A gradient method for the counter-current separation of alkaloids using a heavy organic phase

Nigritanine as a New Potential Antimicrobial Alkaloid for the Treatment of Staphylococcus aureus-Induced Infections

Staphylococcus aureus is a major human pathogen causing a wide range of nosocomial infections including pulmonary, urinary, and skin infections. Notably, the emergence of bacterial strains resistant to conventional antibiotics has prompted researchers to find new compounds capable of killing these pathogens. Nature is undoubtedly an invaluable source of bioactive molecules characterized by an ample chemical diversity. They can act as unique platform providing new scaffolds for further chemical modifications in order to obtain compounds with optimized biological activity. A class of natural compounds with a variety of biological activities is represented by alkaloids, important secondary metabolites produced by a large number of organisms including bacteria, fungi, plants, and animals. In this work, starting from the screening of 39 alkaloids retrieved from a unique in-house library, we identified a heterodimer -carboline alkaloid, nigritanine, with a potent anti-Staphylococcus action. Nigritanine, isolated from Strychnos nigritana, was characterized for its antimicrobial activity against a reference and three clinical isolates of S. aureus. Its potential cytotoxicity was also evaluated at short and long term against mammalian red blood cells and human keratinocytes, respectively. Nigritanine showed a remarkable antimicrobial activity (minimum inhibitory concentration of 128 µM) without being toxic in vitro to both tested cells. The analysis of the antibacterial activity related to the nigritanine scaffold furnished new insights in the structure-activity relationships (SARs) of -carboline, confirming that dimerization improves its antibacterial activity. Taking into account these interesting results, nigritanine can be considered as a promising candidate for the development of new antimicrobial molecules for the treatment of S. aureus-induced infections.

Studies on an abnormally sharpened elution peak observed in counter-current chromatography

Counter-current chromatography (CCC) of the bromoacetylation product of 3,3',5-triiodo-L-thyronine (T3) produced an unusually sharp peak for the desired product, N-bromoacetyl T3 (BrAcT3). A series of experiments revealed that bromoacetic acid, probably present as a side reaction product in the sample solution, was responsible. This compound repressed the ionization of the carboxyl group of BrAcT3 forcing it into the less polar stationary phase until the bromoacetic acid had eluted completely from the apparatus. At this point, the sudden increase of pH and consequent ionization of the BrAcT3 allowed the ammonium salt of the latter to enter the more polar moving phase where it eluted rapidly from the column as a sharp peak. The same phenomenon was observed in the CCC fractionation of a series of indole auxins where addition of trifluoroacetic acid to the sample caused peak sharpening by the same process. The phenomenon recalls pH gradient elution and isoelectric focussing except that the substance responsible for the pH range here is added along with the sample in one bolus forming a sharp pH gradient at its trailing edge. As with gradient elution, the technique is of practical interest since it permits collection of the eluting compounds with increased detectability in fewer fractions. The technique can also enhance separation of compounds whose partition coefficients differ with a change in pH.

New trends in the separation of active principles from plants

The extraction and separation of active principles of plant constituents is a difficult task, even for specialized chemists, when many similar substances are found in a single plant. In the case of the presence of alkaloids a separation method based on counter-current distribution with a discontinuous variation of pH can be used for the resolution of very complex mixtures. The alkaloids separated from twenty-seven medicinal plants are reported.