Conclusions
Recent work has clearly shown that it is possible to explain the different lifetimes of a single Trp-residues in terms of the possibility of having different rotameric states that interconvert only slowly if at all. The dead end elimination method has allowed us to explore these rotamers and in many cases links could be made with lifetimes on the basis of differences in accessibility. Experimental indications for the existence of different rotameric states are becoming available. The ultimate proof for the existence of different microstates that slowly interconvert would come from lifetime measurements on isolated (single) molecules of single-tryptophan proteins. But this is not an easy task, in view of the low photostability of Trp. Finally the analysis of the electron transfer as presented here can only be considered to be a pragmatic description and the parameters obtained have to used with great care in view of the selection of “simple” Trp-environments. For more complex environments a thorough quantum mechanical calculation, using the charge transfer model presented by Callis and Vivian might be necessary for each individual case.