Conformational energetics of interpolyelectrolyte complexation between ι-carrageenan and poly(methylaminophosphazene) measured by high-sensitivity differential scanning calorimetry

The interaction of poly(methylaminophosphazene) hydrochloride (PMAP·HCl) of varying degrees of ionization (f) with the potassium salt of ι-carrageenan was studied by high-sensitivity differential scanning calorimetry at a KCl concentration of 0.15 M, which is included for the purpose of stabilizing the helix conformation of the polysaccharide up to 55 °C. The conditions of strong (pH 3.8, I = 0.15), moderate (pH 7.4, I = 0.15), and weak (pH 7.4, I = 0.25) electrostatic interactions of the polyelectrolytes were considered. The thermodynamic parameters of the helix-coil transition of ι-carrageenan were determined as a function of the polycation/polyanion ratio. We show that the interpolyelectrolyte reaction between PMAP·HCl and ι-carrageenan results in a complete unfolding of the polysaccharide helix under conditions of strong electrostatic interaction and increases its stability under conditions of medium and weak electrostatic interactions. The formation of stoichiometric PMAP-carrageenan interpolyelectrolyte complexes proceeded via a cooperative mechanism at pH 3.8 (f = 0.5) and pH 7.4 (f = 0.2) at an ionic strength of 0.15. In contrast, the complexation at pH 7.4 and an ionic strength of 0.25 could be considered to be a consecutive competitive binding of charged units of poly(methylaminophosphazene) to the oppositely charged polysaccharide matrix in the helix or coil conformation. Binding constants of the polycation to the helix and coil forms of ι-carrageenan were estimated. They revealed a preferential binding of the polycation to the helix form of the polysaccharide.

Trapped water of human erythrocytes and its application in cryopreservation

The novel differential scanning calorimetry method as a technique for determining human red cell volume during freezing process has been reexamined and has been shown to provide a final erythrocyte volume to be 53% of its isotonic value after freezing from 0 to -40 degrees C. A new type of electronic particle counter (Multisizer 3, Beckman Coulter Inc., USA) was used to measure cell volume changes in response to equilibration in anisotonic media, and which gave out an equilibrated volume to be 57% of cell isotonic value in solution of 3186 mOsm. Both of these results indicate that 34-40% of intracellular water is trapped and is unavailable for participation in osmotic shifts. These findings are consistent with the published data that at least 20-32% (v/v) of the isotonic cell water is retained within RBCs. Then the application of trapped water in both simulation of freezing models and freezing-drying control was pointed out.

A modified differential scanning calorimetry for determination of cell volumetric change during the freezing process

A modified analytical and experimental method using differential scanning calorimeter (DSC) was developed to determine the cell volume change during the freezing process. Two cell types were used in the study: human platelets and erythrocytes (red blood cells). Isotonic cell suspensions with different cytocrits were prepared and used in the DSC experiments. Low cooling rates were used to avoid intracellular ice formation. Cell suspensions were cooled from room temperature to -40 degrees C. Latent heat release from the freezing of cell suspensions was shown to be a linear function of cytocrit. From slope and intercept of the linear function, cell volume change was determined based on a developed theoretical model. From experimental data and theoretical analyses, it was revealed that (a) the final volume of a human platelet at -40 degrees C was 33.7% of its isotonic volume, and 15.2% of the original (at isotonic condition) intracellular water remained unfrozen inside platelets, and (b) the final volume of human erythrocyte at -40 degrees C was 50.0% of its isotonic volume, and 30.3% of the original intracellular water was kept inside cells as residual unfrozen water.

Quantitative Assays of the Amount of Diethylenetriaminepentaacetic Acid Conjugated to Water-Soluble Polymers Using Isothermal Titration Calorimetry and Colorimetry

The level of conjugation of diethylenetriaminepentaacetic acid (DTPA) to the polysaccharide sodium hyaluronan (HA) has been measured by a colorimetric assay, isothermal titration calorimetry (ITC), and (1)H NMR spectroscopy. The colorimetric assay is based on the red shift, upon complexation with gadolinium ion (Gd3+), of the wavelength of maximum absorption of the dye arsenazo III. It can be performed in a few minutes using as little as 10 microg of polymer with a detection limit of approximately 0.03 mmol of DTPA (gram of polymer)-1. The ITC measurements yield values of the amount of DTPA linked to HA identical to those obtained by colorimetry. The levels of DTPA conjugation calculated by integration of signals at 3.1-3.2 ppm (DTPA protons) and at 2.0 ppm (HA acetamide protons) in the 1H NMR spectrum of HA-DTPA are consistently overestimated by a factor of approximately 2, compared to the data obtained by ITC and colorimetry. The longer relaxation times of protons of the polymer backbone, compared to those of protons attached to the freely moving DTPA side-chains may account for the discrepancy.

Thermal analysis methods for pharmacopoeial materials

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are commonly used for purity and solvent determinations, polymorphism studies and quantitative analysis. These methods are now simple to carry out. Furthermore, automation with robotics allows the use of DSC as a routine control. It is proposed to introduce DSC and TGA in pharmacopoeial monographs as a replacement or alternative to routine melting-point determinations and loss on drying assays. Furthermore, polymorphism and purity may be determined, which is often lacking in monographs. Results of many commercial batches or reference substances of pharmacopoeial raw materials and examples of the use of DSC robotics for quality control are given.