Theoretical Study of Trapping Sites for Muon and Muonium in the Heme Group of Cytochrome c and Associated Shifts in Muon Spin Rotation Frequencies

D. Cammarere1,2, R. H. Scheicher1, T. M. Briere3, N. Sahoo1,4, T. P. Das1, F. L. Pratt5, K. Nagamine3,6

1 Department of Physics, State University of New York at Albany, Albany, New York, 12222, USA
2 Tamarac High School, Brittenkill, New York, USA
3 Meson Science Laboratory, High Energy Accelerator Research Organization (KEK), Tsukuba Shi, Ibaraki, 305, Japan
4 Department of Radiation Oncology, Albany Medical College, Albany, New York, 12208, USA
5 RIKEN-RAL, Didcot, UK; Clarendon Laboratory, Oxford, UK
6 Muon Science Laboratory, Institute for Physical and Chemical Research (RIKEN), Wako Shi, Saitama, 351-01, Japan

Muon Spin Rotation (mSR) experiments are currently being conducted on the important electron transfer molecule Cytochrome c with the goal to find out about microscopic details of the path the moving electron is taking. Simultaneously we are using the Unrestricted Hartree-Fock Cluster Procedure to determine the trapping sites for muon (m+) and muonium (Mu) in the heme unit of Cyt c and the associated hyperfine interactions. For the trapping sites with the highest binding energies for m+, namely the nitrogen and the carbon of the pyrrole rings, we have used the available magnetic susceptibility data [1] together with our calculated hyperfine fields to predict the Knight-shifts. At room-temperature we found 88.4 ppm and 79.0 ppm for the most attractive nitrogen- and adjacent carbon-site respectively. At 150 K, these shifts increase to 172.7 ppm for the N-site and 153.7 ppm for the C-site. The predicted shifts for m+ at the other trapping sites that we have explored so far demonstrate the loss of tetragonal symmetry of the heme unit due to the influence of the methionine ligand. A report on the trapping strengths and expected mSR frequency shifts for m+ and Mu at various sites on the heme will be presented.

 [1] A. Tasaki et al., Biochim. Biophys. Acta, 140, 284 (1967)

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