Application of Angle-Selected Electron Nuclear Double Resonance to Characterize Structured Solvent in Small Molecules and Macromolecules

Abstract

Applications of angle-selected electron nuclear double resonance (ENDOR) spectroscopy using the vanadyl (VO 2+ ) cation or nitroxyl spin-labels as paramagnetic probes are reviewed in which structured solvent in small molecule and macromolecular complexes have been identified and characterized. By determination of the principal hyperfine (hf) coupling components (A) of magnetic nuclei in the vicinity of the paramagnetic probe, electron-nucleus distances are estimated according to the dipolar equation, and the relative coordinates of the nuclei are assigned on the basis of the orientation dependence of magnetic interactions. The precision in determining electron-nucleus distances over an approximate 3{\textendash}8 {Å} range for VO 2+ and 4{\textendash}11 {Å} range for spinlabels is generally S5% based on ENDOR line widths and is exceeded only by that associated with X-ray diffraction of single crystals. The detailed structure of metal-bound or hydrogen-bonded solvent in small molecules, nucleotide complexes, and proteins in frozen glassy solutions is described. In particular, comparison of the structural details of solvent hydrogen-bonded to spin-labeled \ensuremath{\beta}-lactam antibiotics free in solution and sequestered in the active site of a spinlabeled penicilloyl acylenzyme reaction intermediate of TEM-1 \ensuremath{\beta}-lactamase is shown to bring improved understanding of the origin of the reactivity of \ensuremath{\beta}-lactam antibiotics and the mechanism of action of \ensuremath{\beta}-lactamases. Future directions that may allow a richer level of structural detail for assessment of macromolecular structure are also briefly discussed.