In class 1a ribonucleotide reductase (RNR), a substrate-based radical is generated in the α2 subunit by long-distance electron transfer involving an essential tyrosyl radical (Y122O·) in the β2 subunit. The conserved W48 β2 is∼10 Å from Y122OH; mutations at W48 inactivate RNR. Here, we design a beta hairpin peptide, which contains such an interacting tyrosine-tryptophan dyad. The NMR structure of the peptide establishes that there is no direct hydrogen bond between the phenol and the indole rings. However, electronic coupling between the tyrosine and tryptophan occurs in the peptide. In addition, downshifted ultraviolet resonance Raman (UVRR) frequencies are observed for the radical state, reproducing spectral downshifts observed for β2. The frequency downshifts of the ring and CO bands are consistent with charge transfer from YO· to W or another residue. Such a charge transfer mechanism implies a role for the β2 Y-W dyad in electron transfer.
Bibliographical noteFunding Information:
Supported by NSF CLP CHE-1213350 (B.A.B.) and NIH GM-100310 (G.V.). T.G.M. was partially supported by a GAANN program in Molecular Biophysics and Biotechnology from the Department of Education. We thank Professor N. Hud and Professor J. Reynolds for the use of the CD and the potentiostat, respectively. We are grateful to Dr A. Österholm for advice concerning the DPV measurements.