The stability of gingerol and shogaol in aqueous solutions.

Gingerols, pungent principles of ginger (the rhizome of Zingiber officinale), are biologically active components that may make a significant contribution towards medicinal applications of ginger and some products derived from ginger. Gingerols, however, are thermally labile due to the presence of a beta-hydroxy keto group in the structure, and undergo dehydration readily to form the corresponding shogaols. This study investigated the stability of [6]-gingerol [5-hydroxy-1-(4-hydroxy-3-methoxyphenyl)decan-3-one] at temperatures ranging from 37 to 100 degrees C in aqueous solutions, at pH 1, 4, and 7. Quantitative measurements of [6]-gingerol and its major degradation product [6]-shogaol [1-(4-hydroxy-3-methoxyphenyl)decan-4-ene-3-one] were performed by HPLC. Kinetics of [6]-gingerol degradation was characterized by least square fitting of a rate equation. It was found that gingerol exhibited novel reversible kinetics, in which it undergoes dehydration-hydration transformations with shogaol, the major degradation product. Degradation rates were found to be pH dependent with greatest stability observed at pH 4. The reversible degradation of [6]-gingerol at 100 degrees C and pH 1 was relatively fast and reached equilibrium within 2 h. Activation energies for the forward and reverse reactions for [6]-gingerol were calculated from the Arrhenius equation using reaction rates obtained at temperatures ranging from 37 to 100 degrees C.     

Ascorbic acid stability in aqueous solutions

Different water purity provokes a great variation of the stability of ascorbic acid and isoascorbic acid solutions. The effect of temperature on ascorbate aerobic oxidation was assessed by means of Arrhenius plots from which thermodynamic parameters were derived. The presence of bovine serum albumin drastically reduces the vitamin oxidation rate regardless of stereoisomerism. On the other hand the interaction with alkaline phosphatase, an enzyme inhibited by preincubation with vitamin C, does not modify significantly the stability in the experimental conditions used.     

The stability of trifluorothymidine: hydrolysis in buffered aqueous solutions

The decomposition of trifluorothymidine in aqueous solution, and borate and phosphate buffers has been studied using an h.p.l.c. method. In aqueous solution accelerated studies demonstrate that a storage life, to 10% loss, of more than 30 years at 4°C is predicted, but steam sterilization causes extensive breakdown. Considerable differences occur in the kinetics and routes of decomposition in borate and phosphate buffers in the pH range 4–8. Hydroxide ion attack producing 5-carboxy-2′-deoxyuridine is suppressed in borate buffers. Complex kinetics of decomposition in phosphate buffer are found and possible explanations suggested. Steam sterilization in buffered solution also leads to unacceptable breakdown.     

The stability of trifluorothymidine: hydrolysis in buffered aqueous solutions

The decomposition of trifluorothymidine in aqueous solution, and borate and phosphate buffers has been studied using an h.p.l.c. method. In aqueous solution accelerated studies demonstrate that a storage life, to 10% loss, of more than 30 years at 4 degrees C is predicted, but steam sterilization causes extensive breakdown. Considerable differences occur in the kinetics and routes of decomposition in borate and phosphate buffers in the pH range 4-8. Hydroxide ion attack producing 5-carboxy-2'-deoxyuridine is suppressed in borate buffers. Complex kinetics of decomposition in phosphate buffer are found and possible explanations suggested. Steam sterilization in buffered solution also leads to unacceptable breakdown.     

Study of the stability of tylosin A in aqueous solutions

The decomposition of the 16-membered ring macrolide antibiotic tylosin A in aqueous buffers has been investigated in the pH range 2–13, by means of a liquid chromatographic assay with ultraviolet detection at 280 nm. In acidic medium, tylosin A is converted into tylosin B, while in neutral and alkaline medium, tylosin A aldol is formed together with a number of polar decomposition products of unknown identity. The decomposition kinetics have been studied as a function of the type and concentration of the buffer, ionic strength, pH and temperature.     

The aqueous stability of bupropion

In this study the aqueous stability of bupropion was determined and the pH-degradation profile was obtained. The effects of hydrogen ion, solvent and hydroxide ion concentration are discussed with particular emphasis on the kinetics of degradation of bupropion. Kinetics and degradation of bupropion were determined by HPLC-UV and LC–MS analysis both utilising high pH chromatographic methods. Degradation of bupropion in aqueous solutions follows first-order reaction kinetics. The pH-degradation profile was determined using non-linear regression analysis. The micro- and macro-reaction constants for degradation are presented. Bupropion was most stable in aqueous solutions below pH 5. Degradation was catalyzed by water but mainly by hydroxide ion on the unionised form of bupropion. The energy of activation for decomposition in aqueous solution pH 10.7 I = 0.055 was determined to be 53 kJ mol⁻¹ with a frequency factor of 6.43 × 10¹⁰ s⁻¹. The degradants of bupropion were positively identified and a mechanism of degradation is proposed. The inherent instability of bupropion above pH 5 has implications for its therapeutic use, formulation, pharmacokinetics and use during analysis and storage.