Spirocyclic lactams and curvulinic acid derivatives from the endophytic fungus Curvularia lunata and their antibacterial and antifungal activities.

Curvularia lunata, isolated from the capitula of Paepalanthus chiquitensis (Eriocaulaceae), was cultured in potato dextrose broth (PDB) medium. The ethyl acetate extract yielded two new spirocyclic γ-lactams (3 and 4), and five known compounds, namely: triticones E (1) and F (2), 5-O-methylcurvulinic acid (5), curvulinic acid (6) and curvulin (7). Their structures were elucidated by spectroscopic analysis and by the comparison with literature data. Besides, a computational study was used to elucidate the absolute configuration of the C - 3' in the compounds (3) and (4). The extract and the compounds (1 and 2), (6) and (7) were assayed against gram-positive and gram-negative bacteria and fluconazole-resistant yeast. The triticones (1) and (2) showed good antibacterial activity for Escherichia coli, with a minimum inhibitory concentration of 62.5 µg/mL.

A semiempirical study of acetylcholine hydrolysis catalyzed by Drosophila melanogaster acetylcholinesterase.

The enzymatic mechanism of acetylcholine hydrolysis was evaluated by semiempirical molecular orbital calculations with a model constructed with the coordinates of sixteen amino acids and four water molecules from the crystallographic structure of Drosophila melanogaster acetylcholinesterase (AChE, entry 1QO9 in the Protein Data Bank). Nine proposed reaction points for the hydrolysis mechanism were obtained, including four for the acylation step and five for the deacylation step. Our results indicate that in the Michaelis complex of the acylation step, a looser interaction between the substrate and the oxyanion hole may result from an amino acid change in the acyl pocket observed in insect as compared to the vertebrate enzyme. Detailed descriptions of the reaction profile for the formation of both acylation and deacylation tetrahedral intermediates were obtained. The results indicate the occurrence of partially concerted mechanisms, with deprotonation of the nucleophiles (Ser238 in the acylation step and a water molecule in the deacylation step) by His480 facilitating the nucleophilic additions. Both processes were completed by enthalpically favorable steps, formation of choline in the acylation step and of acetic acid in the deacylation step.