Structural analysis of rebaudioside A derivatives obtained by Lactobacillus reuteri 180 glucansucrase-catalyzed trans-α-glucosylation Gerwig, Gerrit; te Poele, Evelien; Dijkhuizen, Lubbert; Kamerling, Johannis P
a b s t r a c tThe wild-type Gtf180-DN glucansucrase enzyme from Lactobacillus reuteri 180 was found to catalyze the a-glucosylation of the steviol glycoside rebaudioside A, using sucrose as glucosyl donor in a transglucosylation process. Structural analysis of the formed products by MALDI-TOF mass spectrometry, methylation analysis and NMR spectroscopy showed that rebaudioside A is specifically a-D-glucosylated at the steviol C-19 b-D-glucosyl moiety (55% conversion). The main product is a mono-(a1 / 6)-glucosylated derivative (RebA-G1). A series of minor products, up to the incorporation of eight glucose residues, comprise elongations of RebA-G1 with mainly alternating (a1 / 3)-and (a1 / 6)-linked glucopyranose residues. These studies were carried out in the context of a program directed to the improvement of the taste of steviol glycosides via enzymatic modification of their naturally occurring carbohydrate moieties.
IntroductionThe leaves of the Stevia rebaudiana BERTONI plant contain a high variety of sweet substances, called steviol glycosides [1,2], and so far more than 40 different structures have been elucidated (see review Ref. [3]). Stevioside (~5e20% w/w of dried leaves) and rebaudioside A (~2e5% w/w of dried leaves) are the most abundant components (Fig. 1), tasting about 200e300 times sweeter than sucrose (0.4% aqueous solution). Structurally, steviol glycosides have ent-13-hydroxykaur-16-en-19-oic acid as aglycone, called steviol, but are differing in carbohydrate composition at the C-13-tert-hydroxyl and C-19-carboxyl functions.Due to the growing awareness and concerns for human health related to excessive consumption of sugar (sucrose), the application of steviol glycosides as non-caloric bio-alternatives for sucrose and as substitutes for artificial (synthetic) sweeteners is strongly promoted nowadays [4e8]. Since a couple of years, steviol glycosides have been permitted for use as food additive and sweetener in the USA [9] and in Europe (E960) [10,11]. However, despite their intense sweetness and diverse beneficial pharmacological properties [12e15], the main drawback for successful commercialization of Stevia sweeteners is their slight bitterness and unpleasant (metallic) aftertaste, experienced by more than half of the human population.For natural steviol glycosides with b-D-glucopyranosyl units as constituents, it has been reported that the ratio of the number of glucose units at the C-13 site to that at the C-19 site of the steviol core has a relationship with the sweetness as well as with the quality of taste of the steviol glycosides [16,17]. To improve the Abbreviations: FID, free induction decay; FWHM, full width at half maximum; GLC-EI-MS, gas-liquid chromatography electron impact mass spectrometry; HPAEC, high-pH anion-exchange chromatography; HSQC, 1 H-detected hetero-nucl...