INFLUENCE OF CATIONIC ANTIBIOTICS ON PHASE-BEHAVIOR OF ROUGH-FORM LIPOPOLYSACCHARIDE

The rough-form lipopolysaccharide (LPS) interacted with cationic antib iotic polymyxin B and gramicidin S in solution, and showed altered the rmotropic phase behavior and viscoelasticity. The phase behavior was m easured by differential scanning calorimetry and quartz crystal microb alance (QCM). Addition of polymyxin B of up to 0.5 mg/mL to the 5.0 mg /mL LPS solution increased gel-to-liquid crystalline phase transition enthalpy (Delta H) and raised the transition temperature (t(max)). The further addition of polymyxin B reduced the Delta H value. Gramicidin S produced a different effect, whereby a minor addition reduced t(max ) and Delta H value of the LPS. The LPS film on the platinum electrode of the QCM indicated a downward shift of resonant frequency and an up ward shift of resonant resistance when in contact with the antibiotic solution. An interpretation of these variations is that the LPS on the QCM electrode changed not only film weight, but also viscoelasticity owing to contact with the antibiotic solution. The different effects b etween the antibiotics between polymyxin B and gramicidin S on the LPS are induced by the difference of the governing effect. Polymyxin B in teracts with the LPS electrostatically, whereas gramicidin S interacts by hydrophobic moieties.