1-[4-(Trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH), a cationic analogue of diphenylhexatriene (DPH), has photophysical properties that are generally similar to those of DPH. In solution the fluorescence lifetime (r) of TMA-DPH is short (<1.5 ns), but r increases to ~7 ns when the probe is embedded in lipid bilayers at temperatures less than the thermal transition temperature (Tc) of the lipid. The cationic charge ensures that the probe is anchored at the lipid-water interface, most likely with the DPH moiety intercalated between the upper portions of the fatty acyl chains. The profiles of changes in steady-state anisotropies (rss) and limiting hindered anisotropies (r∞) are similar for both TMA-DPH and DPH embedded in lipid bilayers, but values for TMA-DPH even at T >> Tc are generally >0.14, e.g., at 35 °C in 1,2-dimyristoylglycero-3- phosphocholine (DMPC) (cf. 0.03 for DPH in DMPC at 35 °C). Electrostatic interactions of the cationic probe with head groups of phospholipids do not appear to significantly influence the apparent dynamics of the probe. TMA-DPH should prove useful in the study of the dynamics of phospholipid monolayers, e.g., in native or reconsituted lipoproteins.
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