Ultrafast Energy Transfer from Photoexcited Tryptophan to the Haem in Cytochrome c

We report femtosecond Fe K-edge absorption (XAS) and nonresonant X-ray emission (XES) spectra of ferric cytochrome C (Cyt c) upon excitation of the haem (>300 nm) or mixed excitation of the haem and tryptophan (<300 nm). The XAS and XES transients obtained in both excitation energy ranges show no evidence for electron transfer processes between photoexcited tryptophan (Trp) and the haem, but rather an ultrafast energy transfer, in agreement with previous ultrafast optical fluorescence and transient absorption studies. The reported (J. Phys. Chem. B 2011, 115 (46), 13723-13730) decay times of Trp fluorescence in ferrous (similar to 350 fs) and ferric (similar to 700 fs) Cyt c are among the shortest ever reported for Trp in a protein. The observed time scales cannot be rationalized in terms of Fo''rster or Dexter energy transfer mechanisms and call for a more thorough theoretical investigation.

Automated summary

We report femtosecond X-ray absorption and emission spectra of ferric cytochrome C upon excitation of the haem or mixed excitation of the haem and tryptophan. The results suggest an ultrafast energy transfer between tryptophan and the haem, which is consistent with previous studies. The observed decay times of tryptophan fluorescence in ferrous and ferric cytochrome C are among the shortest reported and call for further theoretical investigation.