Dr. Laura F. Cooley

I am interested in using different aspects of fluorescence spectroscopy to deduce and better understand processes that occur in or between molecules. When molecules absorb ultraviolet light, they may be raised to excited states. Being in an excited state opens up new avenues to the molecule – for example a reaction could occur that might not happen from the ground state. The extra energy may be released as light. This is called fluorescence, or more generally, luminescence. The frequency and intensity of the emitted light can yield interesting information about what is going on in the molecules. At the moment, I am using fluorescence from the amino acid tryptophan to deduce the strength of the binding of tryptophan to micelles. Micelles are spherical aggregates of detergent molecules. (We are currently using sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) detergents) These aggregates have a nonpolar interior and a charged exterior, and so can solvate nonpolar molecules in their interior while remaining soluble in water.  
In this study, fluorescence is simply the tool we are using to measure the equilibrium constant for binding of tryptophan to micelles. Fluorescence is being used, but with a twist – a method known as fluorescence anisotropy. In this method, the sample (tryptophan and SDS or CTAB micelles) is excited with polarized light. Then, the intensity of emitted light that is both parallel and perpendicular to the excitation light is measured. If the tryptophan is not bound to a micelle, it will rotate in solution very quickly, and this rotation will cause the emitted light to be polarized equally in all directions, or be “depolarized”. In this case, there is no anisotropy of fluorescence. If, however, the tryptophan is bound to a micelle, it will rotate much more slowly. In this case, more of the light that it emits will be in a direction parallel to the direction of the excitation light and less will be in the perpendicular direction. Thus fluorescence anisotropy is a measure of binding of the tryptophan to micelles, and with some analysis should yield a binding constant. 

My interests are in using physical measurements like this to deduce interesting and important properties of molecular systems. Micelles resemble cell membranes with the polar and nonpolar properties described earlier, so this system may be considered a model for how proteins attach to cell membranes, giving this work relevance to important biological systems. ​​​​