In the present study, the structural characteristics that impart antibacterial activity to C16 alkynoic fatty acids (aFA) were further investigated. The syntheses of hexadecynoic acids (HDA) containing triple bonds at C‐3, C‐6, C‐8, C‐9, C‐10, and C‐12 were carried out in four steps and with an overall yield of 34–78%. In addition, HDA analogs containing a sulfur atom at either C‐4 or C‐5 were also prepared in 69–77% overall yields, respectively. Results from this study revealed that the triple bond at C‐2 is pivotal for the antibacterial activity displayed by 2‐HDA, while the farther the position of the triple bond from the carbonyl group, the lower its bactericidal activity against gram‐positive bacteria, including clinical isolates of methicillin‐resistant Staphylococcus aureus (CIMRSA) strains. The potential of 2‐HDA as an antibacterial agent was also assessed in five CIMRSA strains that were resistant to Ciprofloxacin (Cipro) demonstrating that 2‐HDA was the most effective treatment in inhibiting their growth when compared with either Cipro alone or equimolar combinations of Cipro and 2‐HDA. Moreover, it was proved that the inhibition of S. aureus DNA gyrase can be linked to the antibacterial activity displayed by 2‐HDA. Finally, it was determined that the ability of HDA analogs to form micelles can be linked to their decreased activity against gram‐positive bacteria, since critical micellar concentrations (CMC) between 50 and 300 μg/mL were obtained.

The naturally occurring (6Z)-(±)-2-methoxy-6-hexadecenoic acid (1) and (6Z)-(±)-2-methoxy-6-octadecenoic acid (2) were synthesized in 7–8 steps with 38% and 13% overall yields, respectively, by using an acetylide coupling approach, which made it possible to obtain a 100% cis-stereochemistry for the double bonds. In a similar fashion, the acetylenic analogs (±)-2-methoxy-6-hexadecynoic acid (3) and (±)-2-methoxy-6-octadecynoic acid (4) were also synthesized in 6–7 steps with 48% and 16% overall yields, respectively. The antibacterial activity of acids 1–4 was determined against clinical isolates of methicillin-resistant Staphylococcus aureus (ClMRSA) and E. coli. Among the series of compounds, acid 4 was the most active bactericide towards CIMRSA displaying IC50s (half maximal inhibitory concentrations) between 17–37 µg/mL, in sharp contrast to the 6-octadecynoic acid, which was not bactericidal at all. On the other hand, acids 1 and 3 were the only acids that displayed antibacterial activity towards E. coli, but 1 stood out as the best candidate with an IC50 of 21 µg/mL. The critical micelle concentrations (CMCs) of acids 1–4 were also determined. The C18 acids 2 and 4 displayed a five-fold lower CMC (15–20 µg/mL) than the C16 analogs 1 and 3 (70–100 µg/mL), indicating that 4 exerts its antibacterial activity in a micellar state. None of the studied acids were inhibitory towards S. aureus DNA gyrase discarding this type of enzyme inhibition as a possible antibacterial mechanism. It was concluded that the combination of α-methoxylation and C-6 unsaturation increases the bactericidal activity of the C16 and C18 FA towards the studied bacterial strains. Acids 1 and 4 stand out as viable candidates to be used against E. coli and CIMRSA, respectively.

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