John McCracken
Professor / ChairpersonOffice: 20 Chemistry & 320 F Chemistry
Phone: 517-355-9715 269 /
Websites: Research Group - Area
Awards & Honors
Genealogy/Graduates
Biological Magnetic Resonance
(Research Description PDF - 1214 kb)My research interests involve the development and use of Electron Paramagnetic Resonance (EPR) Spectroscopy for the structural characterization of paramagnetic centers in biological systems. Most often these developments are driven by the desire to understand the relationship between the molecular structure of a paramagnetic center and its chemical function. The catalytic sites of enzymes are a common target of our work.
The structural information housed in an EPR spectrum is usually obtained through the measurement of weak spin-spin couplings, or hyperfine couplings, that arise from magnetic interactions between the magnetic moment of the paramagnetic center and the magnetic moments of nearby nuclei. In metalloenzymes, transition metal ions typically comprise the paramagnetic center and the structure viewed by EPR allows one to determine the ligands bound to the metal. Using the advanced EPR techniques of Electron Spin Echo Envelope Modulation (ESEEM) and Electron Nuclear Double Resonance (ENDOR), the identity, number and relative orientation of these ligands can be determined. Recently, we have used these methods to gain information on the catalytic mechanism of a non-heme Fe(II) enzyme, Taurine/a-ketoglutarate Dioxygenase (TauD) that plays a role in sulfur metabolism in E. coli. To accomplish this task, the ESEEM method was used to measure 2H-hyperfine coupling between the Fe(II) center and specifically deuterated substrate taurine. Measurements of 2H hyperfine couplings at different magnetic field positions across the EPR spectrum, allowed us to determine the distance between coupled deuterons and Fe(II) and the orientation of these nuclei with respect to Fe(II)'s magnetic axes. Our ability to measure 2H and 1H distances with ± 0.2Å accuracy makes this EPR method invaluable for the study of catalytic mechanism.
Current work on TauD is focused on using the 2-dimensional ESEEM method of HYSCORE to follow coordination chemistry at the Fe(II) site. The figure shows a 1D ESEEM spectrum (top) collected at g = 4 along with the corresponding HYSCORE spectrum (bottom). The HYSCORE experiment is the EPR equivalent of the NMR COSY measurement and shows correlated peaks about the frequency diagonal that belong to the same hyperfine interaction. The broad arcs that span the frequency range from 3-13 MHz are from a strong 1H coupling that is only resolved when both taurine and a-ketoglutarate are bound at the active site. The cross peaks in the low frequency region from 0.5 - 3.0 MHz are from the 14N of a coordinated histidine.
Selected Publications
Topography of the Prostaglandin Endoperoxide H2 Synthase-2 in Membranes, MirAfzali, Z.; Leipprandt, J.R.; McCracken, J.L.; and DeWitt, D.L., J. Biol. Chem. 2006, 281, 28354-29364.Probing the Fe-substrate orientation for taurine/a-ketoglutarate dioxygenase using deuterium ESEEM spectroscopy, Muthukumaran, R.B.; Grzyska, P.K.; Hausinger, R.P. and McCracken, J. Biochemistry 2007, 46, 5951-5959.
ESEEM Studies of Peptide Nitrogen Hyperfine Coupling in Tyrosyl Radicals and Model Peptides, McCracken, J.; Vassiliev, I.R.; Yang, E.C.; Range, K. and Barry, B.A. J. Phys. Chem. B 2007, 111, 6586-6592.
A Novel Pathway for Cancer Chemoprevention : Role of Cytochrome c, Valayutham, M.; Muthukumaran, R.B.; Sostaric, J.Z.; McCracken, J. and Zweier, J.L. Free Radical Biology and Medicine 2007, 43, 1076-1085.
Electron Spin Echo Envelope Modulation, McCracken, J., in Encyclopedia of Inorganic Chemistry, volume 2, R.A. Scott and C.M. Lukehart, eds., John Wiley & Sons, Ltd, West Sussex, UK, March 15, 2008, 55-78.
