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

• Published in: Science (American Association for the Advancement of Science) 2013-03, Vol.339 (6127), p.1586-1589
• Main Authors: Consani, Cristina, Auböck, Gerald, van Mourik, Frank, Chergui, Majed
• Format: Article
• Language: English
• Subjects: Absorption spectra; Amino acids; Animals; Broadband; Chromophores; Dadaism; Electron transfer; Electron Transport; Endangered Species Act; Energy; Energy transfer; Excitation spectra; Fluorescence Resonance Energy Transfer; Fretting; Heme - chemistry; Horses; Macromolecules; Molecular biology; Myoglobin; Myoglobin - chemistry; Protein Structure, Secondary; Signals; Spectral energy distribution; Spectrophotometry, Ultraviolet - methods; Spectroscopy; Spectrum analysis; Tryptophan; Tryptophan - chemistry; Ultraviolet spectroscopy; Visible spectrum; Wave excitation; Wavelengths
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1230758

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 add 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.