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

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Science  07 Feb 2013:
DOI: 10.1126/science.1230758

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Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. Using ultra-broadband, ultrafast two-dimensional (2D) spectroscopy in the ultraviolet 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 > 60%, over an edge-to-edge distance below ~10 Å, out-competing the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.

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