Article pubs.acs.org/JAFC
Synthesis and Characterization of Isomaltulose-Derived Oligosaccharides Produced by Transglucosylation Reaction of Leuconostoc mesenteroides Dextransucrase Montserrat Barea-Alvarez,† Maria Teresa Benito,§ Agustín Olano,† Maria Luisa Jimeno,§ and F. Javier Moreno*,† †
Departamento Bioactividad y Análisis de Alimentos, Instituto de Investigación en Ciencias de la Alimentación, CIAL (CSIC-UAM), CEI (UAM+CSIC), c/Nicolás Cabrera 9, 28049 Madrid, Spain § Centro Quı ́mica Orgánica “Lora-Tamayo” (CSIC), c/Juan de la Cierva 3, 28006 Madrid, Spain S Supporting Information *
ABSTRACT: This paper reports the efficient enzymatic synthesis of a homologous series of isomaltulose-derived oligosaccharides with degrees of polymerization ranging from 3 to 9 through the transglucosylation reaction using a dextransucrase from Leuconostoc mesenteroides B-512F. The total oligosaccharide yield obtained under optimal conditions was 41−42% (in weight with respect to the initial amount of isomaltulose) after 24−48 h of reaction. Nuclear magnetic resonance (NMR) structural characterization indicated that dextransucrase specifically transferred glucose moieties of sucrose to the C-6 of the nonreducing glucose residue of isomaltulose. Likewise, monitoring the progression of the content of each individual oligosaccharide indicated that oligosaccharide acceptor products of low molecular weight acted in turn as acceptors for further transglucosylation to yield oligosaccharides of a higher degree of polymerization. The produced isomaltulose-derived oligosaccharides can be considered as isomalto-oligosaccharides (IMOs) because they are linked by only α-(1→6) bonds. In addition, having isomaltulose as the core structure, these IMO-like structures could possess appealing bioactive properties that could find potential applications as functional food ingredients. KEYWORDS: isomaltulose, bioactive oligosaccharides, transglycosylation, dextransucrase, Leuconostoc mesenteroides
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INTRODUCTION Isomaltulose is a reducing disaccharide (6-O-α-D-glucopyranosyl-D-fructose) occurring naturally in honey or sugar cane juice and derived products.1 It is considered as a multifunctional carbohydrate ingredient with beneficial properties, such as noncariogenicity or reduction of postprandial glycemic responses among others, which may be particularly favorable for both diabetics and prediabetics.2,3 These properties make isomaltulose suitable for sucrose replacer in a number of foods and beverages by combining the functional benefits mentioned above with its organoleptic and technological properties such as improved texture and good stability under acidic conditions or thermal processing, as well as mild and sweet taste.3,4 Isomaltulose is approved for the use in foods and beverages in the United States, European Union, Japan, and elsewhere. In addition, regulatory authorities such as EFSA or FDA have ruled this ingredient as noncariogenic or with the capacity to induce lower postprandial glycemic and insulinemic responses.5,6 These facts explain that there is a renewed interest in isomaltulose commercialization, which is normally marketed under the trade name Palatinose. Commercial isomaltulose is produced enzymatically from food-grade sucrose by rearrangement of the glycosidic linkage from a 1,2-fructoside to a 1,6fructoside followed by crystallization.1 Isomaltulose can be considered as an isomalto-oligosaccharide (IMO).7 IMOs involve a heterogeneous group of glucosyl saccharides with the predominant α-(1→6) linkage, and these types of oligosaccharides are of interest in the fields of foods, © 2014 American Chemical Society
pharmaceuticals, and cosmetics due to their unique properties, including prebiotic character, cholesterol regulation, immunomodulatory activity, or prevention of and resistance to various diseases.8 In this context, the development of simple and convenient methods for the efficient synthesis of oligosaccharides derived from isomaltulose could offer opportunities to develop new derivatives with improved or additional functionalities, as well as to further expand the use of isomaltulose as a high-value-added ingredient. Of particular interest are the so-called acceptor reactions of microbial glucansucrases, including dextransucrases (EC 2.4.1.5) and alternansucrases (2.4.1.140). This type of enzyme polymerizes the glucosyl moiety of sucrose to form dextran, an α-(1→6)-linked glucan with optionally α-(1→2), α-(1→3), or α-(1→4) branch linkages depending on the origin of the enzyme.9 When, in addition to sucrose, other carbohydrates (mainly mono- or disaccharides) are present, D-glucosyl units are transferred from sucrose to the carbohydrate through the acceptor reaction, diverting the dextran formation to yield α-Dglucopyranosyl acceptor products.8,10−14 Whereas maltose has proven to be an excellent acceptor for a variety of bacterial glucansucrases to yield gluco-oligosaccharides,15−19 isomaltulose has been hardly used to act as acceptor in this type of Received: Revised: Accepted: Published: 9137
July 16, 2014 August 29, 2014 August 29, 2014 August 29, 2014 dx.doi.org/10.1021/jf5033735 | J. Agric. Food Chem. 2014, 62, 9137−9144
Journal of Agricultural and Food Chemistry
Article
reaction. Concretely, Demuth et al.12 and Côté et al.14,17 showed isomaltulose to be a moderate acceptor for glucansucrases, and only the corresponding trisaccharide acceptor product could be structurally characterized as isomaltotriulose following transglucosylation reaction catalyzed by an alternansucrase from L. mesenteroides NRRL B-21297.17 However, the rest of the acceptor products were neither structurally characterized nor quantified. Finally, isomaltotriulose was also detected in trace amounts in broths of dextranproducing cultures of Streptococcus bovis containing sucrose as substrate.20 In this work the efficient enzymatic synthesis of a homologous series of isomaltulose-derived oligosaccharides catalyzed by a dextransucrase from L. mesenteroides B-512F, as well as their comprehensive NMR structural characterization, is reported.
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quantify the isomaltulose-derived oligosaccharides with the same degree of polymerization (DP). Maltohexaose was also used for the quantification of oligosaccharides with DP above 6. All analyses were carried out in triplicate. Determination coefficients obtained from these calibration curves, which were linear over the range studied, were high (R2 > 0.999). Reproducibility of the method was estimated on the basis of the intraday and interday precision, calculated as the relative standard deviation (RSD) of concentrations of oligosaccharide standards obtained in n ≥ 6 independent measurements, obtaining RSD values below 10% in all cases. Purification and Structural Characterization of the Isomaltulose-Derived Oligosaccharides by Nuclear Magnetic Resonance (NMR). Isomaltulose-derived oligosaccharides from DP3 to DP7 were isolated and purified by LC-RID from sucrose/ isomaltulose mixtures after 24 h of transglucosylation reaction and using a semipreparative column Kromasil (100-NH2) column (250 × 10 mm, 5 μm particle size) (Akzo Nobel, Brewster, NY, USA). Thus, 500 μL of reaction mixtures (7.5 mg of total carbohydrates) was repeatedly eluted with acetonitrile/water (70:30, v/v) as the mobile phase at a flow rate of 5 mL min−1, and fractions corresponding to the main synthesized oligosaccharide were manually collected, pooled, evaporated in a rotatory evaporator R-200 (Büchi, Flawil, Switzerland) below 25 °C/ and freeze-dried for their subsequent characterization. Structure elucidation of the purified oligosaccharides was accomplished by NMR. NMR spectra were recorded at 298 K, using D2O as solvent, on a Varian System 500 NMR spectrometer (1H 500 MHz, 13C 125 MHz) equipped with a 5 mm HCN cold probe. Chemical shifts of 1H (δH) and 13C (δC) in parts per million were determined relative to an external standards of sodium [2,2,3,3-2H4]-3(trimethylsilyl)propanoate in D2O (δH 0.00) and 1, 4-dioxane (δC 67.40) in D2O, respectively. One-dimensional (1D) NMR experiments (1H and 13C) were performed using standard Varian pulse sequences. Two-dimensional (2D) (1H, 1H) NMR experiments [gradient correlation spectroscopy (gCOSY) and total correlation spectroscopy (TOCSY)] were carried out with the following parameters: delay time of 1 s, spectral width of 1675.6 Hz in both dimensions, 4096 complex points in t2, 4 transients for each of 128 time increments, and linear prediction to 256. The data were zero-filled to 4096 × 4096 real points. 2D (1H−13C) NMR experiments [gradient heteronuclear single-quantum coherence (gHSQC) and gradient heteronuclear multiple-bond correlation (gHMBC)] used the same 1H spectral window, a 13C spectral window of 30165 Hz, 1 s of relaxation delay, 1024 data points, and 128 time increments, with a linear prediction to 256. The data were zero-filled to 4096 × 4096 real points. Typical numbers of transients per increment were 4 and 16, respectively.
MATERIALS AND METHODS
Chemicals, Reagents, Standards, and Enzymes. Fructose, glucose, sucrose, maltotriose, maltotetraose, maltopentaose, and maltohexaose were purchased from Sigma-Aldrich (Steinheim, Germany), whereas isomaltulose was bought from Carbosynth (Compton, UK). Acetonitrile (HPLC grade) was obtained from Lab-scan (Gliwice, Poland) and deuterium oxide from Merck Millipore (Darmstadt, Germany). Ultrapure water quality (18.2 MΩ cm) with 1−5 ppb of total organic carbon (TOC) and
