# Question 19.5: You are designing a FRET experiment to determine the magnitu...

You are designing a FRET experiment to determine the magnitude of the structural change introduced by substrate binding to an enzyme. Using site specific mutagenesis, you have constructed a mutant form of the enzyme that possesses a single tyrosine residue and a single tryptophan residue, and these residues are separated by $11 \mathring{A}$. You would like to determine if the distance between these residues changes with substrate binding. The fluorescence of tyrosine overlaps with the tryptophan absorption; therefore, these two amino acids form a FRET pair for which $r_{0}=9 \mathring{A}$, determined using the absorption and emission spectra in combination with Equation (19.181). Calculate the FRET efficiency at $11 \mathring{A}$ separation and how much this distance must increase in order for the efficiency to decrease by $20\%$, the experimental detection limit.

$r_{0}\left(\mathring{A}\right)=8.79 ×10^{-5}\left( \frac{k^{2}J\Phi_{f}}{n^{4}} \right)^{1/6}$                 (19.181)

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