Xiaoqi Xu

Crystalline structure and thermal property characterization of chitin from Antarctic krill (Euphausia superba)

Antarctic krill (Euphausia superba) has been widely studied and extensively recognized as a target for commercial fishing. In this study, Antarctic krill chitin was extracted from defatted Antarctic krill shell, and its crystalline structure and thermal properties were characterized by employing Fourier transform infrared spectroscopy, X-ray diffractometry, scanning electron microscopy, thermogravimetry, and differential scanning calorimetry. Results showed that Antarctic krill chitin corresponded to the α-polymorph, and was composed of small, stable, and uniform microcrystals. The degree of N-deacetylation was 11.28 ± 0.86%. The d-spacings of Antarctic krill chitin were 9.78 Å and 4.63 Å at (020) and (110) planes. The crystalline sizes were 6.07 nm and 5.16 nm at (020) and (110) planes, respectively. The activation energy of the polysaccharide chain decomposition was 123.35 kJ/mol and the glass transition (T(g)) of Antarctic krill chitin was 164.96 °C.

Structural study of fucoidan from sea cucumber Acaudina molpadioides: A fucoidan containing novel tetrafucose repeating unit

Structure of fucoidan from sea cucumber Acaudina molpadioides (Am-FUC) was investigated in this study. Low molecular weight polysaccharide of Am-FUC (Am-LMW) was prepared by enzymatic degradation and assayed by nuclear magnetic resonance. According to the structure of Am-LMW, and referring to previously reported information of Am-FUC oligosaccharides, the structure of Am-FUC was elucidated as [→3-α-l-Fucp-1→3-α-l-Fucp2,4(OSO3(-))-1→3-α-l-Fucp-1→3-α-l-Fucp2(OSO3(-))-1→]n. Compared to other investigated fucoidans, which also consist of tetrafucose repeating units with 1→3-linkages, Am-FUC contains a novel repeating unit distinctive in sulphation pattern.

Enzymatic preparation and structural determination of oligosaccharides derived from sea cucumber (Acaudina molpadioides) fucoidan

Sea cucumber fucoidan is a major bioactive component of sea cucumber. Sea cucumber is widely consumed in East Asian countries as healthy food. Employing the degrading enzyme from the marine bacterium strain Flavobacteriaceae CZ1127, sea cucumber (Acaudina molpadioides) fucoidan oligosaccharides were prepared by enzymatic hydrolysis. The oligosaccharide profile of the hydrolysate was determined by liquid chromatography coupled with mass spectrometry (LC-MS). With the assistance of LC-MS, four major oligosaccharides in the hydrolysate were purified. By using tandem mass spectrometry and nuclear magnetic resonance, delicate structures of the oligosaccharides were verified as α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp, α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp-1→3-α-l-Fucp, α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp-1→3-α-l-Fucp-1→3-α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp and α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp-1→3-α-l-Fucp-1→3-α-l-Fucp-1→3-α-l-Fucp(2,4OSO3(2-))-1→3-α-l-Fucp-1→3-α-l-Fucp.

Structure elucidation of fucoidan composed of a novel tetrafucose repeating unit from sea cucumber Thelenota ananas

Thelenota ananas is one of the most popular edible sea cucumber species consumed in China and some Southeast Asian countries. In this study, the delicate structure of fucoidan from T. ananas (Ta-FUC) was clarified. Oligosaccharides and low molecular weight polysaccharides of Ta-FUC were prepared by enzymatic degradation. Their structures, which retained primary structural features of Ta-FUC, were investigated using tandem mass spectrometry and nuclear magnetic resonance. As a result, Ta-FUC was found to be composed of a novel tetrafucose repeating unit [→3-α-L-Fucp-1→3-α-L-Fucp-1→3-α-L-Fucp2,4(OSO3(-))-1→3-α-L-Fucp2(OSO3(-))-1→]. Meanwhile, it was proven to possess a significant inhibitory effect on superoxide radicals with an IC50 of 17.46±0.14μg/ml. This effect was related to its sulphation pattern.

Structure and rheological characteristics of fucoidan from sea cucumber Apostichopus japonicus.

Sea cucumber is a traditional health food consumed in East Asia. In this study, fucoidan from sea cucumber Apostichopus japonicus (Aj-FUC) was isolated, and its structure and rheological characteristics were elucidated for the first time. Aj-FUC was a branched polysaccharide mainly composed of a novel repeating unit [α-L-Fucp2(OSO3(-))-1 → 3,(α-L-Fucp-1 → 4-α-L-Fucp-1 →)4-α-L-Fucp2(OSO3(-))-1 → 3-α-L-Fucp2(OSO3(-))], clarified by using a combination of infrared spectroscopy, methylation analysis, enzymatic degradation and nuclear magnetic resonance. In steady shear measurement, Aj-FUC manifested a non-Newtonian shear-thinning behaviour at low shear rate (1-100 S(-1)) while exhibiting a non-Newtonian shear-thickening behaviour at high shear rate (100-1000 S(-1)); salts had limited impact on its flow curve. Comparative study on viscosity and rheological behaviour of Aj-FUC and a linear fucoidan extracted from sea cucumber Acaudina molpadioides suggested that the presence of branch structure might significantly influence the rheological characteristics of fucoidan.

Primary structure and chain conformation of fucoidan extracted from sea cucumber Holothuria tubulosa.

Knowledge of the structure of polysaccharides is essential for understanding and controlling their functional properties. In this study, fucoidan was extracted from Holothuria tubulosa, a species of sea cucumber that has recently become commercially important. The primary structure of the H. tubulosa fucoidan was clarified using a combination of enzymatic degradation, methylation analysis, and NMR, and its chain conformation was further elucidated by utilizing high performance size exclusion chromatography combined with multiple angle laser light scattering and viscometry. The fucoidan was composed of a tetrafucose repeating unit [ → 3-α-L-Fucp2(OSO3(-))-1 → 3-α-L-Fucp2,4(OSO3(-))-1 → 3-α-L-Fucp-1 → 3-α-L-Fucp2(OSO3(-))-1 → ]. Its sulfate content was determined to be 31.2 ± 1.6% and the weight-average molecular mass was 1567.6 ± 34.1 kDa. The molecule adapted a random coil conformation in 0.15M NaCl solution (pH 7.4) at 25 °C, with a root-mean-square radius of 63.9 ± 1.8 nm and a hydrodynamic radius of 44.5 ± 4.5 nm. This is the first report on the chain conformation of sea cucumber fucoidan.

Conformational and physicochemical properties of fucosylated chondroitin sulfate from sea cucumber Apostichopus japonicus

This study aimed at investigating the chain conformation and physicochemical properties of fucosylated chondroitin sulfate extracted from sea cucumber Apostichopus japonicas (Aj-fCS). By using HPSEC-MALLS-Visc-RI, Mw, z(1/2), Rh and [η] for Aj-fCS were determined as 58.0±4.4kDa, 21.8±1.3nm, 12.5±1.3nm and 27.8±0.5mL/g respectively. Conformation parameter αs derived from the relationship of Mw-z(1/2) (0.39) and structure-sensitive parameter ρ (1.74) consistently indicated that Aj-fCS adopted a random coil conformation in solution, which was also supported by atomic force microscopy. Stiffness parameters of Aj-fCS chains including q (2.72nm), d (1.03nm) and C∞ (5.28) were furthermore deduced from the worm-like cylinder model. Aj-fCS demonstrated a shear-thinning rheological behavior, relatively low apparent viscosity, negative charge in wide pH and ionic strength ranges, and favorable thermostability. These results have important implications for designing and fabricating functional foods or drugs based on Aj-fCS.

Chain conformational and physicochemical properties of fucoidans from sea cucumber

Although fucoidans from sea cucumber (SC-FUCs) have been proven as potential bioactive polysaccharides and functional food ingridents, their chain conformation and physicochemical properties were still poorly understood. This study investigated the chain conformation of fucoidans from sea cucumber Acaudina molpadioides (Am-FUC), Isostichopus badionotus (Ib-FUC) and Apostichopus japonicus (Aj-FUC), of which primary structure has been recently clarified. Chain conformation parameters demonstrated that studied SC-FUCs adopted random coil conformation in 150mM NaCl solution (pH 7.4). Based on the worm-like cylinder model and atomic force microscopy, the chain stiffness of SC-FUCs was further evaluated as Am-FUC≈Ib-FUC>Aj-FUC. It was suggested that the existence of branch structure increased the chain flexibility, while sulfated pattern exerted limited influence. SC-FUCs demonstrated shear-thinning rheological behavior and negative charge. Am-FUC possessed a higher thermostability than Ib-FUC and Aj-FUC. These results have important implications for understanding the molecular characteristics of SC-FUCs, which could facilitate their further application.

Phase behaviors involved in surimi gel system: Effects of phase separation on gelation of myofibrillar protein and kappa-carrageenan

Phase behaviors of mixtures of myofibrillar protein and κ-carrageenan at different mixing ratios and temperatures were examined by digital images and confocal scanning laser microscopy, showing that that the extent of phase separation was enhanced as the ratio of polysaccharides and temperature increased. The zeta potential of the mixtures became less negative as the protein ratio increased, and the complex became saturated at or above the protein/κ-carrageenan ratio of R4 (3.2%:0.8%). Gelation process performed by dynamic rheological analysis demonstrated that the presence of carrageenan decreased the gelation temperature but increased the storage modulus. Analysis of the microstructures of the mixed gels showed that the networks were significantly influenced by the concentrations of κ-carrageenan. The present work could be applied to evaluate the mechanism of competition between phase separation and gelation in mixtures of proteins and polysaccharides.

Fucosylated chondroitin sulfate is covalently associated with collagen fibrils in sea cucumber Apostichopus japonicus body wall

Fucosylated chondroitin sulfate (fCS) is the major carbohydrate constituent of sea cucumber. However, the distribution of fCS in the sea cucumber body wall has not been fully described. We addressed this in the present study employing Apostichopus japonicus as the material, a sea cucumber species with significant commercial importance. It was found that fCS was covalently attached to collagen fibrils via O-glycosidic linkages. Transmission electron microscopy analysis revealed that fCS precipitate was present in gap regions of collagen fibrils as roughly globular or ellipsoidal dots. The fCS dots arranged circumferentially around the fibrils with an axial repeat period that matched the periodicity of the fibrils. Physicochemical analysis indicated that the presence of fCS significantly increased the negative charge of the fibrils. These findings provide novel insight into fCS distribution in the sea cucumber body wall and its supramolecular organization with other macromolecules.