Imaging membrane order using environmentally sensitive fluorophores

In the lipid raft hypothesis, ordered and disordered lipid membranes are responsible for regulating the distribution, dynamics, and interactions of membrane associated proteins. Ordered and disordered bilayers may be distinguished by the degree of order in their acyl tails (the order parameter) which in turn affects lipid mobility and lipid packing. Low density lipid packing in the disordered phase allows polar water molecules to penetrate into the usually non-polar bilayer interior. Transition to the ordered phase causes condensation of the membrane, tighter lipid packing, and more complete exclusion of polar water. This process can be measured and quantified using polarity sensitive fluorophores embedded within the bilayer which then have different emission properties depending on membrane phase. Two examples of these are Laurdan and di-4-ANEPPDHQ which can be used to image membrane order distributions in live cells via a variety of microscopy techniques.