Macroscopic characterization of bilayer membranes composed of triglyceride and phosphatidylcholine investigated using high-pressure ESR spin probe technique

The effect of high static pressure on the microscopic properties of triglyceride (HCO-10) and phosphatidylcholine (DLPC) membranes was investigated by ESR (electron spin resonance). The external pressure was varied to ascertain the distribution equilibrium constant between the aqueous and vesicle phases, depending on the molecular sizes of various nitroxide probes relative to that of the interstitial space of the HCO-10 and DLPC membranes. The distribution equilibrium of any probe for the HCO-10 membrane was shifted to the aqueous phase side with increasing the external pressure. However, the distribution equilibrium of di-tert-butyl nitroxide (DTBN) for the DLPC membrane was shifted to the vesicle phase side with increasing external pressure. Furthermore, information on the rotational motion of the nitroxide probes in the HCO-10 and DLPC vesicle was obtained from the anisotropic ESR signals. Rotational correlation times (τ_R) of the nitroxide probes in the vesicle phases increased with increasing external pressure. The magnitude of the activation volume (ΔV^‡) obtained using τ_R in both vesicles is TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)>DTBN>DMDEN (2,2-dimethyl-5,5-diethylpyrroline N-oxide), which might be attributed to the interaction between the probes and the membrane chains.