The aqueous stability of N-acyl-L-homoserine lactones, their affinity for cyclodextrin inclusion, and the investigation of a catalyst for their hydrolysis

Abstract

Many bacteria demonstrate population-density dependent gene expression, a phenomenon known as quorum sensing, to regulate phenotypes that, if expressed by an individual bacterium, would be ineffective. While some of these phenotypes are innocuous (e.g., exhibiting bioluminescence), certain bacteria employ quorum sensing regulated phenotypes in their pathogenesis. Traditionally, infections by these pathogens have been treated with antibiotic therapies. However, with the emergence of multidrug resistant strains, alternative strategies are being considered for attenuation of quorum sensing regulated traits that do not apply selective pressures. Macrocycles — in particular the cyclodextrins — that are capable of forming inclusion complexes with Gram-negative quorum sensing signal molecules (N-acyl-L-homoserine lactones, AHLs), which ultimately regulate phenotype ex-pression have been investigated for their anti-quorum sensing capabilities; however, the conditions employed in numerous such assays with respect to pH have been suspect. In this work, we monitored the hydrolysis kinetics of AHLs at different pHs using1H-NMR to evaluate previous AHL-centric studies, defining conditions under which the signal molecules can be confidently studied. Then, using AHL analogues, we evaluated how the hydrolysis of AHLs affects their affinity for macrocycle-inclusion. Having found that AHLs are preferentially bound byβ-cyclodextrin over their hydrolyzed forms, we evaluated a known ester hydrolysis catalyst for activity against AHLs in hopes that it could be appended to the macrocycle, generating an artificial lactonase. While our pursuit was unsuccessful in that our candidate catalyst did not successfully accelerate AHL hydrolysis, we remain confident that a chemical species capable of independently hydrolyzing lac-tones under aqueous conditions exists, can be appended to cyclodextrin, and serve as an artificial lactonase to better quench quorum sensing regulated phenotypes and be used to improve outcomes of organisms adversely affected by them.