Two-step energy transfer enables use of phenylalanine in action-EET for distance constraint determination in gaseous biomolecules

Two-step energy transfer is potentially useful for exploring macromolecular structure, but it has not been observed previously in the gas-phase. Single step excitation energy transfer (EET) has been recently documented for tyrosine and tryptophan containing peptides, but not for phenylalanine. Herein, we report sequential energy transfer from phenylalanine to tyrosine to a disulfide, resulting in homolytic cleavage of a sulfur–sulfur bond. Interestingly, energy transfer from phenylalanine is only observed in the presence of tyrosine and only occurs within certain distance constraints. Isolated, electronically excited phenylalanine is known to have an extremely long lifetime in the gas phase, potentially suggesting quicker relaxation occurs via energy transfer to tyrosine. Alternatively, the direct overlap of states between phenylalanine and disulfide bonds is predicted to be poor, in which case tyrosine would serve to bridge the gap. In either case, the distance constraints imposed by this two-step EET are shown to be useful for evaluation and determination of gaseous biomolecular structure.