Difference in chiral recognition of gel and liquid-crystalline phases of phosphatidylcholine vesicles as determined by circular dichroism spectroscopy

The circular dichroism (CD) spectra of three types of (L)phosphatidylcholine (PC) vesicles in aqueous solution showed differences in the sign and intensity of the Cotton effect compared with those of monomers in ethanol, indicating the existence of chiral environments in these vesicles. From the temperature dependence of CD intensities, the main phase transition temperatures between gel (Gel) and liquid-crystalline (LC) phases of the vesicles were estimated to be 40, 23, and 55 degrees C for dipalmitoyl PC, dimyristoyl PC and distearoyl PC, respectively. Furthermore, both low-fluidity Gel and high-fluidity LC phases recognized the chirality of incorporated 2-hydroxymethyl[5]thiaheterohelicene (5HM) with a helical structure, which undergoes a rapid racemization owing to a weak repulsion between the terminal hydrogen atoms. The ability for chiral recognition was evaluated using thermodynamic parameters for the equilibrium between P and M enantiomers of 5HM in the vesicles; the Gel phase manifested a higher recognition ability than the LC phase.