Michèle Mestdagh

Physico-chemical properties and rheology of alginate gel beads formed with various divalent cations

The physico-chemical parameters influencing the behaviour of alginate beads have not been deeply studied in the literature. In this communication, the formation and the mechanical and structural properties of alginates with different molecular weights and M/G ratio were investigated. The results obtained showed that (1) to produce spherical beads, the product of alginate concentration and intrinsic viscosity C[eta] must be higher than a threshold value and the accurate value is a function of the gelling cation type; (2) the nature of the cation, the polymer and cation concentration, and the ionic strength influence the mechanical properties of alginate beads; (3) critical point drying and cryogenic preparation are equivalent and less disturbing methods than lyophilization for studying the structural properties of alginate beads in SEM; and (4) calcium-alginate beads are smooth and uniform whereas copper-alginate beads are rough and seem to be formed of associations of independent microgels

Reversible inhibition and irreversible inactivation of catalase in presence of hydrogen peroxide.

Spectroscopic and kinetic investigations have been carried out on catalase from bovine liver and from Aspergillus niger to address the mechanism of activity loss at high hydrogen peroxide concentrations (0.01 to 2 M). The mammalian enzyme was both reversibly inhibited and irreversibly inactivated in the presence of hydrogen peroxide, whereas the fungal enzyme did not show any reversible inhibition. A comparison of reaction rates with catalase preparations containing different proportions of Compound III indicated that the formation of Compound III is responsible for the reversible inhibition of bovine liver catalase at high H2O2 concentrations. Superoxide radical did not appear to be the inactivating species in this mechanism. Kinetic modelling emphasises the role of Compound III in both types of activity loss. It shows that the higher activity of A. niger catalase at high substrate concentration, compared to bovine liver catalase, the lack of reversible inhibition of the former and its lower rate of irreversible inactivation may be attributed both to a high rate of conversion of Compound III into native form and to a low rate of conversion of Compound I to Compound II.

Spectroscopic study (u.v.-visible and electron paramagnetic resonance) of the interactions between synthetic polycarboxylates and copper ions

The interactions of poly(methacrylic acid) (PMA), poly(acrylic acid) (PAA) and a copolymer of acrylamide and acrylic acid (CAMAA) with copper (II) ions in aqueous solutions were studied by electron paramagnetic resonance (e.p.r.) spectroscopy as a function of the degree of neutralization cr. A procedure for decomposition of the e.p.r. spectra was developed in order to evaluate the fraction of free copper and bound copper ions. When a loss of the e.p.r. signal intensity is observed in the solutions containing polymer with respect to the one of the reference copper salt solutions without polymer, this effect is attributed to the formation of a binuclear polymer/copper complex (copper: carboxylates, 2/4), inactive in e.p.r. The complex visible in e.p.r, is assumed to be mononuclear (copper: carboxylates, 1/2). This indicates that the main PMA/copper complex is binuclear. PAA and copper interactions lead to both bi-and mono-nuclear complexes, but this last species is preponderant at the higher ct values. In the case of CAMAA of lower charge density and where the charges are quenched, only a mononuclear complex was found. The constants of formation of the mono-and bi-nuclear complexes were evaluated by assuming that the system was ruled by a series of equilibrium laws: copper hydrolysis, polymer ionization and polymer/copper complexation. The peculiar behaviour for PAA may be explained by considering that the constant of formation for the binuclear complex decreases when the intramolecular monomer concentration is lowered by the chain expansion

Irradiation Modes' Impact on Radical Entrapment in Photoactive Resins.

Different irradiation protocols are proposed to polymerize dental resins, and discordances remain concerning their impact on the material. To improve this knowledge, we studied entrapment of free radicals in unfilled Bis-GMA/TEGDMA (50:50 wt%) resin after light cure. The tested hypothesis was that various irradiation parameters (curing time, irradiance, and radiant exposure) and different irradiation modes (continuous and pulse-delay) led to different amounts of trapped free radicals. The analysis of cured samples (n = 3) by electron paramagnetic resonance (EPR) revealed that the concentrations of trapped free radicals significantly differed according to the curing protocol. When continuous modes with similar radiant exposure were compared, higher concentrations of trapped free radicals were measured for longer times with lower irradiance. Concerning pulse modes, the delay had no influence on trapped radical concentration. These results give new insights into the understanding of the photopolymerization process and highlight the relevance of using EPR when studying polymerization of dimethacrylate-based materials.

Kinetic study of free radicals trapped in dental resins stored in different environments.

In this work, we used electron paramagnetic resonance to follow the decrease kinetics of free radicals trapped in an experimental resin (ER) and in a commercial composite (Charisma (Ch)) stored under different conditions (in air at 25 and 37 degrees C; in argon, oxygen and water at 25 degrees C). During the first day, the decay was fast (0-24h-rate of decay of allylic radical: 1700-1000a.u. for Ch, 1700-1500a.u. for ER) and the storage conditions had no influence on the kinetics. This phase was ascribed to a post-polymerization phenomenon. From 1day to 1month, the rate of decay depended on the storage environment. In argon, free radicals were quite stable (1day to 1month-rate of decay of allylic radical: 1200-1000a.u. for Ch, 1400-1200a.u. for ER). For the other storage environments, in ER, the rate of decay was higher in water than in oxygen and in air (1day to 1month-rate of decay of allyl radical: 1400a.u. to 100, 500 and 800a.u., respectively). In Ch, free radicals faded quicker than in ER, as undetectable levels were reached before 1month, which attests to the influence of fillers on radical decrease kinetics. Heating experiments were also performed, and free radical concentrations decreased faster at higher temperatures, especially above the glass transition temperature. In conclusion, ambient oxygen is mainly involved in the termination process of free radicals. Therefore, conditions influencing oxygen diffusion have an impact on radical kinetics as well.

Protein Adsorption in Calcium Alginate Gel Beads

A Iginate is a natural polysaccharide consisting of a linear chain of (1-4)-linked residues of ~3-D-mannuronic acid (M) and a-L-guluronic acid (G) in different proportions and sequential arrangements. The most common arrangement is that of a block copolymer, in which long, homopolymeric sequences of guluronic acid residues (G-G blocks) and similar sequences of mannuronic acid residues (M-M blocks) are intercalated between sequences of mixed composition (M-G blocks). Calcium alginate gel beads are often used as a matrix to entrap proteins, enzymes, whole microbial, plant or animal cells. The loaded beads are used then as immobilized biocatalysts or for controlled release of the entrapped molecules. A systematic study of calcium alginate gel properties is necessary to define the parameters which influence the performance of the gel in such applications. Martinsen et al. [1,2] recently reported that mechanical strength, swelling, shrinking, and resistance to interference by monovalent cations, are highly dependent upon the molecular weight of the alginate, its M/G ratio, and especially its contents of G-G blocks. Since many applications are based on the diffusion properties of solu-

An E.P.R. and potentiometric study of the complexation of copper ions by galacturonic acid and galacturonans

Abstract E.p.r. spectroscopy and copper potentiometry indicated the stepwise formation of copper-galacturonate complexes, with successive formation constants log K 1 ∼2.16 and log K 2 ∼2.06. Comparison of the magnetic parameters g | of these complexes (2.403 ± 0.003 and 2.380 ± 0.003, respectively) with those of copper complexes formed with pectins of various origins (2.390 ±0.003) showed that weaker bonds were formed with the polymers than with the monomers. The greater affinity of copper for the polymers is therefore due mainly to entropic effects. The presence of aquo-complexes in frozen solutions, easily detectable by e.p.r. spectroscopy, was observed, for all the pectins studied, when the metal-to-ligand ratio ( N ) was >0.5. The increase in line-width with N , up to N ∼0.5, was more important with unesterified pectins (∼200%) than with esterified pectins (∼40%).

Modifications induced by swift heavy ions on poly(hydroxybutyrate-hydroxyvalerate) (PHB/HV) and poly(epsilon-caprolactone) (PCL). Part 2. Radicals characterization

Modifications induced by different energetic heavy ions (Ar-40(9+), Kr-80(15+), Xe-129(24+), Pb-208(53+) and Pb-20(56+)) on poly(epsilon -caprolactone) (PCL) and poly(hydroxybutyrate-hydroxyvalerate) (PHB/HV) have been investigated by electron spin resonance (ESR). Indeed, film irradiation by heavy ions leads to, among other phenomena, the formation of radicals in the ion tracks. Thanks to ESR, it is possible to detect these radicals and to identify them or at least to characterize them by following the evolution of the radical signal as a function of parameters, like temperature, or the kinetic of disappearance of the radical species at ambient temperature in vacuum or ambient atmosphere. This study confirmed the generation of radicals by the irradiation of PHB/HV samples with energetic heavy ions reported in the literature. The study on PCL was not pursued after a few preliminary studies, revealing the presence of an ESR signal in the nonirradiated sample. Electronic stopping power has a major influence on radical decrease at ambient temperature. The ion used for the irradiation did not modify very much the radical signal and the evolution of the radicalar signal intensity with temperature. Different reasoning and experiments revealed that the glass transition temperature is a key temperature above which irreversible recombinations of the most stable radicals take place. A simulation study indicated that the most stable radical produced was probably a tertiary radical formed by the stabilization of the secondary radical resulting from the abstraction of a highly mobile hydrogen adjacent to the carbonyl