A closer look into laurdan as a probe to monitor cationic DODAB bilayers

Abstract

Laurdan is a fluorescent molecule largely used to probe the lipid packing and hydration of membranes. In phospholipid bilayers that undergo the gel-fluid thermal transition, Laurdan fluorescent band displays a significant red shift upon the transition. However, Laurdan in DODAB bilayers does not show this characteristic shift of the emission band. Hence, the analysis of the membrane fluidity through the so-called Generalized Polarization (GP) cannot be used with DODAB. Nevertheless, both static and time resolved fluorescence attest that the Laurdan fluorescence in DODAB, like in phospholipids, is composed of two emission bands. The emission bands are associated with excited states with different lifetimes, and the relative intensity of the bands is sensitive to the bilayer thermal phase. Analyses were performed by decomposing the emission spectrum into two Gaussian bands and by computing the Decay Associated Spectra (DAS), the latter with time resolved fluorescence. We conclude that Laurdan in DODAB bilayers is in a shallower position as compared with the probe in phospholipids, possibly due to the small DODAB head group. Hence, Laurdan could be a useful probe to monitor interactions at the surface of DODAB bilayers, for instance with genetic material, quite often associated with cationic vesicles in lipoplexes, with broad medical applications.