Phosphorescence Lifetimes

Glutamine 320 forms a hydrogen bond with the enamine of tryptophan 109 which stabilizes the indole ring. Replacing this glutamine with a leucine removes this hydrogen bond and therefore results in more 'flexibility' for the tryptophan. We would expect that this increased flexibility should result in a shorter RTP lifetime, which is what we observe [Figure 1, below]. If we replace the glutamine with a glycine we are also creating an additional cavity around the tryptophan which should impart even more flexibility and therefore we would expect an even shorter RTP lifetime. Again, this is what we observe [Figure 1, below].

Figure 1: Plot of the phosphorescence lifetime as a function of temperature for the glutamine mutants studied in these experiments.

Hydrogen Exchange Theory