Ultrafast Tryptophan-to-Heme Electron Transfer in Myoglobins Revealed by UV 2D Spectroscopy

Fretting About Electrons The FRET technique (Fluorescence Resonance Energy Transfer) is widely used to probe structural dynamics in large macromolecules such as proteins. Essentially, the technique relies on photoexciting a donor chromophore and then watching for signs of energy transfer to an acceptor chromophore elsewhere in the framework. Consani et al. (p. 1586, published online 7 February; see the Perspective by Winkler) now show, using a sophisticated type of broadband time-resolved ultraviolet spectroscopy, that in myoglobin, an excited tryptophan residue relaxes by electron, rather than energy, transfer. The distinction is generally difficult to observe but has strong bearing on the applicability of FRET in this and analogous systems. Relaxation in a photoexcited protein by electron transfer may limit the generality of a common energy transfer–based probe. [Also see Perspective by Winkler] Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. By using ultra-broadband ultrafast two-dimensional (2D) spectroscopy in the ultraviolet (UV) and transient absorption in the visible range, we have disentangled the excited state decay pathways of the tryptophan amino acid residues in ferric myoglobins (MbCN and metMb). Whereas the more distant tryptophan (Trp7) relaxes by energy transfer to the heme, Trp14 excitation predominantly decays by electron transfer to the heme. The excited Trp14→heme electron transfer occurs in <40 picoseconds with a quantum yield of more than 60%, over an edge-to-edge distance below ~10 angstroms, outcompeting the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.