Lasalocid Acid Antibiotic at a Membrane Surface Probed by Sum Frequency Generation Spectroscopy

Carboxyl polyether ionophores (CPIs) are widely used as veterinary antibiotics and to increase food utilization in ruminating animals. Furthermore, CPIs can target drug-resistant bacteria, but detailed knowledge about their mode-of-action is needed to develop agents with a reasonable therapeutic index. It has been suggested that ionophores bind to membranes and incur large structural changes to shield a bound ion from the hydrophobic environment of the lipid bilayer for transport. One crucial piece of information is missing, however: Is it necessary for the free ionophore to adsorb on the membrane surface before interacting with a cation to facilitate cross-membrane ion transport? To answer this question, we applied sum-frequency generation (SFG) vibrational spectroscopy and surface tensiometry to identify the interaction between the prototypical CPI lasalocid acid (LA) and a model membrane. Observed changes in the surface pressure demonstrate that the free LA undergoes a self-assembly process with the lipid monolayer. Spectra taken from the lipid monolayer show that the free acid inserts partially into the lipid monolayer and then after complexation with sodium chloride disrupts the lipid monolayer. Overall, this study strongly suggests that this must be the crucial step of LA and metal ion complexation that allows the ionophore to traverse a lipid membrane.