Biodegradation of random co-polypeptide hydrogels consisting of N-hydroxypropyl l-glutamine as one component

Two component random co-polypeptide hydrogels consisting of N-hydroxypropyl l-glutamine and l-alanine (Ala) or l-phenylalanine (Phe) were prepared by performing aminolysis reactions with 3-amino-1-propanol together with crosslinking reaction with 1,8-octamethylenediamine on hydrogels of the starting co-polymers consisted of γ-benzyl l-glutamate and Ala or Phe. The relationship between their bulk structure and properties was evaluated with regard to the swelling ratio in water (q), the rate of water vapor permeability (V_f), tensile properties, and enzymatic degradation behaviors of hydrogels in a pseudo-extracellular fluid (PECF). The tensile property of the hydrogels was highly dependent on q in PECF, and on the hydrophobicity of the side chains. A relationship was obtained between the V_f and q of hydrogels in PECF regardless of the differences in the nature of the side chains. Biodegradation of the hydrogels in vitro by bromelain indicated that degradation took place in bulk rather than on the surface, and that the rate of degradation was also highly dependent on q in the samples as well as on the hydrophobicity of the side chains of the samples.     

Surface properties and enzymatic degradation of end-capped poly(l-lactide)

Surface properties and enzymatic degradation of poly(l-lactide) (PLLA) end-capped with hydrophobic dodecyl and dodecanoyl groups were investigated by means of advancing contact angle (θ_a) measurement, quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The θ_a values of end-capped PLLA films were larger than those of non-end-capped PLLA films, suggesting that the hydrophobic dodecyl and dodecanoyl groups were segregated on the film surface. The weight changes of end-capped PLLA thin films during enzymatic degradation in the presence of proteinase K were monitored by using a QCM technique. The relatively fast weight loss of PLLA film occurred during first few hours of degradation, followed by a decrease in the erosion rate. The erosion rate of PLLA films at the initial stage of degradation was dependent on the chain-end structure of PLLA molecules, and the value decreased with an increase in the amount of hydrophobic functional groups. The surface morphologies of PLLA thin films before and after degradation were characterized by AFM. After the enzymatic degradation, the surface of non-end-capped PLLA films was blemished homogeneously. In contrast, the end-capped PLLA thin films were degraded heterogeneously by the enzyme, and many hollows were formed on the film surface. From these results, it has been concluded that the introduction of hydrophobic functional groups at the chain-ends of PLLA molecules depressed the erosion rate at the initial stage of enzymatic degradation.     

Investigation of the degradation behaviour of poly(ethylene glycol-co-d,l-lactide) copolymer

The aim was to investigate the degradation behaviour of poly(ethylene glycol-co-d,l-lactide) (PEG-d,l-PLA) multiblock copolymer, in bulk and as microspheres, in aqueous medium. The degradation behaviour of PLA homopolymers in bulk and microspheres was evaluated as comparison.Microsphere preparation was performed by the double emulsion solvent evaporation method. Physical-chemical characterization of the raw polymers and the microspheres was performed by nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry (MDSC). Polymer molecular weight, before and after incubation in aqueous environment, was evaluated by GPC; water uptake and mass loss were determined gravimetrically.The presence of PEG segments inside PLA chains gave a characteristic spongy structure to the microspheres. A significant increase in polymer T_g values was found for the microsphere formulations compared to polymer in bulk. After 63 days of incubation in the aqueous environment, the PEG-d,l-PLA microspheres achieved an average M_w reduction of 47% compared to 20% for PLA microspheres. The corresponding M_w decrease of the polymers in bulk was significantly higher: 72% and 41% for PEG-d,l-PLA and PLA, respectively.The data show how the degradation behaviour of polymer in bulk in an aqueous environment is significantly different from the behaviour of the corresponding microspheres. These results highlight the importance of performing a thorough physical-chemical characterization on microsphere formulations.     

Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s

Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M_n) and d-lactide unit contents (X_d) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X_d, while the k values did not depend on M_n. This means that the effects of X_d on the hydrolytic degradation rate of the films are higher than those of M_n. In contrast, in the period of 32-60 weeks neither X_d nor M_n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks.     

Poly l-lactide-layered double hydroxide nanocomposites via in situ polymerization of l-lactide

The use of clay nanofillers offers a potential route to improved barrier properties in polylactide films. Magnesium-aluminium layered double hydroxides (LDHs) are interesting in this respect and we therefore explored synthesis of PLA-LDH nanocomposites by ring-opening polymerization. This method is attractive because it should ensure good dispersion of LDH in the polymer. The effect of adding either LDH carbonate (LDH-CO₃) or laurate-modified LDH (LDH-C₁₂) was investigated. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy revealed that exfoliated nanocomposites were obtained when using LDH-C₁₂ but that LDH-CO₃ gave a partly phase-separated morphology. Thermogravimetric analysis showed that PLA-LDH combinations exhibited higher degradation onset temperatures and differential scanning calorimetry confirmed that LDHs can act as nucleating agents. However, PLA molecular weight was significantly reduced when in-situ polymerization was conducted in the presence of the LDHs and we suggest that chain termination via LDH surface hydroxyl groups and/or metal-catalyzed degradation could be responsible.     

Racemization behavior of l,l-lactide during heating

To control the depolymerization process of poly(l-lactic acid) into l,l-lactide for feedstock recycling, the racemization of l,l-lactide as a post-depolymerization reaction was investigated. In the absence of a catalyst, the conversion to meso-lactide increased with increase in the heating temperature and time at a higher rate than the conversion into oligomers. The resulting high composition of meso-lactide suggests that the direct racemization of l,l-lactide had occurred in addition to the known racemization mechanism that occurs on the oligomer chains. In the presence of MgO, the oligomerization rapidly proceeded to reach an equilibrium state between monomers and oligomers. The equilibrium among l,l-, meso-, and d,d-lactides was found to be a convergent composition ratio l,l-:meso-:d,d-lactides = 1:1.22:0.99 (wt/wt/wt) after 120 min at 300 °C. This composition ratio also indicates that in addition to the known racemization reaction on the oligomer chains, direct racemization among the lactides is also a frequent occurrence.     

Phase behavior of biodegradable amphiphilic poly(l,l-lactide)-b-poly(ethylene glycol)-b-poly(l,l-lactide)

This study describes the miscibility phase behavior in two series of biodegradable triblock copolymers, poly(l-lactide)-block-poly(ethylene glycol)-block-poly(l-lactide) (PLLA-PEG-PLLA), prepared from two di-hydroxy-terminated PEG prepolymers (M_n = 4000 or 600 g mol⁻¹) with different lengths of poly(l-lactide) segments (polymerization degree, DP = 1.2-145.6). The prepared block copolymers presented wide range of molecular weights (800-25,000 g mol⁻¹) and compositions (16-80 wt.% of PEG). The copolymer multiphases coexistance and interaction were evaluated by DSC and TGA. The copolymers presented a dual stage thermal degradation and decreased thermal stability compared to PEG homopolymers. In addition, DSC analyses allowed the observation of multiphase separation; the melting temperature, T_m, of PLLA and PEG phases depended on the relative segment lengths and the only observed glass transition temperature (T_g) in copolymers indicated miscibility in the amorphous phase.     

Effects of mechanical load on the degradation of poly(d,l-lactic acid) foam

Degradable behaviors of polymer for implantation in body should be evaluated before clinical application. The effect of continuous mechanical load on the degradation progress of poly(d,l-lactic acid) (PDLLA) foam gasket was investigated in detail by specially designed load-providing devices. While PDLLA degraded in the PBS solution (pH, 7.4) at 37 °C for 3 months, the changes of surface morphology, molecular weight, elastic modulus, tensile strength and mass loss were recorded. The results revealed that the degradation rates of PDLLA under continuous loads were obviously quicker than those without load. Moreover, the influence of tensile plus compressive load was larger than that of tensile load. It was indicated that in vivo degradation of PDLLA would not only be influenced by the local solution, but also by the surrounding load. When regulating the degradation rate of bioabsorbable polymer, one should consider the indispensable effect of load where implanted.     

Poly(l,l-lactide-co-glycolide)/tricalcium phosphate composite scaffold and its various changes during degradation in vitro

A poly(l,l-lactide-co-glycolide) (70/30)/(tricalcium phosphate) (PLGA/TCP) composite scaffold was fabricated by low-temperature deposition (LDM) and its degradation performed in vitro for 22 weeks. Various changes during degradation in vitro, which included changes in acidity of the degradation medium, morphology, weight, composition, molecular weight of the PLGA component and mechanical properties of the scaffold, were investigated. It was found that the acidity of degradation medium of the PLGA(70/30)/TCP composite scaffolds reduced and became much lower than that of TCP-free scaffold. With degradation, the volume and porosity of the PLGA(70/30)/TCP composite scaffold reduced at first then increased slowly, while the surface morphology of the scaffold changed from smooth to rough. The weight loss of the scaffold increased by dissolution of the degraded products and TCP component, but mainly by dissolution of the glycyl-rich degraded products of the PLGA component. The molecular weight of the PLGA component reduced with time, but the molecular weight distribution increased at first and then reduced. The compressive strength and modulus of the scaffold increased at first and then reduced with further degradation. The effect of degradation on modulus was much bigger than that on compressive strength. Based on excellent cell affinity of the PLGA(70/30)/TCP composite scaffold, a potentially useful bone tissue engineering scaffold is proposed.     

Hydrolytic degradation of carbohydrate-based polyamides of the AABB type derived from l-arabinose and d-xylose

The hydrolytic degradation of a series of aregic carbohydrate-based polyamides derived from l-arabinose and d-xylose is described. These polyamides are those that are fully sugar-based (PA-SuSu), those derived from aldaric acids and polyalkylene diamines (PA-mSu), and those derived from diamine sugars and polyalkylene dicarboxylic acids (PA-Sun). Their physical properties and crystal structures depend on their constitution and the configuration of the carbohydrate-based moiety. The feasibility of the hydrolysis of these polyamides was, in general, related with such structural properties. Thus, the fully sugar-based PA-SuSu were amorphous, water-soluble materials, and were hydrolysed in water at 70 °C. PA-mSu were crystalline and more resistant to hydrolysis — they were degraded at pH 2 and 70 °C [T_g(s) 60-90 °C]. PA-Sun were amorphous and highly hygroscopic materials — they were hydrolysed in water at 37 °C [T_g(s) 25-40 °C].     

Intermolecular/interionic interactions in l-isoleucine-, l-proline-, and l-glutamine-aqueous electrolyte systems

Speed of sound and density values for ternary systems (amino acid + salt + water): l-isoleucine/l-proline/l-glutamine in aqueous solutions of 1.5 M KCl, 1 M KNO₃, and 0.5 M K₂SO₄ have been measured for several concentrations of amino acids at different temperatures (303.15, 308.15, 313.15, 318.15, and 323.15 K). Using speed of sound and density data, the thermodynamic parameters such as isentropic compressibility (κ_s), change in isentropic compressibility (Δκ_s) and relative change (Δκ_s/κ₀) in isentropic compressibility have been computed. The isentropic compressibility values decrease with increase in the amino acid concentration as well as with temperature. The decrease in κ_s values with increase in concentration of l-isoleucine/l-proline/l-glutamine in 1.5 M KCl, 1 M KNO₃, and 0.5 M K₂SO₄ has been ascribed to an increase in the number of incompressible zwitterions in solutions, and the formation of ‘zwitterions-ions’ and ‘zwitterions-water dipole’ entities in solutions. The decrease in κ_s values with increase in temperature has been attributed to the corresponding decrease of κ_relax (a relaxational part of compressibility), which is dominant over the corresponding increase of κ_∞ (an instantaneous part of compressibility). The trends of variation of Δκ_s and Δκ_s/κ₀ with variations in solute concentration and temperature have also been discussed in terms of solute-solute and solute-solvent intermolecular/interionic interactions operative in the systems.     

Influence of hydrolytic degradation on the surface properties of poly-5d/95l-lactide resorbable bone plates

This study explores in vitro aging effects on the surface properties of resorbable PLA95 (poly-5d/95l-lactide) bone plates. The in vitro degradation of injection molded PLA95 bone plates was undertaken by soaking them in a PBS solution. Specimens were harvested at 0, 4, 6, 8, 12, 20, and 26 weeks. After each in vitro aging period, the surface morphology, viscosity, chemical structure, wettability, and thermal properties of the PLA95 bone plates were examined by scanning electron microscopy (SEM), capillary viscometers, attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR), contact angle, and modulated differential scanning calorimetry (MDSC), respectively. The surface morphology of aged PLA95 bone plates exhibited bulk erosion. As hydrolysis progressed, the inherent viscosity (I.V.) of the PLA95 plates gradually decreased from 0.83 ± 0.01 dL/g at week 0-0.46 ± 0.03 dL/g at week 26. However, the absorbance peak intensity ratio between δ_{as CH3} (A_1452 cm⁻¹) and ν_CO (A_1750 cm⁻¹) and the contact angle reveal different tendencies than that of molecular weight, which decreases. The contact angle of the PLA95 plates decreased until week 4, increased until week 8, and subsequently decreased again. Peak separation analysis reveals that the equilibrium part of the modulated DSC overlapped curves exhibit triple endothermic peaks. Over time, in vitro degradation changes the position and area of the individual peaks. After different time periods of degradation, the variation of wettability shows a tendency similar to the change of PLA95 plates crystallinity; the intensity ratio of A_1452 cm⁻¹ and A_1750 cm⁻¹ (δ_{as CH3}/ν _CO) absorbance peaks varied like the ratio of β/α-crystal heat of fusion. Results also show a similarity in the degradation time dependence in MDSC, contact angle, and ATR-FTIR measurements. During the in vitro aging process, the breakdown and subsequent recrystallization of PLA95 molecular chains might be attributed to a progressive change in wettability and the molecular conformation between δ_{as CH3} and ν_CO.     

Immobilization of bromelain onto porous copoly(γ-methyl-l-glutamate/l-leucine) beads

Water-insoluble bromelain was prepared by immobilizing bromelain onto the surface of porous copoly(γ-methyl-l-glutamate/l-leucine) (ML) beads with and without spacer. The mode of the immobilization between bromelain and porous copolypeptide ML beads was covalent fixation. The relative activity and the stability of the immobilized bromelain was investigated. The retained activity of the bromelain covalently immobilized by the azide method was found to be excellent toward a small ester substrate, N-benzyl-l-arginine ethyl ester, but rather low toward casein, a high molecular weight substrate. The values of the Michaelis constant K_m and the maximum reaction velocity V_m for free and immobilized bromelain on the porous copolypeptide ML beads were estimated. Apparent K_m was larger for immobilized bromelain than for the free one, while V_m was smaller for the immobilized bromelain. The thermal stability of the covalently immobilized bromelain was higher than that of the free bromelain. The initial enzymatic activity of the immobilized bromelain remained approximately unchanged with storage time, when the batch enzyme reaction was performed repeatedly, indicating the excellent durability.     

Prevailing mechanisms of the hydrolytic degradation of oligo(d,l-lactide)-grafted dextrans

The predominant mechanism of the hydrolytic degradation of oligo(d,l-lactide)-grafted dextrans in phosphate buffer was followed by quantifying both released dextran and lactic acid from the copolymers. The studied amphiphilic copolymers, with well-defined structure, exhibited various oligo(d,l-lactide) weight fractions (F^OLA) while having a quite high extent of free hydroxyl groups (>90%). Depending on their F^OLA, oligo(d,l-lactide)-grafted dextrans were soluble either in water or in organic solvents (THF, toluene, …) and different prevailing mechanisms of hydrolytic degradation were observed. The copolymer soluble in THF, with longer oligo(d,l-lactide) grafts and higher F^OLA, was found to degrade via a particular mechanism by which the greatest part of dextran was released into buffer medium during the first two weeks of degradation. During the initial stage of degradation, the hydrophilicity of dextran backbone was considered to be the main driving force for the hydrolytic cleavage of the ester linkage between backbone and grafts. Released oligo(d,l-lactide) grafts were found to be degraded via chain-end degradation or random degradation depending on their solubility in buffer medium. In case of water-soluble copolymers with shorter oligo(d,l-lactide) grafts and lower F^OLA, the chain-end degradation was exclusively observed.     

Novel stereocontrolled approach to conformationally constrained analogues of l-glutamic acid and l-proline via stereoselective cyclopropanation of 3,4-didehydro-l-pyroglutamic ABO ester

A new stereocontrolled approach to l-(carboxycyclopropyl)glycines (l-CCGs) and 3,4-methano-l-prolines, conformationally constrained analogues of l-glutamic acid and l-proline, respectively, was developed using a 3,4-didehydro-l-pyroglutamate derivative as a common chiral template. The unsaturated l-pyroglutamate derivative employed in this work is a novel chiral synthon in which the carboxyl functionality is protected as a 2,7,8-trioxabicyclo[3.2.1]octyl group (ABO ester). Stereospecific cyclopropanation of the olefin using diazomethane followed by appropriate functional group interconversion gave l-CCG-III and trans-3,4-methano-l-proline with complete stereocontrol. Synthesis of other diastereomers of l-CCG and cis-3,4-methano-l-proline was accomplished by alteration of the 3,4-methanoglutamic acid framework via carboxycyclopropanation of the olefin with sulfur ylide and subsequent Barton decarboxylation reaction of the original γ-carboxyl group included in the pyroglutamate skeleton.     

Synthesis of a fluoroalkene peptidomimetic precursor of N-acetyl-l-glutamyl-l-alanine

Fluoroolefin peptidomimetics of the dipeptide N-acetyl-Glu-Ala were synthesized via an Evans asymmetric aldol reaction and an Overman rearrangement. This dipeptide is the N-terminal of an octapeptide implicated in the signal transduction via PKCε.     

Synthesis of d-glucose and l-phenylalanine substituted phenylene-thiophene oligomers

Phenylene-thiophene oligomers bearing peracetylated β-d-glucose or N-BOC-l-phenylalanine as chiral substituents were synthesized in good yields by a versatile protocol based on the Suzuki-Miyaura cross-coupling reaction. Aryl iodides bearing the chiral biomolecules as substituents efficiently reacted with pinacol boronates of bi- or terthiophenes leading to the bio-functionalized oligomers in good yields.     

An efficient synthesis of d-ribo-C18-phytosphingosine and l-arabino-C18-phytosphingosine from d-fructose

d-ribo-C₁₈-phytosphingosine and l-arabino-C₁₈-phytosphingosine were synthesised starting from commercially inexpensive d-fructose. Metal-mediated fragmentation and stereoselective reduction were used as key steps to provide the hydrophilic portion of d-ribo and l-arabino phytosphingosines. Grubbs’ cross-metathesis and hydrogenation allowed the incorporation of hydrophobic tail.     

Kinetics of thermo-oxidative and thermal degradation of poly(d,l-lactide) (PDLLA) at processing temperature

Poly(d,l-lactide) (PDLLA) degraded at processing temperature under air and nitrogen. A random chain scission model was established and used to determine the activation energy E_a, and FT-IR, ¹H and ¹³C NMR were used to elucidate the degradation behavior under different atmospheres. Results showed that there were two to three stages. The 1st stage was dominated by the oligomers containing carboxylic acid groups and hydroxyl groups, during which oxygen and nitrogen had little effect on the degradation, thus they share similar E_a. When the oligomers were consumed over or evaporated, the 2nd stage began, and oxygen had a promoting effect on the thermo-oxidation process, resulting in the great decrease in E_a. The third stage of PDLLA was observed when it degraded under nitrogen over 200 °C, which was caused by the appearance of carboxylic acid substance.     

Synthesis of l-aminohomohistidine (l-Ahh)

A short synthesis of l-aminohomohistidine (l-Ahh), which starts from readily available δ-hydroxy-l-lysine is described. The embedding of the basic guanidino moiety in the aromatic imidazole lowers the basicity of the side chain to a pK_a of 8.3. It is proposed that l-Ahh may be employed as an arginine-mimetic in medicinal chemistry.