There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
The sensitivity of Laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) excitation and
emission spectra to the physical state of the membrane arises from dipolar relaxation
processes in the membrane region surrounding the Laurdan molecule. Experiments performed
using phospholipid vesicles composed of phospholipids with different polar head groups
show that this part of the molecule is not responsible for the observed effects. Also,
pH titration in the range from pH 4 to 10 shows that the spectral variations are independent
of the charge of the polar head. A two-state model of dipolar relaxation is used to
qualitatively explain the behavior of Laurdan. It is concluded that the presence of
water molecules in the phospholipid matrix are responsible for the spectral properties
of Laurdan in the gel phase. In the liquid crystalline phase there is a relaxation
process that we attribute to water molecules that can reorientate during the few nanoseconds
of the excited state lifetime. The quantitation of lipid phases is obtained using
generalized polarization which, after proper choice of excitation and emission wavelengths,
satisfies a simple addition rule.