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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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78 Weljie and Vogel<br />

Fig. 3. Steady-state tryptophan fluorescence spectra of a synthetic peptide encompassing<br />

the CaM-binding domain of skeletal myosin light chain kinase alone (filled<br />

rectangles), and in complexes with wild-type CaM (filled triangles), a mutant<br />

calmodulin where the C-terminal methionine residues were replaced with leucine<br />

(filled ovals), and a CaM where the methionine residues were replaced by<br />

selenomethionine. Note that there is a blue-shift in the observed peak intensity of tryptophan<br />

fluorescence once the fluorophore is sequestered into the hydrophobic binding<br />

pocket. Also, changing the envrionment of the tryptophan by altering the chemical<br />

nature of the CaM side chains produces remarkable intensity variations (10,12).<br />

Once such a complex is formed and the peak fluorescence known, one can<br />

establish the degree of solvent exposure via quenching experiments. Fluorescence<br />

is said to be quenched when a species proximal to the fluorophore provides<br />

an alternate relaxation pathway for the excited electronic state to return<br />

to the ground state. Common quenching agents include potassium iodide,<br />

cesium chloride, and acrylamide. The degree of quenching is generally given<br />

using a Stern-Volmer plot, from which one obtains Stern-Volmer quenching<br />

constants (K SV) (see Fig. 4).<br />

Detailed below is a method for acquiring simple tryptophan emission spectra,<br />

and subsequently obtaining Stern-Volmer quenching constants. Selected<br />

examples of other applications of fluorescence to calcium-binding proteins<br />

examples are provided in Subheading 4.2.

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