On the Behavior of Indole-Containing Species Sequestered within Reverse Micelles at Sub-Zero Temperatures

We report on the effects of temperature (+30 to −100 °C) on the fluorescence from N-acetyl tryptophanamide (NATA) and human serum albumin (HSA) sequestered within Aerosol-OT (AOT) reversed micelles. NATA reports simultaneously from the polar and non-polar side of the reverse micelle interface. As the sample temperature decreases, the relative fraction of NATA molecules associated with the polar side increases. This redistribution process is characterized by ΔH = −14.8 ± 0.6 kJ/mol and ΔS = −54 ± 2 J/(K mol). The activation energy for thermal quenching (Ea,TQ) associated with the polar side NATA molecules is 6.7 kJ/mol before the micelles have shed water and 1.0 kJ/mol after water shedding (below approximately −20 °C). The time-resolved fluorescence intensity decay for tryptophan-214 in HSA is triple exponential. We suggest that these lifetimes arise from three indole residue conformations in equilibrium. Cooling the sample causes a freezing-in of the least quenched conformer; the other conformers are frozen out. The Ea,TQ value for the shortest lifetime component is 6 kJ/mol. The Ea,TQ for the long and intermediate lifetime components are equivalent (∼1.5 kJ/mol).