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

Published Online February 7 2013
Science 29 March 2013:
Vol. 339 no. 6127 pp. 1586-1589
DOI: 10.1126/science.1230758

1Laboratory of Ultrafast Spectroscopy, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

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.