Benzyl Glutamate Research Articles

Chemical structures and compositions of peptide copolymer films affect their functional properties for cell adhesion and cell viability

In this study we synthesized peptide copolymers containing benzyl glutamate (BG) moiety (to enhance cell viability) and lysine derivatives (to improve cell adhesion) for use in tissue engineering. We examined the effects of two lysine derivatives: Nε-t-butyloxycarbonyl lysine (BOCL) and Nε-carbobenzoxy lysine (CBZL). The cell behavior on the copolymer films was influenced by the functional characteristics of the copolymers, governed by their chemical structures and compositions. A BOC unit on the lysine side chain is bulkier than a CBZ unit, resulting in a rougher film surface displaying poorer adhesion. The film's hydrophilicity and cell adhesion properties were improved by increasing the CBZL content in the copolymers, due to the presence of the amide linkages of the CBZ units. The copolymer film prepared with an equal molar ratio of CBZL and BG exhibited the best cell performance, resulting from a balance in the copolymer's functional properties. The cell behavior on this film was further improved by partially hydrolyzing the copolymer to produce net positive charge, thereby increasing the hydrophilicity and cell recognition. These novel copolymers appear suitable for use as films for enhancing cell culturing during tissue engineering.

Meticulous Doxorubicin Release from pH-Responsive Nanoparticles Entrapped within an Injectable Thermoresponsive Depot.

The dual stimuli-controlled release of doxorubicin from gel-embedded nanoparticles is reported. Non-cytotoxic polymer nanoparticles are formed from poly(ethylene glycol)-b-poly(benzyl glutamate) that, uniquely, contain a central ester link. This connection renders the nanoparticles pH-responsive, enabling extensive doxorubicin release in acidic solutions (pH 6.5), but not in solutions of physiological pH (pH 7.4). Doxorubicin-loaded nanoparticles were found to be stable for at least 31 days and lethal against the three breast cancer cell lines tested. Furthermore, doxorubicin-loaded nanoparticles could be incorporated within a thermoresponsive poly(2-hydroxypropyl methacrylate) gel depot, which forms immediately upon injection of poly(2-hydroxypropyl methacrylate) in dimethyl sulfoxide solution into aqueous solution. The combination of the poly(2-hydroxypropyl methacrylate) gel and poly(ethylene glycol)-b-poly(benzyl glutamate) nanoparticles yields an injectable doxorubicin delivery system that facilities near-complete drug release when maintained at elevated temperatures (37 °C) in acidic solution (pH 6.5). In contrast, negligible payload release occurs when the material is stored at room temperature in non-acidic solution (pH 7.4). The system has great potential as a vehicle for the prolonged, site-specific release of chemotherapeutics.

Copolymacrolactones Grafted with l-Glutamic Acid: Synthesis, Structure, and Nanocarrier Properties.

The enzymatic ring-opening copolymerization (eROP) of globalide (Gl) and pentadecalactone (PDL) was performed in solution from mixtures of the two macrolactones at ratios covering the whole range of comonomeric compositions. The resulting P(Glx-r-PDLy) random copolyesters were aminofunctionalized by thiol-ene reaction with aminoethanethiol. ROP of γ-benzyl-l-glutamate N-carboxyanhydride initiated by P(Glx-r-PDLy)-NH2 provided neutral poly(γ-benzyl-l-glutamate)-grafted copolyesters, which were converted by hydrolysis into negatively charged hybrid copolymers. Both water-soluble and nonsoluble copolymers were produced depending on copolymer charge and their grafting degree, and their capacity for self-assembling in nano-objects were comparatively examined. The emulsion solvent-evaporation technique applied to the chloroform-soluble copolymers grafted with benzyl glutamate rendered well-delineated spherical nanoparticles with an average diameter of 200–300 nm. Conversely, micellar solutions in water were produced from copolyesters bearing grafted chains composed of at least 10 units of glutamic acid in the free form. The copolymer micelles were shown to be able to load doxorubicin (DOX) efficiently through electrostatic interactions and also to release the drug at a rate that was markedly pH dependent.

Nanomicelles based on a boronate ester-linked diblock copolymer as the carrier of doxorubicin with enhanced cellular uptake

This study sought to develop a new type nanomicelle based on boronate ester-linked poly(ethylene glycol)-b-poly(benzyl glutamate) (PEG-BC-PBLG) diblock copolymer as the carrier of doxorubicin (Dox) to achieve acid-induced detachment of PEG shells and subsequent boronic acid-mediated enhanced endocytosis. In vitro studies revealed that the PEG-BC-PBLG copolymer was stable in neutral solutions but tend to hydrolysed under acidic conditions, which was attributed to the acid-sensitive properties of boronate ester bonds. The formation of PEG-BC@PBLG micelles was confirmed based on critical micelle concentration (CMC), particle size, and morphology observations. It was observed that these micelles were spherical with an average particle size of approximately 80nm, as measured by dynamic laser scattering (DLS), suggesting their passive targeting to tumour tissue and endocytosis potential. Dox-loaded PEG-BC@PBLG micelles (PEG-BC@PBLG·Dox) showed sustained drug release profiles over 9h, and their cumulative drug release was dependent on the pH value of the environment. Remarkably, cellular uptake ability of PEG-BC@PBLG micelles was found to be higher than that of non-boronate ester-linked PEG@PBLG micelles due to boronic acid-mediated endocytosis, as revealed by confocal laser scanning microscopy (CLSM) imaging of fluorescein isothiocyanate (FITC) green-conjugated micelles, thereby providing higher cytotoxicity against HepG2 cells. The antitumour activity and toxicity of PEG-BC@PBLG·Dox micelles in vivo were evaluated in BLAB/c mice against HepG2 cell-derived tumours. Compared with Dox, PEG-BC@PBLG·Dox showed reduced toxicity, whereas its tumour growth inhibition rate was 17% higher than that of free Dox. These results indicate the great potential of PEG-BC@PBLG micelles as the carrier of various lipophilic anticancer drugs with improved anti-tumour efficacy.

Block copolypeptide nanoparticles for the delivery of ocular therapeutics.

Self-assembling block copolypeptides were prepared by sequential ring-opening polymerization of N-carboxyanhydride (NCA) derivatives of γ-benzyl-L-glutamic acid and ε-carbobenzyloxy-L-lysine, followed by selective deprotection of the benzyl glutamate block. The synthesized polymers had number average molecular weights close to theoretical values, and had low dispersities (ĐM = 1.15-1.28). Self-assembly of the amphiphilic block copolymers into nanoparticles was achieved using the "solvent-switch" method, whereby the polymer was dissolved in THF and water and the organic solvent removed by rotary evaporation. The type of nanostructures formed varied from spherical micelles to a mixture of spherical and worm-like micelles, depending on copolymer composition. The spherical micelles had an average diameter of 43 nm by dynamic light scattering, while the apparent diameter of the mixed phase system was around 200 nm. Reproducibility of nanoparticle preparation was demonstrated to be excellent; almost identical DLS traces were obtained over three repeats. Following qualitative dye-solubilization experiments, the nanoparticles were loaded with the ocular anti-inflammatory drug dexamethasone. Loading efficiency of the nanoparticles was 90% and the cumulative drug release was 94% over 16 d, with a 20% burst release in the first 24 h.

New amphiphilic glycopolypeptide conjugate capable of self-assembly in water into reduction-sensitive micelles for triggered drug release

For the development of biomimetic carriers for stimuli-sensitive delivery of anticancer drugs, a novel amphiphilic glycopolypeptide conjugate containing the disulfide bond was prepared for the first time by the ring-opening polymerization of benzyl glutamate N-carboxy anhydride in the presence of (propargyl carbamate)ethyl dithio ethylamine and then click conjugation with α-azido dextran. Its structure was characterized by Fourier-transform infrared spectroscopy and nuclear magnetic resonance analyses. Owing to its amphiphilic nature, such a conjugate could self assemble into nanosize micelles in aqueous medium, as confirmed by fluorometry, transmission electron microscopy and dynamic light scattering. For the resultant micelles, it was found to encapsulate poorly water-soluble anticancer drug (methotrexate, MTX) with the loading efficiency of 45.2%. By the in vitro drug release tests, the release rate of encapsulated MTX was observed to be accelerated significantly in the presence of 10mM 1,4-dithio-DL-threitol (DTT), analogous to the intracellular redox potential.

Preparation and properties of poly(benzyl glutamate)–poloxamer–poly(benzyl glutamate) and poly(glutamic acid)–poloxamer–poly(glutamic acid) triblock polymers

AbstractThe novel block copolymer poly(benzyl glutamate) (PBLG)–polomamer–PBLG were synthesized from glutamic acid and poloxamer in six steps with three different molecular weights, and another new block copolymer, poly(glutamic acid) (PGA)–poloxamer–PGA, was obtained by the benzyl deprotection of PBLG–poloxamer–PBLG. The obtained compounds were characterized by IR spectroscopy, gel permeation chromatography, and 1H‐NMR. The in vitro biological degradation and water absorption of PBLG showed that a greater proportion of PBLG in the copolymer led to a slower degradation and weaker water absorption, so the speed of degradation and water absorption could be adjusted through adjustment of the ratio of poloxamer. Both PBLG–poloxamer–PBLG and PGA–poloxamer–PGA exhibited lower cytotoxicity and good biocompatibility in the methyl thiazolyl tetrazolium (MTT) assay. The results show that both block polymers are promising as drug‐carrier materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Bioreducible Block Copolymers Based on Poly(Ethylene Glycol) and Poly(γ-Benzyl l-Glutamate) for Intracellular Delivery of Camptothecin

Poly(ethylene glycol)-b-poly(γ-benzyl L-glutamate)s bearing the disulfide bond (PEG-SS-PBLGs), which is specifically cleavable in intracellular compartments, were prepared via a facile synthetic route as a potential carrier of camptothecin (CPT). Diblock copolymers with different lengths of PBLG were synthesized by ring-opening polymerization of benzyl glutamate N-carboxy anhydride in the presence of a PEG macroinitiator (PEG-SS-NH(2)). Owing to their amphiphilic nature, the copolymers formed spherical micelles in an aqueous condition, and their particle sizes (20-125 nm in diameter) were dependent on the block length of PBLG. Critical micelle concentrations of the copolymers were in the range 0.005-0.065 mg/mL, which decreased as the block length of PBLG increased. CPT, chosen as a model anticancer drug, was effectively encapsulated up to 12 wt % into the hydrophobic core of the micelles by the solvent casting method. It was demonstrated by the in vitro optical imaging technique that the fluorescence signal of doxorubicin, quenched in the PEG-SS-PBLG micelles, was highly recovered in the presence of glutathione (GSH), a tripeptide reducing disulfide bonds in the cytoplasm. The micelles released CPT completely within 20 h under 10 mM GSH, whereas only 40% of CPT was released from the micelles in the absence of GSH. From the in vitro cytotoxicity test, it was found that CPT-loaded PEG-SS-PBLG micelles showed higher toxicity to SCC7 cancer cells than CPT-loaded PEG-b-PBLG micelles without the disulfide bond. Microscopic observation demonstrated that the disulfide-containing micelle could effectively deliver the drug into nuclei of SCC7 cells. These results suggest that PEG-SS-PBLG diblock copolymer is a promising carrier for intracellular delivery of CPT.

Synthesis and physicochemical characterization of amphiphilic block copolymers bearing acid-sensitive orthoester linkage as the drug carrier

Amphiphilic block copolymers bearing an acid-sensitive orthoester linkage, composed of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(γ-benzyl L-glutamate) (PBLG), were prepared as the carrier capable of selectively releasing the hydrophobic drug at the mildly acidic condition. Diblock copolymers with various lengths of PBLG were synthesized via ring opening polymerization of benzyl glutamate NCA in the presence of the acid-labile PEG as a macroinitiator. Owing to their amphiphilicities, the copolymers formed spherical micelles in aqueous conditions, and their particle sizes (22–106 nm in diameter) were dependent on the block length of PBLG. These nanoparticles were stable in the physiological buffer (pH 7.4), whereas they were readily decomposed under the acidic condition. In particular, the block copolymer with a smaller hydrophobic portion was rapidly disassembled under the acidic condition. Doxorubicin (DOX), chosen as the model anti-cancer drug, was effectively encapsulated into the hydrophobic core of the micelles using the solvent casting method. The loading efficiency depended on the hydrophobic block length of the copolymer; i.e., the longer hydrophobic block allowed for loading of larger amounts of the drug. In vitro release studies demonstrated that DOX was slowly released from the pH-sensitive micelles in the physiological buffer (pH 7.4), whereas the release rate of DOX significantly increased under the acidic condition (pH 5.0). From the in vitro cytotoxicity test, it was found that DOX-loaded pH-sensitive micelles showed higher toxicity to SCC7 cancer cells than DOX-loaded micelles without the orthoester linker. These results suggest that the amphiphilic block copolymer bearing the orthoester linkage is useful for pH-triggered delivery of the hydrophobic drug.

High-Vacuum Vapor Deposition and in Situ Monitoring of N-Carboxy Anhydride Benzyl Glutamate Polymerization

A high-vacuum physical vapor deposition (PVD) method was used to grow polypeptide chains from gold surfaces coupled with in situ monitoring using surface plasmon resonance spectroscopy (SPR). Polymerization of N-carboxy anhydride of benzyl-L-glutamate (BLG-NCA) was demonstrated, forming grafted polybenzyl-L-glutamate (PBLG) films. 2-Aminoethanethiol (AET) was used as the initiating site either deposited as a self-assembled monolayer (SAM), as a PVD film, or codeposited on the gold substrates. PBLG films up to 30 nm thick in which the formation of different secondary structures was monitored by in situ SPR and ex situ Fourier transform infrared spectroscopy (FT-IR) methods based on the parameters for deposition were prepared. Furthermore, surface topology and film morphology properties were determined on the basis of AFM measurements and were found to depend largely on the deposition process of the AET. Besides investigation of the AET surface density effects, the orientation of the PBLG chains was also characterized using ex situ FT-IR.

Synthesis of poly(benzyl glutamate‐b‐styrene) rod‐coil block copolymers by dual initiation in one pot

AbstractWe have developed a metal free synthetic pathway to homopolypeptide rod‐coil block copolymers. The concept was proven for the synthesis of poly(benzyl‐L‐glutamate‐b‐styrene). A dual initiator containing a primary amine and a nitroxide group was used in a macroinitiation approach with high initiation efficiency. Good control over the molecular weight in the ring opening polymerization of benzyl‐L‐glutamate N‐carboxyanhydride was obtained in DMF at 0 °C yielding poly(benzyl‐L‐glutamates) with low polydispersities around 1.1. The almost quantitative incorporation of the dual initiator was confirmed by MALDI‐ToF analysis. Macroinitiation of styrene by nitroxide‐mediated controlled radical polymerization yielded the block copolymer with high structural control. The diblock structure was confirmed by molecular weight increase upon macroinitiation by size exclusion chromatography and retention time comparison with homopolymers using gradient polymer elution chromatography. Both polymerizations were also successfully conducted in one pot without intermediate isolation owing to the high compatibility of both polymerization techniques. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3068–3077, 2008

Micelles Based on Biodegradable Poly(l-glutamic acid)-b-Polylactide with Paramagnetic Gd Ions Chelated to the Shell Layer as a Potential Nanoscale MRI−Visible Delivery System

There is much interest in the development of a nanoscale drug delivery system with MRI visibility to optimize the delivery efficiency and therapeutic efficacy under image guidance. Here we report on the successful fabrication of nanoscale micelles based on biodegradable poly( L-glutamic acid)- b-polylactide (PG- b-PLA) block copolymer with paramagnetic Gd3+ ions chelated to their shell. PG- b-PLA was synthesized by sequential polymerization reactions: anionic polymerization of L-lactide followed by ring-opening polymerization of benzyl glutamate N-carboxylic anhydride. The metal chelator p-aminobenzyldiethylenetriaminepenta(acetic acid) (DTPA) was readily conjugated to the side chain carboxylic acids of poly( L-glutamic acid). The resulting copolymer formed spherical micelles in aqueous solution with an average diameter of 230 nm at pH 7.4. The size of PG(DTPA)- b-PLA micelles decreased with increasing pH value. DTPA-Gd chelated to the shell layer of the micelles exhibited significantly higher spin-lattice relaxivity (r1) than a small-molecular-weight MRI contrast agent, indicating that water molecules could readily access the Gd ions in the micelles. Because of the presence of multiple carboxylic acid functional groups in the shell layer, polymeric micelles based on biodegradable PG(DTPA-Gd)- b-PLA may be a suitable platform for the development of MRI-visible, targeted nanoscale drug delivery systems.

Preparation of Hydrophilic Polyhydroxyalkyl Glutamine Crosslinked Films and its Biodegradability

Polybenzyl glutamate (PBLG) or polymethyl glutamate (PMLG) films have been aminolyzed with amino alcohol and crosslinked with aliphatic diamine at 60 degrees C for 48 h simultaneously which led to the formation of crosslinked films of polyhydroxyalkyl glutamine (PHAG). ATR-IR indicates that for the aminolysis of PBLG with 2-amino-1-ethanol or 3-amino-1-propanol, benzyl glutamate almost completely turned to hydroxyalkyl glutamine, however for the aminolysis of PMLG with 5-amino-1-pentanol, methyl glutamate partially turned to hydroxypentanyl glutamine. The water-swelling test shows that water-swelling ratio Q of PHAG films from amino alcohol with longer carbon chain was smaller, the PHAG films crosslinked by 1,2-diamino ethane have the higher water-swelling ratio Q, but the PHAG films crosslinked by 1,8-diamino octane have the lower water swelling ratio Q; and PHAG films with a greater amount of crosslinking agents have high crosslinking density or the low water swelling-ratio Q for same amino alcohol and diamine. It is obvious from in vitro enzymatic hydrolysis test that specimens with smaller swelling ratio Q displayed larger T(1/2), time for half weight digestion of PHAG film, that is, less biodegradability. Therefore, biodegradability of the crosslinked PHAG films can be controlled by changing amino alcohol and diamine.

A Dendron Based on Natural Amino Acids: Synthesis and Behavior as an Organogelator and Lyotropic Liquid Crystal

From linear to dendritic: Glycine is part of the predominant sequence in collagen proteins, and aspartate is similar to the repeating unit of poly(benzyl glutamate)s (PBG)s. Collagen and PBGs are linear peptides that self-organize into fibers and form liquid-crystalline phases and gel solvents. How would a molecule self-assemble if it was built by arranging Gly-Asp units in a dendritic way (see picture)? A dendron as both gelator and liquid crystal?

TRANSIENT DIRECTOR ORIENTATION PATTERN OF CREEPING PLANE POISEUILLE FLOW OF A NEMATIC POLYMER

The time-dependent Leslie-Ericksen equation in numerically solved for creeping plane poiseuille flow of a nematic polymer (Poly Benzyl Glutamate). The Leslie coefficients and Frank orientation curvature elasticity constants of the nematic polymer reported in the literature were used for the computations of three dimensional director time (t) evolution over the two-dimensional (x-y) plane. The flow is along the x-axis, and the thickness axis the y-axis. On the bounding plates assumed strong homeotropic anchoring condition. On the inlet and outlet flow boundaries, zero surface couple condition is used. Although poiseuille flow has a zero shearing line in the middle with changing sing of shear rate when going through the mid gap region, unlike simple shear flow, directors escape the shear (x-y) plane as flow rate increases, which this computation study shows in accordance with various experimental studies. As the center velocity (U) increases, complex 3-D orientation structure emerges. When U is further increased, tube orientation walls reminiscent of the tube walls of shear flow emerges, not through pinching and reconnection of orientation wall pair, but through emanation from the central backbone orientation wall structure.

Real-Time 1−2 Plane SAXS Measurements of Molecular Orientation in Sheared Liquid Crystalline Polymers

We report studies on the average molecular orientation state in steady and transient shear flow of two lyotropic liquid crystalline polymers: poly(benzyl glutamate) [PBG] and hydroxypropylcellulose [HPC], both in m-cresol solution. An annular cone and plate X-ray shear cell is used to probe molecular orientation in the “1−2” plane, allowing simultaneous measurements of the degree of anisotropy and the average orientation angle relative to the flow direction. In steady shear flow, molecular orientation increases with shear rate in both materials. Comparisons with separate measurements in the “1−3” plane indicate that both materials exhibit a macroscopically biaxial orientation distribution function. The orientation angle is always small and exhibits a sign change from positive to negative values with increasing shear rate. In transient flows, anisotropy and orientation angle both exhibit damped oscillations that scale with shear strain. The Larson−Doi polydomain model is in qualitative agreement with data...

Tumbling dynamics in a nematic surfactant solution in transient shear flows

We report rheological and structural studies of a nematic surfactant solution (CPCl/hex) in transient shear flows. Upon step changes in steady shear flow conditions, the shear stress exhibits damped oscillations that scale with shear strain, attributed to director tumbling under shear. In situ small-angle x-ray scattering spectroscopy is used to measure changes in average micellar orientation during similar transient shear protocols; the measured orientation parameter also exhibits damped oscillations reflecting the underlying tumbling dynamics. Stress and structure data are compared with predictions of the Larson–Doi tumbling polydomain model. The model predictions are qualitatively similar to the experimental observations. In particular, orientation is observed to initially decrease upon shear flow reversal in CPCl/hex, in agreement with the Larson–Doi predictions but in contrast to previous results for model lyotropic solutions of poly(benzyl glutamate). Shear stress and average orientation are closely...

Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers

This article summarizes recent experiments relating measurements of molecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques are flow birefringence and x-ray scattering. Since LCPs usually exhibit a “polydomain” texture, measurements of flow-induced orientation reflect both the local distribution of molecular orientation around the director and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Deborah number to flow alignment at high Deborah number. Rheo-optical measurements of the full refractive index tensor in PBG allow the orientation predictions of microstructural theories for LCP rheology to be quantitatively tested. At low shear rates the two model materials differ: PBG solutions exhibit significant orientation, while HPC solutions show little orientation. This is correlated with the presence of so-called “Region I” shear thinning in HPC solutions. Conversely, in PBG solutions of high concentration, x-ray scattering measurements demonstrate that Region I arises from the presence of a hexagonal phase. The model systems are further differentiated in relaxation. Molecular orientation increases in PBG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of dynamic properties. Finally, structural investigations of a PBG solution and a nematic surfactant solution during step changes in shear conditions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.

Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers

This article summarizes recent experiments relating measurements of molecular orientation to bulk rheological behavior in liquid crystalline polymers (LCPs) under shear. The principal experimental techniques are flow birefringence and x-ray scattering. Since LCPs usually exhibit a “polydomain” texture, measurements of flow-induced orientation reflect both the local distribution of molecular orientation around the director and the heterogeneous distribution of director orientations in the sheared LCP. In model lyotropic solutions of poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC), there are clear structural signatures of a transition from director tumbling dynamics at low Deborah number to flow alignment at high Deborah number. Rheo-optical measurements of the full refractive index tensor in PBG allow the orientation predictions of microstructural theories for LCP rheology to be quantitatively tested. At low shear rates the two model materials differ: PBG solutions exhibit significant orientation, while HPC solutions show little orientation. This is correlated with the presence of so-called “Region I” shear thinning in HPC solutions. Conversely, in PBG solutions of high concentration, x-ray scattering measurements demonstrate that Region I arises from the presence of a hexagonal phase. The model systems are further differentiated in relaxation. Molecular orientation increases in PBG solutions, but decreases in HPC solutions upon flow cessation; these differences are manifested in the evolution of dynamic properties. Finally, structural investigations of a PBG solution and a nematic surfactant solution during step changes in shear conditions are used to interrogate tumbling dynamics at low shear rates and test microstructural tumbling models.

X-ray scattering investigation of highly concentrated poly(benzyl glutamate) solutions under shear flow

In situ x-ray scattering techniques are used to measure molecular orientation and investigate the coexistence of hexagonal and nematic phases in highly concentrated PBG/cresol solutions under shear flow. Beyond a threshold concentration, the diffuse nematic lobe in x-ray scattering patterns is accompanied by sharp reflections indicating lateral packing of molecules in a hexagonal arrangement. The threshold concentration for formation of this hexagonal phase depends on molecular weight, but occurs near 35 wt % PBG and corresponds directly to the onset of region I shear thinning. Applied shear acts to reduce the amount of hexagonal phase and ultimately, at high enough shear rates, return the solution to a fully nematic state. Relaxation experiments at high shear rates indicate that the final state of the solution is highly sensitive to the continued presence of the hexagonal phase during shear. Specifically, a much higher final orientation results if the hexagonal phase—even a very small amount—persists during the preceding shear. We postulate a nucleation mechanism to explain this behavior, in which regrowth of the hexagonal phase during relaxation is promoted in such a way that molecular orientation is favored in a direction parallel to the residual hexagonal phase in the sample at the onset of relaxation. This results in increased molecular orientation in both the hexagonal and nematic phases. Finally, we show examples of erratic relaxation behavior observed in PBG/cresol at concentrations immediately below the threshold of hexagonal phase formation. The degree of this erratic behavior appears to depend on the amount of strain applied before flow cessation.