Evaluation of dielectric relaxation spectrum of phospholipids in solution by time domain reflectometry

Time domain reflectometry (TDR) was applied to dielectric relaxation measurements and relaxation spectra of four kinds of phospholipids (dipalmitoyl phosphatidylcholine, egg lecithin, phosphatidyl ethanolamine, and sphingomyelin) and were evaluated in chloroform solutions. Fast measurements covering a wide frequency range from 1 MHz to 10 GHz could be done easily for materials having multiple relaxations and dc conductivity. A procedure of extracting individual spectra from the complicated complex permittivity has been presented. Reasonable assignment of molecular motion could be given to each spectrum. The first spectrum observed at about 1.5 ns is caused by an overall rotation of the whole molecule in the solution. The second spectrum at 0.2–0.4 ns is related to a motion of the zwitterionic head group. The third spectrum observed in lecithin at 50–80 ps is mainly caused by internal motions of the ester groups. It is concluded that the TDR method is quite suitable for the investigation of molecular dynamics of biomaterials.