James F McCabe | PubMed Health

PubMed2002

Effect of moisture on polyvinylpyrrolidone in accelerated stability testing.

Accelerated stability studies are a common approach for predicting the long-term stability of pharmaceutical formulations. However, in this study, a slowing of dissolution was observed for a formulation following storage at elevated temperature and humidity. The moisture sorption isotherm for the binder, polyvinylpyrrolidone (PVP), shows absorption of a significant quantity of water on exposure to elevated humidity. Modulated temperature differential scanning calorimetry (mDSC) has been used to demonstrate that moisture uptake will depress the glass transition temperature (Tg) of PVP to the conditions used in accelerated stability studies. Exposure to elevated temperature and humidity resulted in a change in the PVP from the glassy to the rubbery state. This conversion produces a change in the dissolution profile. Long-term stability studies conducted at temperatures and humidities below the Tg, would not have induced this change.

Cellulose↗

PubMed2002

Substituent effects on edge-to-face aromatic interactions.

Chemical double mutant cycles have been used to measure the magnitude of edge-to-face aromatic interactions in hydrogen-bonded zipper complexes as a function of substituents on both aromatic rings. The interaction energies vary depending on the combination of substituents from +1.0 kJ mol-1 (repulsive), to -4.9 kJ mol-1 (attractive). The results correlate with the Hammett substituent constants which indicates that electrostatic interactions are responsible for the observed differences in interaction energy. The experiments can be rationalised based on local electrostatic interactions between the protons on the edge ring and the pi-electron density on the face ring as well as global electrostatic interactions between the overall dipoles on the two aromatic groups.

Amides↗