Dynamic hydration of phospholipid films in aqueous environments

A dynamic study of the hydration of phospholipid films attached to solid substrates when exposed to liquid water at room temperature is presented. The phospholipids used were 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (DPPE), and a modification of DPPE with a fluorescent molecular probe: n-(5-fluoresceinthicarbamoyl)-1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine, triethylammonium salt (FITC-DPPE). Three different experimental techniques were used: rates of hydration were measured by depositing the phospholipid film on a quartz crystal microbalance (QCM), total internal reflection fluorescence (TIRF) studies were carried out with FITC-DPPE films deposited on the internal surface of a cylindrical quartz tube, and X-ray diffraction analysis was used to determine possible changes of the film’s crystalline structure during hydration. The DPPE films exhibited different successive hydration stages: within the first 2 h, the films uniformly hydrate towards a limiting water uptake (short-time behavior); however, at one point, hydration rates suddenly increase and the hydration process continues for longer periods of time, of the order of 24 h. No evidence of change in the film crystalline structure was found when dry and hydrated DPPE films were analyzed by wide-angle X-ray diffraction. The FITC-DPPE films showed a completely different hydration pattern: water uptake reached a maximum value at short times and then decreased continuously until an asymptotic value was reached. The TIRF results on FITC-DPPE films show that the evolution of fluorescence with time closely resembles the hydration results obtained in the QCM. This is attributed to the self quenching occurring in the phospholipid films.