Partition of Amphiphilic Molecules to Lipid Bilayers by ITC: Low-Affinity Solutes

A protocol is developed to allow
the accurate characterization
of partition to lipid bilayers for solutes with low affinity, using
isothermal titration calorimetry. The methodology proposed is suitable
for studies using complex membranes, such as intact biomembranes or
whole cells. In the method developed, the association is characterized
at increasing solute concentrations. This allows the characterization
of solute partition into unperturbed membranes, as well as effects
induced by high solute concentrations. Most druglike molecules are
expected to interact with low-to-moderate affinity with relevant cell
membranes. This is due to both the need for a relatively high aqueous
solubility of the drug and the poor binding properties of the cell
membranes. The methodology is applied to characterize the interaction
of antibiotic Rifampicin with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and with lipid bilayers representative
of bacterial membranes.