Competition between tryptophan fluorescence and electron transfer during unfolding of the villin headpiece.

The 35-residue, C-terminal headpiece subdomain of the protein villin folds to a stable structure on a microsecond time scale and has served as a model system in numerous studies of protein folding. To obtain a convenient spectroscopic probe of the folding dynamics, Kubelka et al. introduced an ionized histidine residue at position 27, with the expectation that it would quench the fluorescence of tryptophan 23 in the folded protein by extracting an electron from the excited indole ring [Kubelka, J., et al. (2003) J. Mol. Biol. 329, 625-630]. Although the fluorescence yield decreased as anticipated when the protein folded, it was not clear that the side chains of the two residues were sufficiently close together for electron transfer to compete effectively with fluorescence. Here, hybrid classical-quantum mechanical molecular dynamics simulations are used to examine the rates of transfer of an electron from the excited tryptophan to various possible acceptors in the modified headpiece and a smaller fragment comprised of residues 21-27 (HP7). The dominant reaction is found to be transfer to the amide group on the carboxyl side of W23 (amide a24). This process is energetically favorable and has a large coupling factor in the folded protein at 280 K but becomes unfavorable as HP7 unfolds at higher temperatures. Changes in electrostatic interactions of the solvent and other parts of the protein with the indole ring and a24 contribute importantly to this change in energy.