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

Author:

Munson Chase A.1,Kelepouris Lee1,Baker Gary A.1,Baker Sheila N.1,Blanchard Gary J.1,Bright Frank V.1

Affiliation:

1. Department of Chemistry, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000 (C.A.M., F.V.B.); Current address: US Army Research Laboratory, AMSRD-ARL-WM-BD, Aberdeen Proving Ground, Maryland 21005-5069 (C.A.M.); Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 (L.K., G.J.B.); Current address: MPI Research, 54943 Main St., Mattawan, Michigan 49071 (L.K.); Chemical Sciences Division, Oak Ridge National...

Abstract

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).

Publisher

SAGE Publications

Subject

Spectroscopy,Instrumentation

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