title: Hydrogen bonding of tryptophan radicals revealed by EPR at 700 GHz.
abstract: Redox-active tryptophans are important in biological electron transfer and redox biochemistry. Proteins can tune the electron transfer kinetics and redox potentials of tryptophan via control of the protonation state and the hydrogen-bond strength. We examine the local environment of two neutral tryptophan radicals (Trp108 on the solvent-exposed surface and Trp48 buried in the hydrophobic core) in two azurin variants. Ultrahigh-field EPR spectroscopy at 700 GHz and 25 T allowed complete resolution of all of the principal components of the g tensors of the two radicals and revealed significant differences in the g tensor anisotropies. The spectra together with (2)H ENDOR spectra and supporting DFT calculations show that the g tensor anisotropy is directly diagnostic of the presence or absence as well as the strength of a hydrogen bond to the indole nitrogen. The approach is a powerful one for identifying and characterizing hydrogen bonds that are critical in the regulation of tryptophan-assisted electron transfer and tryptophan-mediated redox chemistry in proteins.
authors: Department of Chemistry, University of California Davis, Davis, California 95616, United States. stst@uw.edu
authors: Stefan
authors: S
authors: Stoll
authors: Hannah S
authors: HS
authors: Shafaat
authors: J
authors: J
authors: Krzystek
authors: Andrew
authors: A
authors: Ozarowski
authors: Michael J
authors: MJ
authors: Tauber
authors: Judy E
authors: JE
authors: Kim
authors: R David
authors: RD
authors: Britt
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D006860:Hydrogen Bonding; D011789:Quantum Theory; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D014364:Tryptophan