Xiuqiang Zhang

Stimuli-Responsive Zwitterionic Block Copolypeptides: Poly(N-isopropylacrylamide)-block-poly(lysine-co-glutamic acid)

Synthesis of novel zwitterionic block copolypeptides, poly(N-isopropylacrylamide)-block-poly(L-glutamic acid-co-L-lysine) [PNiPAM(n)(PLG(x)-co-PLLys(y))m , where n is the number-average degree of polymerization (DP(n)) of PNiPAM block, x and y are the mole fraction of glutamic acid and lysine residues, respectively, and m is the total DP(n) of the peptide block], and their stimuli-responsiveness to temperature and pH variation in aqueous solutions are described. Initiated with the amino-terminated poly(N-isopropylacrylamide) (PNiPAM(n)-NH2), ring-opening polymerization (ROP) of a mixture of gamma-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA), and Boc-L-lysine N-carboxyanhydride (BLLys-NCA) afforded the block copolypeptides PNiPAM(n)(PBLG(x)-co-PBLLys(y))m, with a poly(N-isopropylacrylamide) block together with a random copolypeptide block, which was then deprotected with HBr/trifluoroacetic acid into the double hydrophilic block copolypeptides, PNiPAM(n)(PLG(x)-co-PLLys(y))m. Their block ratios and lengths, as well as the amino acid residue ratios in the random copolypeptide block are varied (n = 360, x = 0.4-0.5, y = 0.4-0.6, and m = 220-252). The secondary structures of the copolypeptides in aqueous solution at different pH conditions were examined. Phase transitions in aqueous solutions induced by both pH and temperature variation were investigated by (1)H NMR spectroscopy. The transitions induced by temperature were also explored by turbidity measurements using UV/vis spectroscopy for their lower critical aggregation temperature (LCAT) determination. Furthermore, these aggregation processes were followed by dynamic light scattering measurements.

Thermoresponsive supramolecular dendronized copolymers with tunable phase transition temperatures

Supramolecular copolymerization from a mixture of comonomers with different hydrophilicities may offer an easy and convenient access to form thermoresponsive supramolecular copolymers with varied phase transition temperatures by simply varying ratios of the comonomers. Herein, supramolecular dendronized copolymers (SDCPs) were constructed via host–guest interaction from a linear polymer carrying β-cyclodextrin units (the host) and a mixture of two adamantyl-cored second generation oligo(ethylene glycol) dendrons (the guests). The phase transition temperatures of these SDCPs can be tuned continuously in the range of 34–56 °C by varying the ratios of the dendritic guests. To check the possible effects of steric hindrance and hydrophilicity of the guests on the supramolecular complexation, isothermal titration calorimetry was applied to follow the complexation process of the first (G1) and second generation (G2) dendritic guests. Proton NMR spectroscopy was utilized to follow dehydration and collapse processes of SDCPs. It was found that the dehydration and collapse of the OEG units initiated decomposition of the dendritic guests from the supramolecular copolymers, and the more hydrophobic guest dissociated at a much lower temperature than that for the more hydrophilic one.

Effect of Neferine on Liver Ischemia-Reperfusion Injury in Rats

INTRODUCTION Liver ischemia/reperfusion leads to the formation of reactive oxygen species (ROS) that cause liver injury, a critical clinical problem during liver surgery and transplantation. The aim of the present study was to investigate the hepatoprotective and antioxidant effects of neferine against liver ischemia/reperfusion injury in rats. MATERIALS AND METHODS Wistar rats were randomly divided into 4 groups (n = 8): sham group; model group, and neferine high and low groups (50 and 25 mg/kg, respectively). After either saline or neferine was orally administered for 5 days rat livers were subjected to 30 minutes of ischemia followed by 6 hours of reperfusion. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), and hydroxyl radical levels were measured in serum. The liver was removed to assay malondialdehyde (MDA) and carbonyl contents, superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities, as well as to evaluate histopathologic changes. RESULTS Neferine significantly prevented AST and ALT elevations, reduced hydroxyl radical release, inhibited SOD and GPx activities, and decreased MDA and carbonyl contents. At the same time, neferine attenuated the histopathologic changes. CONCLUSION Neferine protected against liver ischemia/reperfusion in rats through antioxidant mechanisms. However, further studies are needed to verify whether the hepatoprotection of neferine is correlated with anti-inflammatory and anti-apoptotic effects.

Lysine-based dendronized polymers with preferred chirality

Dendronized polymethacrylates carrying lysine-based third generation (G3) dendrons were efficiently synthesized via a macromonomer route by free radical polymerization. These polymers showed amplified chirality when compared to the corresponding free dendrons, and the handedness of their chiral conformations could be mediated by the diastereomeric dendrons.

Thermoresponsive cyclodextrins with switchable inclusion abilities

Cyclodextrins (CDs) modified with oligoethylene glycols (OEGs) show unprecedented thermoresponsive properties in aqueous solutions and their phase transition temperatures can be mediated in the range of 24–63 °C depending on the ring size of CDs and the structure of the OEG units. Their synthesis, thermoresponsive behavior, and host-guest chemistry are described herein. Similar to the native CDs, these CD derivatives display strong inclusion ability for dyes below the phase transition temperatures. Moreover, the encapsulated dyes can be easily released due to the dehydration of OEG units at elevated temperatures, associated with an obvious color change of the aqueous solutions, which makes these CD species unique as scaffolds for colorimetric sensors.

Liquid Crystalline Period Variations in Self- Assembled Block Copolypeptides-Surfactant Ionic Complexes

We investigate the complexation of ampholytic poly(N-isopropylacylamide)-block-poly- (L-glutamic acid)-block-poly(L-lysine) (PNiPAM-b-PLG-b-PLLys) triblock copolymers and PNiPAM-block-(PLG-co-PLLys) diblock copolymers with counter charged anionic and cationic surfactants. Both triblock and diblock copolymers are able to selectively form complexes through either L-glutamic acid-cationic surfactant or L-lysine-anionic surfactant ionic pairs, depending on the protonated or deprotonated states of the ampholytic peptide units. The complexes show ordering at multiple length scales: i) the block copolymer length scale (10(1)  nm), ii) the liquid crystalline length scale (10(0)  nm), and, iii) the peptidic secondary structures length scale (10(0)  nm). We show that the liquid crystalline period can be tuned by varying the random/block copolypeptide architectures and the composition of the ampholytic amino acid species.

Remarkable Structure Effects on Chiroptical Properties of Polyisocyanides Carrying Proline Pendants

Chiral polymers with simple chemical structures and high helical conformation stabilities are important for their applications as chiral supports and asymmetrical catalysts. We report herein the synthesis of a series of aliphatic polyisocyanides carrying proline pendants of different chiralities, and an investigation of the effects of the chemical structures of these pendants on the chiroptical properties of the polymers. The configuration of the chiral center at the 4-position of the proline pendants was changed from S to R to check its effect on the handedness of the helical conformation. To examine the effects of steric hindrance on the stabilities of the helical conformation for these aliphatic representatives, proline pendants with various substituents at both the carboxyl and amine terminals were designed. To further examine the steric effects of the proline pendants, aromatic counterparts were also prepared. In the latter case, the effects of hydrogen bonds between pendant units on the enhancement and stabilities of the helical conformation were investigated by switching from the ester to an amide linkage. The Cotton effects and signal intensities of both aliphatic and aromatic polyisocyanides from circular dichroism spectroscopy were compared based on the bulkiness of the pendant groups, solvent polarities, and solution temperatures. It was found that highly stable helical conformations of polyisocyanides could be imposed by small bulky monoproline pendants.