PHYSICOCHEMICAL PROPERTIES OF A MURAMYLDIPEPTIDE DERIVATIVE (B30-MDP) RELATED TO MEMBRANE FORMATION

We investigated the physicochemical properties of B30-MDP [6-O-(2-tetradecylhexadecanoyl)-N-acetyl-muramyl-L-alanyl-D-isoglutamine], a muramyldipeptide derivative having immunoadjuvant activity [1], using polarizing optical microscopy, differential scanning calorimetry (DSC), and electron spin resonance (ESR) spectroscopy. Microscopic observations showed that B30-MDP molecules form myelin figures in phosphate buffered saline (PBS). It was revealed that B30-MDP forms membranous structure because of an increase in the hydrophobicity. In the DSC measurements, the B30-MDP membrane in PBS gave no endothermic peak between 5 degrees to 50 degrees C. Enthalpy change upon the phase transition from the gel to liquid crystalline state of dipalmitoylphosphatidylcholine (DPPC) membrane and its phase transition temperature decreased by the addition of B30-MDP. ESR measurements using 5 doxyl stearic acid showed that the fluidity of the B30-MDP membrane was almost comparable to that of DPPC membrane at the temperature below the phase transition temperature of DPPC, while it was lower than that of DPPC at the temperature higher than this point. The fluidity of DPPC membrane increased upon the addition of B30-MDP. These results indicate that B30-MDP forms membranous structure and that the bulky hydrophilic region of B30-MDP influences its membrane structures, thermal behavior, and membrane fluidity.