Quercetin-grafted chitosan prepared by free radical grafting: characterization and evaluation of antioxidant and antibacterial properties.

Chitosan (CS) is considered a versatile biopolymer with promising applications. However, it is not a good chain-breaking antioxidant due to the lack of H-atom donors. In this work, CS was combined with quercetin (Q), a natural antioxidant, via a free radical-mediated procedure to strengthen the antioxidant capacity. The successful formation of Q-grafted CS (Q-CS) was confirmed by ultraviolet-visible absorbance and Fourier transform infrared spectroscopy. After combination, the obtained Q-CS had a phenolic content of 13.9 mg QE/g Q-CS and showed a lower crystallinity and thermal stability than the native CS. The 2,2-diphenyl-1-picrylhydrazyl, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), superoxide, and hydroxyl radical scavenging activities of Q-CS were higher than those of CS, illustrating that grafting with Q is an available way to improve the antioxidant capacity of CS. In addition, Q-CS showed higher minimal inhibitory concentrations against tested bacteria than CS, suggesting that combining with Q has a negative effect on the antibacterial activity of CS. Our results indicate that Q-CS may have great potential for applications in the fields of food and healthcare.

Preparation, characterization, and functional evaluation of proanthocyanidin-chitosan conjugate.

In this study, chitosan (CS) was conjugated with proanthocyanidin (PA) by a free radical grafting reaction. The successful synthesis of PA-CS conjugate was confirmed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance. The optimal molar ratio of PA to CS repeat unit for the preparation of PA-CS was 0.13:1, which led to a high PA content of 381.76 mg PAE/g in PA-CS. The antioxidant assays demonstrated that PA-CS had much stronger radical scavenging activity and reducing power than the native CS. Especially, the half-inhibition concentrations of PA-CS against DPPH and ABTS radicals were only 6.2 µg/mL and 5.9 µg/mL, respectively. In addition, PA-CS showed an alteration in antibacterial activity compared with CS, and the alteration varied with bacterial strain.

Optimized preparation, characterization, and antioxidant activity of chitooligosaccharide-glycine Maillard reaction products.

In this study, chitooligosaccharide (COS) and glycine (Gly) were selected to prepare Maillard reaction products, which were designated COS-Gly-MRPs. Changes in the FTIR and fluorescence spectra confirmed the formation of the COS-Gly-MRPs. Using ferric reducing antioxidant power (FRAP) as a response, the optimal reaction conditions, i.e., a time of 107 min, temperature of 121 °C, pH of 6.0, and nCOS:nGly = 2.5:1, were obtained by one-variable-at-a-time method and by response surface methodology. The resulting COS-Gly-MRPs exhibited much stronger antioxidant activity than their substrates. The FRAP of COS-Gly-MRPs was 32.14 mmol Fe2+/L, and the radical scavenging activity of COS-Gly-MRPs reached 78.6, 89.0, 92.3, and 86.0% for ABTS, superoxide, DPPH, and hydroxyl radicals, respectively. After 7 days of storage, COS-Gly-MRPs-treated fruit juices showed higher antioxidant capacity than those treated with a mixture of COS and Gly.

The effective and selective separation of (-)-epigallocatechin gallate by molecularly imprinted chitosan beads.

The (-)-epigallocatechin gallate (EGCG) imprinted chitosan beads (EICBs) were fabricated for the effective and selective separation of EGCG. The EGCG molecules interacted with the amino groups of chitosan in the imprinting process, resulting in a highly porous structure of EICBs and more adsorption sites. Consequently, EICBs exhibited better adsorption performance than non-imprinted chitosan beads. The maximum adsorption capacity of EGCG onto EICBs reached 135.50 mg/g at 313 K. The imprinting factor of EICBs was 4.22, indicating that EICBs possess good recognition ability and selectivity for EGCG. After five cycles of reuse, only a slight decrease (7.77%) in the adsorption capacity was observed, demonstrating the satisfactory reusability of EICBs. Furthermore, the adsorption of EGCG onto EICBs is deduced to be the monolayer adsorption on an energetically homogeneous surface; the hydrogen bonding between EGCG and EICBs is the main driving force for the adsorption. Our studies suggest that EICBs have a great potential for the effective and selective separation of EGCG.

A serpin released by an entomopathogen impairs clot formation in insect defense system.

Steinernema carpocapsae is an entomopathogenic nematode widely used for the control of insect pests due to its virulence, which is mainly attributed to the ability the parasitic stage has to overcome insect defences. To identify the mechanisms underlying such a characteristic, we studied a novel serpin-like inhibitor (sc-srp-6) that was detected in a transcriptome analysis. Recombinant Sc-SRP-6 produced in Escherichia coli had a native fold of serpins belonging to the α-1-peptidase family and exhibited inhibitory activity against trypsin and α-chymotrypsin with Ki of 0.42 × 10(-7) M and 1.22 × 10(-7) M, respectively. Functional analysis revealed that Sc-SRP-6 inhibits insect digestive enzymes, thus preventing the hydrolysis of ingested particles. Moreover, Sc-SRP-6 impaired the formation of hard clots at the injury site, a major insect defence mechanism against invasive pathogens. Sc-SRP-6 does not prevent the formation of clot fibres and the activation of prophenoloxidases but impairs the incorporation of the melanin into the clot. Binding assays showed a complex formation between Sc-SRP-6 and three proteins in the hemolymph of lepidopteran required for clotting, apolipophorin, hexamerin and trypsin-like, although the catalytic inhibition occurred exclusively in trypsin-like. This data allowed the conclusion that Sc-SRP-6 promotes nematode virulence by inhibiting insect gut juices and by impairing immune clot reaction.

Cloning, characterisation and heterologous expression of an astacin metalloprotease, Sc-AST, from the entomoparasitic nematode Steinernema carpocapsae.

Steinernema carpocapsae is a parasitic nematode that is high virulent to insects. The parasitic juvenile reaches the insect haemocoelium by passing through mid-gut barriers and develops there. During invasion, the nematode was predicted to express a large set of proteases, including metalloproteases, one of which was sequenced and expressed in this work. A 1583-nucleotide cDNA encoding a putative metalloprotease containing a 28-aa signal peptide, a 79-aa propeptide and a 311-aa mature protease with a predicted molecular mass of 35.2 kDa and a theoretical pI of 5.9 was cloned from the parasitic stage of the nematode. Sequence analyses predicted signature sequences of the astacin metalloprotease family, an astacin domain, a zinc-binding motif and a methionine turn motif; therefore, this protein was identified as an astacin and designated as Sc-AST. The astacin domain of Sc-AST has an amino acid sequence homology of 46% to prototypical astacin from Astacus astacus and 82% to Caenorhabditis elegans NAS-8. Like NAS-8 of C. elegans, Sc-AST has a C-terminal ShK toxin domain. Recombinant Sc-AST was produced in an Escherichia coli system and was purified by affinity chromatography. Maldi-MS/MS analysis of purified recombinant protein matched the Sc-AST sequence with a significance score of 499. Sc-AST was produced in the correct folding conformation, showed activities against gelatin and azocasein substrates and was inhibited by divalent metal-chelating agents. Sc-AST presented an optimum pH of 7.5 and temperature of 37°C and K(m), V(max) and k(cat) values of 1.86 mM, 0.281 µM/min and 27.9 s(-1), respectively. Expression analyses indicated that Sc-AST is up-regulated in the parasitic stage and is strongly induced in vitro by insect tissues, thus suggesting that it plays a role in the parasitic process.

Variable volume fed-batch fermentation for nisin production by Lactococcus lactis subsp. lactis W28.

A feeding technology that was suitable for improving the nisin production by Lactococcus lactis subsp. lactis W28 was established. The effects of initial sucrose concentration (ISC) in the fermentation broth, feeding time, and feeding rate on the fermentation were studied. It was observed that a fed-batch culture (ISC = 10 g l(-1)) with 100 ml sucrose solution (190 g l(-1)) being evenly fed (9-10 ml h(-1)) into the fermenter after 3-h fermentation gave the best performance in terms of biomass and nisin yield. Under these conditions, the total biomass and the total nisin yield were approximately 23% and 51% higher than those in batch fermentation, respectively. When the sucrose concentration was controlled at 5-10 g l(-1) in variable volume intermittent fed-batch fermentation (VVIF) with ISC = 10 g l(-1), the total biomass and the total nisin yield were 29% and 60% above those in batch fermentation, respectively. The VVIF proved to be effective to eliminate the substrate inhibition by maintaining sucrose at appropriate levels. It is also easy to be scaled up, since various parameters involved in industrial production were taken into account.