High-frequency ESR at ACERT.

High-field ESR offers many advantages in exploring fundamental questions of structure and dynamics in chemical, biological and physical samples. We provide a review of recent work performed at ACERT demonstrating the utility and flexibility of our methods for extracting both qualitative and quantitative information from a variety of systems. In particular, we emphasize the utility of multi-frequency ESR techniques for unraveling the details of the complex dynamical modes of proteins in solution and in heterogeneous systems such as lipid bilayers. We also include indications of directions for future work where appropriate.

Displacement of the tyrosyl radical cofactor in ribonucleotide reductase obtained by single-crystal high-field EPR and 1.4-A x-ray data.

The R2 protein of class I ribonucleotide reductase generates and stores a tyrosyl radical essential for ribonucleotide reduction and, thus, DNA synthesis. X-ray structures of the protein have enabled detailed mechanistic suggestions, but no structural information has been available for the active radical-containing state of the protein. Here we report on methods to generate the functional tyrosyl radical in single crystals of R2 from Escherichia coli (Y122(*)). We further report on subsequent high-field EPR experiments on the radical-containing crystals. A full rotational pattern of the spectra was collected and the orientation of the g-tensor axes were determined, which directly reflect the orientation of the radical in the crystal frame. The EPR data are discussed in comparison with a 1.42-A x-ray structure of the met (oxidized) form of the protein, also presented in this paper. Comparison of the orientation of the radical Y122(*) obtained from high-field EPR with that of the reduced tyrosine Y122-OH reveals a significant rotation of the tyrosyl side chain, away from the diiron center, in the active radical state. Implications for the radical transfer connecting the diiron site in R2 with the substrate-binding site in R1 are discussed. In addition, the present study demonstrates that structural and functional information about active radical states can be obtained by combined x-ray and high-field EPR crystallography.