative proton (1 H) nuclear magnetic resonance (qHNMR) methods for purity assessments of aliphatic and indole glucosinolates (1-5). METHOD: Several NMR parameters such as pulse program, relaxation time, and delay time were optimised. Three qHNMR methods were developed using gluconapin (3), neoglucobrassicin (4), and sinigrin (5) for method validation and with maleic acid as internal standard. RESULTS: The quantification was based on the integrated area ratios of an olefinic proton (H-4 for 1-3; H-6 for 4; and H-3 for 5) of the side chain from glucosinolates relative to the olefinic proton from the internal standard using deuterated water (D2 O) as the solvent. The qHNMR methods were successfully applied for purity assessments of four aliphatic glucosinolates (1-3 and 5: progoitrin, epiprogoitrin, gluconapin, and sinigrin), and an indole glucosinolate (4: neoglucobrassicin). CONCLUSION: The purity of glucosinolates isolated from I. indigotica and commercial sinigrin was accurately assessed using the developed qHNMR method. The qHNMR provides a reliable and superior means to determine the purity of glucosinolates.