Abstract:
Chlorhexidine is an effective antiseptic used widely in disinfecting products (hand
soap), oral products (mouthwash), and is known to have potential applications in the
textile industry. Chlorhexidine has been studied extensively through a biological and
biochemical lens, showing evidence that it attacks the semipermeable membrane in
bacterial cells. Although extremely lethal to bacterial cells, the present understanding
of the exact mode of action of chlorhexidine is incomplete.
A biophysical approach has been taken to investigate the potential location of
chlorhexidine in the lipid bilayer. Deuterium nuclear magnetic resonance was used
to characterize the molecular arrangement of mixed phospholipid/drug formulations.
Powder spectra were analyzed using the de-Pake-ing technique, a method capable of
extracting both the orientation distribution and the anisotropy distribution functions
simultaneously. The results from samples of protonated phospholipids mixed with
deuterium-labelled chlorhexidine are compared to those from samples of deuterated
phospholipids and protonated chlorhexidine to determine its location in the lipid
bilayer.
A series of neutron scattering experiments were also conducted to study the biophysical
interaction of chlorhexidine with a model phospholipid membrane of DMPC,
a common saturated lipid found in bacterial cell membranes. The results found the
hexamethylene linker to be located at the depth of the glycerol/phosphate region of
the lipid bilayer. As drug concentration was increased in samples, a dramatic decrease in bilayer thickness was observed.
Differential scanning calorimetry experiments have revealed a depression of the
DMPC bilayer gel-to-lamellar phase transition temperature with an increasing drug
concentration. The enthalpy of the transition remained the same for all drug concentrations,
indicating a strictly drug/headgroup interaction, thus supporting the
proposed location of chlorhexidine.
In combination, these results lead to the hypothesis that the drug is folded approximately
in half on its hexamethylene linker, with the hydrophobic linker at the depth
of the glycerol/phosphate region of the lipid bilayer and the hydrophilic chlorophenyl
groups located at the lipid headgroup. This arrangement seems to suggest that the
drug molecule acts as a wedge to disrupt the bilayer. In vivo, this should make
the cell membrane leaky, which is in agreement with a wide range of bacteriological
observations.
