Isothermal Microcalorimetry Experiments Research Articles

Nature of the Polyanion Governs the Antimicrobial Properties of Poly(arginine)/Polyanion Multilayer Films

Coatings with antimicrobial properties are garnering interest to prevent implant-associated infections. Recently, we showed that poly(arginine)/hyaluronic acid (PAR/HA) multilayers built with PAR chains constituted from 30 arginine residues (PAR30) have strong antimicrobial properties through a contact-killing mechanism. This property is due to the ability of PAR30 chains, when associated with HA, to diffuse in the multilayer. Here, we investigate the effect of the nature of the polyanion on the antimicrobial activity of (PAR30/polyanion) multilayers. Four polysaccharides, one polypeptide, and one synthetic polyelectrolyte are investigated. Surprisingly, only HA leads to films with antimicrobial character. We relate this property to the strong diffusion capacity of PAR30 chains in (PAR30/HA) multilayers compared to their diffusion ability in the other (PAR30/polyanion) films. Through isothermal microcalorimetry experiments, we also demonstrate that interactions in solutions of PAR30 chains with the differ...

Using the low-frequency Raman spectroscopy to analyze the crystallization of amorphous indomethacin

This paper gives a detailed analysis of the low-frequency Raman spectrum (LFRS) in the 5–250 cm −1 region, corresponding to collective vibrations, in the crystalline forms and in the amorphous state of indomethacin (IMC). This study points out the high sensitivity of the LFRS to detect, identify and evaluate the first traces of crystallization in comparison with high-frequency regions where internal vibration bands are detected. This analysis reveals that amorphous IMC prepared by cryogrinding instantaneously partially crystallizes at room temperature in the stable γ phase, well below T g = 43 °C. A method based on the treatment of the LFRS to determine precise and very low volume of crystallized material within amorphous matrix is described and used to analyze the crystallization kinetics of ground amorphous IMC powder. This study demonstrates that Raman spectroscopy is also a well-adapted technique to point out small amount of amorphous state in crystalline matrix. Crystallization of ground IMC powder was also analyzed by isothermal microcalorimetry experiments, which is one of the most widely used methods to analyze isothermal crystallization and to evaluate crystallinity.

Microcalorimetry and DSC study of the compatibility of energetic materials

This study included accelerated aging of cyclotetramethylene tetranitramine (HMX) and CH-6 (>97% RDX), coating of HTPB propellant with two sealants (A, B) and reaction kinetics between cyclotrimethylenetrinitramine (RDX) and organic salt (ammonium benzoate). Isothermal microcalorimetry (IMC) analyses at 55–75°C and DSC analyses at decomposition temperatures (180–300°C) were in agreement regarding compatibility. IMC heat flow measurements indicated that aging conditions did not affect HMX and CH-6. DSC rates of reaction at 200°C were similar and thus indicated no reaction. DSC onset of reaction and IMC heat flow measurements showed that one sealant (A) had an interaction with the HTPB propellant while the other did not. When the sealants were combined their reaction with HTPB decreased as the amount of sealant B increased. Reruns of HTPB with sealants A and sealants A+B after 3 weeks at room temperature showed that there was still an additional reaction of ∼9%. IMC and DSC kinetic analyses were carried out on HTPB propellant+sealant A admixture. IMC gave a value of 5 kcal/mol for the reaction in the 55–75°C region while DSC measured the decomposition reaction where the activation energy was lowered from 48 kcal/mol for the propellant to 43 kcal/mol for the admixture. DSC kinetic study of RDX+organic salt showed a shift of the RDX peak temperature from 217°C (neat RDX) to 180°C (admixture). The activation energy for decomposition of the admixture was 25 kcal/mol. Literature values for neat RDX is 47 kcal/mol. Thus DSC kinetic data indicated that the admixture was incompatible at elevated temperature. An IMC kinetic study at temperatures from 65 to 77°C for this admixture gave a value of 33 kcal/mol. The reaction mechanism in the DSC and IMC experiments may be different since in the former case it involves a reaction between a gas (NH 3) and a liquid (RDX) while in the latter case it involves a gas (NH 3) and solid RDX which is a much slower reaction as also reflected in the higher activation energy.

Thermodynamic Analysis of the Activation of Glycogen Phosphorylase b Over a Range of Temperatures

Equilibrium dialysis and isothermal microcalorimetry experiments have been carried out to characterize the thermodynamics of the binding of AMP to glycogen phosphorylase b (EC 2.4.1.1) at pH 6.9 over the temperature range of 25-35 degrees C. Thermal titrations were performed at each temperature in various buffer systems, which have afforded the calculation of the number of protons exchanged when the AMP binds to each site in the protein. Thermodynamic parameters were obtained for the binding of AMP to the two nucleotide and the two inhibitor sites of the dimeric enzyme. The former show positive cooperativity while the latter behave as independent binding sites. A positive delta Cp value was obtained for the AMP binding to the two N sites (1.3 and 1.4 kJ K-1 mol-1), while the delta Cp was negative for the binding to the I sites (-1.9 kJ K-1 mol-1). The application of Sturtevant's method to our data (Sturtevant, J. M. (1977) Proc. Natl. Acad. Sci. U. S. A. 74, 2236-2240) and their comparison with a similar analysis undertaken with phosphorylase a (Mateo, P. L., González, J. F., Barón, C., Lopez-Mayorga, O., and Cortijo, M. (1986) J. Biol. Chem. 261, 17067-17072) has opened the way to some understanding of the thermodynamics of the allosteric transition in the protein.