Fluorescence spectra decomposition by asymmetric functions: Laurdan spectrum revisited

Due to their asymmetric nature, complex fluorescence spectra of molecules can be analyzed much better by log-normal distributions than by Gaussian ones. So far, the log-normal function has been used for deconvolution of emission spectra of different fluorescent molecules, such as Tryptophan and Prodan, but to our knowledge it is far less used for Laurdan (2-dimethylamino-6-lauroylnaphthalene). In this article, we present the decomposition of Laurdan emission spectra in large unilamellar vesicles using a procedure that relies on the log-normal asymmetric function. The procedure was calibrated using Laurdan spectra in homogeneous solutions of various solvents. Comparing our results with the ones obtained from a Gaussian fit, we show that (i) the position of the elementary peaks (~440 and 490 nm) is preserved in a large range of temperatures that include the main phase transition of lipid bilayer and (ii) the bilayer hydration, as reported by Laurdan, increases approximately 8 times from the gel phase to the liquid crystalline one, a result that fits with other reports, providing a more realistic description. In addition, we propose a new parameter to globally evaluate Laurdan emission spectra with the prospect of acquiring a larger range of values than the classical "generalized polarization".