Conformational studies of Gram-negative bacterial quorum sensing 3-oxo N-acyl homoserine lactone molecules

In their ¹H NMR spectra in CDCl₃ 3-oxo-N-acyl homoserine lactones (OHLs) show significant downfield chemical shifts of the amide NH proton when compared to the parent N-acyl homoserine lactones (AHLs). NMR spectroscopic and DFT calculation studies have shown that this is most likely due to the presence of a stabilising intramolecular H-bond from the NH to the 3-oxo group. The ¹H NMR spectra also show evidence for the enol tautomers and that the amount of enol present for a range of OHLs is 4.1-4.5% in CDCl₃ and 6.5-7.2% in CD₃CN. In contrast, DFT calculations show that the lowest energy enol tautomer and the keto tautomer are of equal energy in the gas phase, but that the keto tautomer is more stable in chloroform, acetonitrile and water solution. The calculations also show that there is no evidence for any n→π^∗ or C5H-bonding interactions being present in either the lowest energy keto or enol tautomer of the OHLs in solution or the gas phase, which is in contrast to the reported solid-state structure.