Structural modifications of lipid vesicles by 8-methoxypsoralen.

On prolonged UV-A illumination the ESR spectrum of 16-doxylstearic acid in dimyristoyl phosphatidylcholine vesicles loaded with 8-methoxypsoralen changed dramatically as a second broad component gradually appeared. The composite nature of the spectrum was proven by digital subtraction and the two components were simulated using the modified Bloch equations. From the spectral anisotropy of the motionally averaged three-line component, the molecular order was found to decrease during UV-A illumination, and isotropic rotation of the spin probe was observed well below the phase transition temperature. In parallel an exchange averaged component dominated the central part of the spectrum, indicating the segregation of spin probes into separate domains. The build-up of the composite lineshape was followed by fitting weighted sums of the above two components to spectra recorded after various time intervals. It is concluded that PUVA therapy, which is routinely used against psoriasis, might also induce overlooked damage to the saturated lipid species of biological membranes.