Densitometric HPTLC Quantification of 2-Azaanthraquinone Isolated from Mitracarpus scaber and Antimicrobial Activity against Dermatophilus congolensis

Abstract

2-Azaanthraquinone (benzo[g]isoquinoline) isolated from Mitracarpus scaber has been found to have in-vitro antimicrobial activity against Dermatophilus congolensis (MIC 7.5 µg mL–1), the causative agent of bovine dermatophilosis. To quantify this compound in Mitracarpus scaber, a new, simple and rapid high-performance thinlayer chromatographic (HPTLC) method was developed and validated for selectivity, recovery, and repeatability. Compounds were separated on silica gel 60F254 plates with toluene–ethyl acetate– methanol, 80 + 18 + 2, as mobile phase. Detection was performed by densitometric scanning at λ = 310 nm and calibration plots showed the response was linearly dependent on concentration in the range 10–100 µg mL–1, with good values of R2. The method was repeatable and precise with relative standard deviations between 0.98 and 1.59% intra-day and between 3.41 and 5.56% inter-day for concentrations from 10 to 100 µg mL–1. Limits of detection and quantification were 3 and 6 µg mL–1, respectively (corresponding to 15 and 30 ng, respectively, on the plate). Concentrations of azaanthraquinone were found to be 0.009% in Mitracarpus scaber aerial parts, 0.057% in the alcoholic extract, and 1.95% in the alkaloid extract. Azaanthraquinone may explain the antimicrobial activity of this last extract and part of the effect of the alcoholic extract.