Lack of Environmental Sensitivity of a Naturally Occurring Fluorescent Analog of Cholesterol

Dehydroergosterol (DHE, Δ^5,7,9(11),22-ergostatetraen-3β-ol) is a naturally occurring fluorescent analog of cholesterol found in yeast. Since DHE has been shown to faithfully mimic cholesterol in a large number of biophysical, biochemical, and cell biological studies, it is widely used to explore cholesterol organization, dynamics and trafficking in model and biological membranes. In this work, we show that DHE, in spite of its localization at the membrane interface, does not exhibit red edge excitation shift (REES) in model membranes, irrespective of the membrane phase. These results are reinforced by semi-empirical quantum chemical calculations of dipole moment changes of DHE in ground and excited states, which show a very small change in the dipole moment of DHE upon excitation. We conclude that DHE fluorescence exhibits lack of environmental sensitivity, despite its usefulness in monitoring cholesterol organization, dynamics and traffic in model and biological membranes.