On what time scale does solvent relaxation in phospholipid bilayers happen?

Time-resolved emission spectra of seven fluorescent probes in egg-phosphatidylcholine bilayers have been investigated. About 90% of the solvent relaxation monitored by the headgroup labels Prodan, Laurdan, and Patman and by the backbone label 2-AS can be captured with an instrument providing subnanosecond time resolution. In comparison to 2-AS, the transient red-shift of 9-AS is characterized by a larger contribution of a picosecond process and by slower nanosecond dynamics, The major contribution to solvent relaxation probed by C(17)DiFU and Dauda is faster than the ultimate time resolution of the experiment; those chromophores appear to be located within the external interface of the bilayer.