Preparation of superabsorbent cellulosic hydrogels

In this work, milled softwood (SW) bleached kraft fibers were crosslinked by esterification with poly(vinyl methyl ether-co-maleic acid) (PVMEMA) and polyethylene glycol (PEG). The effects of fiber length, crosslinking reaction time, and dosage of PVMEMA on water absorption and retention value (WAARV) for the crosslinked fibers were determined. The results show that as the softwood fiber length is mechanically decreased from 2.41 to 0.50 mm and employing a weight ratio of fiber to polymers equivalent to 1.00:1.28 the WAARV increased from 86.50 to 189.20 g/g. Analysis of the crosslinked fibers by SEM and light microscope indicated that the polymers and fibers form a crosslinked fibrous matrix. FT-IR spectroscopy was employed to detect the ester linkage between PVMEMA and PEG/SW kraft pulp fibers. The results suggested that the ester crosslinked pulps exhibit excellent water absorbent properties and have the potential of utilizing milled bleached SW kraft fibers, such as refiner dust or pulp fines, for novel water absorbent applications.     

Alkali‐catalyzed low temperature wet crosslinking of plant proteins using carboxylic acids

We report the development of a new method of alkali‐catalyzed low temperature wet crosslinking of plant proteins to improve their breaking tenacity without using high temperatures or phosphorus‐containing catalysts used in conventional poly(carboxylic acid) crosslinking of cellulose and proteins. Carboxylic acids are preferred over aldehyde‐containing crosslinkers for crosslinking proteins and cellulose because of their low toxicity and cost and ability to improve the desired properties of the materials. However, current knowledge in carboxylic acid crosslinking of proteins and cellulose requires the use of carboxylic acids with at least three carboxylic groups, toxic phosphorous‐containing catalysts and curing at high temperatures (150–185°C). The use of high temperatures and low pH in conventional carboxylic acid crosslinking has been reported to cause substantial strength loss and/or undesired changes in the properties of the crosslinked materials. In this research, gliadin, soyprotein, and zein fibers have been crosslinked with malic acid, citric acid, and butanetetracarboxylic acid to improve the tenacity of the fibers without using high temperatures and phosphorus‐containing catalysts. The new method of wet crosslinking using carboxylic acids containing two or more carboxylic groups will be useful to crosslink proteins for various industrial applications. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Crosslinking of alginic acid/chitosan matrices using polycarboxylic acids and their utilization for sodium diclofenac release

Three different ratios of alginic acid/chitosan matrices of ratios 3/1, 1/1 and 1/2, respectively, were crosslinked in their dry state using citric acid (CA)/sodium hypophosphite (SHP) at different conditions controlling the crosslinking process such as citric acid concentration, citric acid/sodium hypophosphite molar ratio as well as time and temperature of reaction. Results indicate that such matrices were crosslinked efficiently on curing at 180 °C for 9 min in presence of CA/SHP ratio 1 and the citric acid concentration of 0.6 based on the weight of any matrices. The crosslinked matrices were characterized by investigating their swelling properties, FT-IR and thermalgravimetric analysis. Furthermore, such crosslinked matrices were tested as drug release for sodium diclofenac.     

Covalent crosslinking of Escherichia coli phenylalanyl-tRNA and valyl-tRNA to the ribosomal A site via photoaffinity probes attached to the 4-thiouridine residue

tRNAPhe and tRNAVal of Escherichia coli were derivatized at the S4U8 position with p-azidophenacyl and p-azidophenacylacetate photoaffinity probes. The modified tRNAs could still function efficiently in all of the partial reactions of protein synthesis except for an approximately sevenfold decrease in the rate of translocation. Irradiation (310 to 340 nm) of probe-modified Phe-tRNA or Val-tRNA placed in the ribosomal A site led to crosslinking that was totally dependent on irradiation, the presence of the azido group on the probe, mRNA, and elongation factor Tu (EFTu). Prephotolysis of the modified tRNA abolished crosslinking, but prephotolysis of the ribosomes and factors had little effect. Crosslinking was efficiently quenched by mercaptoethanol or dithiothreitol, demonstrating accessibility of the probe to solvent. Use of GDPCP in place of GTP also blocked crosslinking, probably because of the retention of EFTu on the ribosome. Crosslinking with the p-azidophenacyl acetate (12 A) probe was only half as efficient as with the p-azidophenacyl (9 A) probe, and this ratio was not changed by varying Mg2+ from 5 to 15 mM. The crosslink was from a functional A site, since AcPhePhe-tRNA at the A site could be crosslinked, and it was A site-specific, because neither translocation nor direct non-enzymatic P site binding yielded any significant covalent product. The crosslink was to ribosomal protein(s) of the 30 S subunit. No other ribosomal component was crosslinked. Identification of the protein crosslinked is described in the accompanying paper.     

Properties and potential medical applications of regenerated casein fibers crosslinked with citric acid

Regenerated protein fibers developed from casein using alkaline solutions and crosslinked with citric acid have good tensile properties and water stability but were cytotoxic. Casein is obtained as a byproduct during skim milk production but has limited industrial applications. In this research, casein was dissolved using alkaline solutions and the effects of fiber formation conditions on the tensile properties was studied before and after crosslinking with citric acid. Crosslinked casein fibers had tensile strength as high as 110 MPa and breaking elongation of 13.6%. The fibers were also stable in 90°C water between pH 3 and 9 but were hydrolyzed at pH 11. Casein was cytotoxic and did not support the attachment and growth of mouse fibroblasts.     

An eco-friendly – novel approach for attaining wrinkle – free/soft-hand cotton fabric

A novel approach for upgrading both the wrinkle free and softness properties of cotton fabrics without adversely affecting their strength properties using an eco-friendly finishing regimes was investigated. Factors affecting the performance properties of the finished substrate such as pre-treatment, i.e., carboxymethylation (CMC) or ionic-crosslinking, post-treatment with amino functional silicone softener and its concentration, degree of carboxymethylation as well as thermofixation conditions were studied. The obtained results revealed that post-treatment with the amino based silicone micro emulsion (SiE) up to 30 g/L at pH 4 to a wet pickup of 100% followed by drying at 100 °C for 5 min and curing at 170 °C for 3 min results in a remarkable improvement in fabric resiliency (expressed as dry and wet wrinkle recovery angles), as well as in softness degree, without seriously affecting its retained strength. Improvement extent of the aforementioned properties is governed by the nature of the pre-treatment steps. Fixation of the amino-functional silicone softener onto/or within the modified cellulose structure is accompanied by a formation of semi-inter and/or intra-penetrated network (semi-IPN) thereby enhancing both the extent of crosslinking and networking as well as providing very high softness. FTIR analysis proved the formation of Si–O–Si–cellulose complex. Scanning electron micrograph shows that cotton, CMC and ionic crosslinked cotton fabrics treated with SiE shows higher surface smoothness and considerable reduction in protruding loose fibers, ditches and grooves compared with the untreated one.     

碳化二亚胺对透明质酸进行化学修饰的研究

目的：用水溶性的碳化二亚胺(EDC)作为交联剂对透明质酸(HA)进行化学修饰,研究交联产物(HA-EDC)的物化性质和微观结构。方法：制备HA与EDC的交联产物,对该交联产物进行流变性和热学性能的研究,并对其进行红外光谱、拉曼光谱以及核磁共振的微观结构研究。结果：反应时间和交联剂添加量的不同可以改变交联反应的交联度。不同摩尔比的EDC和HA的交联产物有不同的溶胀性。光谱分析得到交联产物的N-酰脲的结构。结论：可以用碳化二亚胺来对透明质酸进行化学修饰,得到具有很强吸水性并且吸水性可以控制的交联产物,交联反应过程中,产物的结构从不稳定的O-异酰脲转变为稳定的N-酰脲。     

A deep learning model for predicting optimal distance range in crosslinking mass spectrometry data

Macromolecular assemblies play an important role in all cellular processes. While there has recently been significant progress in protein structure prediction based on deep learning, large protein complexes cannot be predicted with these approaches. The integrative structure modeling approach characterizes multi-subunit complexes by computational integration of data from fast and accessible experimental techniques. Crosslinking mass spectrometry is one such technique that provides spatial information about the proximity of crosslinked residues. One of the challenges in interpreting crosslinking datasets is designing a scoring function that, given a structure, can quantify how well it fits the data. Most approaches set an upper bound on the distance between Cα atoms of crosslinked residues and calculate a fraction of satisfied crosslinks. However, the distance spanned by the crosslinker greatly depends on the neighborhood of the crosslinked residues. Here, we design a deep learning model for predicting the optimal distance range for a crosslinked residue pair based on the structures of their neighborhoods. We find that our model can predict the distance range with the area under the receiver-operator curve of 0.86 and 0.7 for intra- and inter-protein crosslinks, respectively. Our deep scoring function can be used in a range of structure modeling applications.     

Comparative analysis of gelatin scaffolds crosslinked by genipin and silane coupling agent

Scaffolds based on gelatin (G) are considered promising for tissue engineering, able to mimic the natural extracellular matrix. G drawback is its poor structural consistency in wet conditions. Therefore, crosslinking is necessary to fabricate stable G scaffolds. In this work, a comparative study between the performance of two different crosslinkers, genipin (GP) and γ-glycidoxypropyltrimethoxysilane (GPTMS), is presented. Flat membranes by solvent casting and porous crosslinked scaffolds by freeze-drying were prepared. Infrared spectroscopy and thermal analysis were applied to confirm G chain crosslinking. Moreover, GP and GPTMS increased the stability of G in aqueous media and improved the mechanical properties. Crosslinking reduced the wettability, especially in the case of G_GPTMS samples, due to the introduction of hydrophobic siloxane chains. Both G_GP and G_GPTMS scaffolds supported MG-63 osteoblast-like cell adhesion and proliferation.     

Finishing of cotton fabrics with poly (N-vinyl-2-pyrrolidone) to improve their performance and antibacterial properties

Cotton fabric was thermally crosslinked with poly (N-vinyl-2-pyrrolidone) (PVP) at different conditions including temperature, time, PVP concentrations and molecular weights. Results indicated that treating the cotton fabrics with 4% aqueous solution of PVP of molecular weight 10,000 Dalton followed by drying at 85 °C for 5 min then curing at 160 °C for 3 min results in crosslinking as will as an improvement in some performance properties of that fabrics such as resiliency, tensile strength, and acid dyeability. Post-treating PVP crosslinked fabric with 5% iodine in ethanol solution for 5 h at 50 °C imparts antibacterial activity against Staphylococcus aureus and Escherichia coli. Moreover, incorporation of PVP in the easy-care finishing of cotton fabrics, as polymer additive, with N,N-dimethylol 4,5-dihydroxyethylene urea as a crosslinker enhances some of the performance properties of finished fabrics such as the nitrogen content, tensile strength and acid dyeability along with decreasing resiliency as well as whiteness index, whereas the ester crosslinking with citric acid, in presence of PVP, enhances resilience, tensile strength and whiteness indices accompanied with a reduction in the %N of the treated fabrics. Infra red spectrum of PVP crosslinked fabric as well as EDX analysis of loaded iodine on PVP crosslinked cotton fabric were investigated.     

Imaging cell interactions with native and crosslinked polyelectrolyte multilayers

The adhesion of primary chondrocytes to polyelectrolyte multilayer films, made of poly(l-lysine) (PLL) and hyaluronan (HA), was investigated for native and crosslinked films, either ending by PLL or HA. Crosslinking the film was achieved by means of a water-soluble carbodiimide in combination with N-hydroxysulfosuccinimide. The adhesion of macrophages and primary chondrocytes was investigated by microscopical techniques (optical, confocal, and atomic), providing useful information on the cell/film interface. Native films were found to be nonadhesive for the, primary chondrocytes, but could be degraded by macrophages, as could be visualized by confocal laser scanning microscopy after film labeling. Confocal microscopy images show that these films can be deformed by the condrocytes and that PLL diffuses at the chondrocyte membrane. In contrast, the cells adhered and proliferated well on the crosslinked films, which were not degraded by the macrophages. These results were confirmed by a MTT test over a 6-d period and by atomic force microscopy observations. We thus prove that chemical crosslinking can dramatically change cell adhesion properties, the cells being more stably anchored on the crosslinked films. Both authors kcontributed equally.     

Comparison of the mutagenic activity of XAD4 and blue rayon extracts of surface water and related drinking water samples

The combination of mutagenicity tests and selective extraction methodologies can be useful to indicate the possible classes of genotoxic organic contaminants in water samples. Treated and source water samples from two sites were analyzed: a river under the influence of an azo dye-processing plant discharge and a reservoir not directly impacted with industrial discharges, but contaminated with untreated domestic sewage. Organic extraction was performed in columns packed with XAD4 resin, that adsorbs a broad class of mutagenic compounds like polycyclic aromatic hydrocarbons (PAHs), arylamines, nitrocompounds, quinolines, antraquinones, etc., including the halogenated disinfection by-products; and with blue rayon that selectively adsorbs polycyclic planar structures. The organic extracts were tested for mutagenicity with the Salmonella assay using TA98 and TA100 strains and the potencies were compared. A protocol for cleaning the blue rayon fibers was developed and the efficiency of the reused fibers was analyzed with spiked samples. For the river water samples under the influence of the azo-type dye-processing plant, the mutagenicity was much higher for both blue rayon and XAD4 extracts when compared to the water from the reservoir not directly impacted with industrial discharges. For the drinking water samples, although both sites showed mutagenic responses with XAD4, only samples from the site under the influence of the industrial discharge showed mutagenic activity with the blue rayon extraction, suggesting the presence of polycyclic compounds in those samples. As expected, negative results were found with the blue rayon extracts of the drinking water collected from the reservoir not contaminated with industrial discharges. In this case, it appears that using the blue rayon to extract drinking water samples and comparing the results with the XAD resin extracts we were able to distinguish the mutagenicity caused by industrial contaminants from the halogenated disinfection by-products generated during water treatment.     

Alginate and pectin composite films crosslinked with Ca ions: Effect of the plasticizer concentration

The manufacture of composite biofilms of alginate and LM-pectin crosslinked with calcium ions requires a two-step contact with Ca²⁺: initially a low-structured pre-film is formatted which is further crosslinked in a second contact with a more concentrated Ca²⁺ solution containing plasticizer. This research evaluated the influence of the plasticizer (glycerol) concentration (1–15% w/v) in this finishing reticulation step on final films characteristics. The results indicated that the extent of the simultaneous Ca²⁺ crosslinking and plasticization with glycerol was determined by the level of structural organization obtained in the pre-reticulation. Increasing the glycerol concentration of the crosslinking solution increased film solubility in water, moisture content, volumetric swelling and flexibility and decreased the resistance to tensile stress. Transparent alginate and pectin composite films with acceptable mechanical properties, low solubility and limited degree of swelling were obtained with 10% glycerol in the second contact solution.     

Mustard gas crosslinking of proteins through preferential alkylation of cysteines

Mustard gas,bis(2-chloroethyl)sulfide, treatment of proteins is shown to generate significant amounts of covalently crosslinked protein dimers. This is due to the preferential alkylation of cysteine residues. Crosslinking does not occur in the model protein staphylococcal nuclease, which has no cysteine residues. Treatment of cysteine-containing mutants of staphylococcal nuclease with this chemical warfare agent did result in crosslinking. However, these dimers are slowly cleaved back to monomers by an unknown mechanism. The alkylation and crosslinking of cysteine-containing proteins by mustard gas may contribute to its toxicity.     

Thermodynamic aspects of the gelatinization and swelling of crosslinked starch

Samples of epichlorohydrin crosslinked potato starch were prepared by using a high ratio of starch to water and alkali concentration below the gelatinization level. This yielded crosslinked samples that were partially crystalline, and where the number of crosslinks could be varied between 1 and 20 crosslinks per 100 anhydroglucose units. The degree of swelling varied regularly with degree of crosslinking, and the molecular weight between crosslinks M_c as derived from swelling data in a good swelling agent compared favorably with M_c derived from chemical analysis. From the heat of gelatinization of the crosslinked starches, as observed in a differential scanning calorimeter, for gelatinization in glycerol, water, and dimethylsulfoxide, a model for the gel state of the crosslinked starch is proposed. It is concluded that the entropy of melting is the determining factor in establishing the temperature of gelatinization.     

Temperature dependence of the elastic modulus of alginate gels

The temperature dependence of the elastic modulus for alginate gels was studied using two different gel systems: covalently crosslinked Na-alginate gels and in-situ prepared Ca-alginate gels. The modulus of physically crosslinked gels showed a complex behaviour. The temperature coefficient of the modulus of covalently crosslink gels changed from positive for the lowest crosslinked gels to negative for the highest crosslinked gels. This suggests a change from rubberlike to enthalpy-driven elasticity with an increasing degree of crosslinking for these gel networks.     

Crosslinking of enzyme coaggregate with polyethyleneimine: A simple and promising method for preparing stable biocatalyst of Serratia marcescens lipase

Crosslinking of enzyme aggregates is a promising method for enzyme immobilization. In this work, crosslinked enzyme coaggregates of Serratia marcescens lipase with polyethyleneimine (CLECAs-SML-PEI) were prepared using polyethyleneimine (PEI) as coprecipitant and glutaraldehyde as crosslinking reagent. The crude lipase solution at a low protein concentration (0.1 mg/ml), with PEI at a mass ratio of 3:1 (PEI/protein, w/w), was found to be most adequate for the coprecipitation of SML. After crosslinking of the coaggregate of SML-PEI with 0.2% (w/v) glutaraldehyde under ambient temperature, over 70% of the total lipase activity was recovered. Compared with the free SML, the optimum temperature of the CLECAs-SML-PEI was enhanced from 50 °C to 60 °C and its thermal stability was also significantly improved. CLECAs-SML-PEI showed excellent operational stability in repeated use in aqueous–toluene biphasic system for asymmetric hydrolysis of trans-3-(4′-methoxyphenyl)glycidic acid methyl ester (MPGM), without significant inactivation after 10 rounds of repeated use.     

A dye-photosensitized reaction that generates stable protein-protein crosslinks

Irradiation of fibrinogen with visible light for 30 s in the presence of 1-1000 microM fluorescein was found to crosslink fibrinogen both inter- and intramolecularly. Optimum crosslinking was achieved at dye concentrations of around 100 microM and the amount of crosslinking was shown to increase with pH. Crosslinking was inhibited in the presence of 50 microM tryptophan or tyrosine and enhanced in the presence of 5 mM histidine. Twice as much crosslinking was found to take place under anaerobic conditions. These observations are consistent with a dye-photosensitized reaction following the hydrogen abstraction pathway. The subunits of eukaryotic ribosomes and those of phosphorylase a were also crosslinked by the method described.     

Crosslinking of hnRNP proteins to pre-mRNA requires U1 and U2 snRNPs.

Proteins interacting with pre-mRNAs during early stages of spliceosome formation in a HeLa nuclear extract were investigated by photochemical RNA-protein crosslinking. The level of protein crosslinking to a beta-globin pre-mRNA was positively correlated with the presence of an intron. Proteins of 110,000, 59,000 and 39,000 mol. wt. were crosslinked to the beta-globin pre-mRNA, the latter of which was identified as the A1 hnRNP protein. Comparable experiments with an adenovirus pre-mRNA revealed crosslinked proteins of 110,000, 56,000 and 45,000 mol. wt., with the latter identified as belonging to the C group hnRNP proteins. Crosslinking of hnRNP proteins to both the beta-globin and adenovirus pre-mRNAs was eliminated by oligodeoxynucleotide-directed RNase H excision of an internal region (nt 28-42) of U2 RNA, but was not affected by oligo/RNase H cleavage of the 5'-terminal 15 nucleotides of U2 RNA. Cleavage of the 5'-terminal 15 nucleotides of U1 RNA preferentially eliminated crosslinking of the hnRNP A1 protein to both pre-mRNAs. The requirement of intact U1 snRNP for A1 protein crosslinking was further demonstrated by the fact that although micrococcal nuclease-treated extracts did not support crosslinking of A1 hnRNP protein to beta-globin pre-mRNA, crosslinking was restored by addition of a U1 snRNP-enriched fraction.     

Crosslinked polyethylenimine: An enzyme carrier with spacers of various lengths introduced in crosslinking reaction

Polyethylenimine (PEI) reacted with epichlorhydrin to various degrees of crosslinking was further activated with thiophosgene or with succinic anhydride; the carboxyl derivatives obtained were converted to hydrazide derivatives. d-Glucose oxidase (β-d-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4), glucoamylase (exo-1,4-α-d-glucosidase, 1,4-α-d-glucan glucohydrolase, EC 3.2.1.3) and cholinesterase (acetyl-, butyryl-) [acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7), butyryl cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8)] were bound in their native forms to the first two types of carriers, while in the case of the hydrazide derivative of crosslinked PEI this occurred just after periodate oxidation of a glycoenzyme. The PEI derivatives prepared from the crosslinked PEI with the lowest degree of crosslinking, i.e. from that having the longest mean length of the spacer (～ 11.1 Å), revealed the best binding properties towards low-molecular thiols, amino acids and proteins/enzymes. After enzyme coupling, the isothiocyanate derivatives of crosslinked PEI gave preparations with the highest residual activities.