Synthesis of Mannich bases: 2-(3-Phenylaminopropionyloxy)-benzoic acid and 3-Phenylamino-1-(2,4,6-trimethoxy-phenyl)-propan-1-one, their toxicity, ionization constant, antimicrobial and antioxidant activities

Mannich bases 2-(3-Phenylaminopropionyloxy)-benzoic acid (A) and 3-Phenylamino-1-(2,4,6-trimethoxy-phenyl)-propan-1-one (B) were synthesized. Structures were confirmed by ultraviolet/visible and infra-red spectroscopies. The ionization constant (pKa) values at 8.3 and 8.0 reported for compounds A and B, respectively, indicated that protonation might occur at physiological pH. The LC50 values of 145,595 μg/ml (A) and 82,526 μg/ml (B) obtained from Brine shrimp lethality testing showed that both compounds were non-toxic. The two compounds possessed significant antimicrobial activity against bacterial and fungal strains; Escherichia coli, Staphylococcus aereus, Bacillus subtilis, Pseudomonas aeruginosa, Klebsiella pneumoniae, Salmonellae typhi, Candida albicans, Rhizopus stolonifer, Aspergillus niger and Penicillium notatum when compared with standards, gentamicin and tioconazole for bacteria and fungi, respectively. In vitro antioxidant screening by the DPPH free radical scavenging method and the scavenging effect on hydrogen peroxide showed that the compounds possessed significant antioxidant activity when compared with antioxidant standards ascorbic acid, butylated hydroxylanisole and α-tocopherol.