Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 51 (2018)

Approach to determination of muon stopping sites in proteinsY. Sugawara,∗1 T. Fujita,∗1 A. D. Pant,∗2 E. Torikai,∗3,∗4 K. Ishida,∗5 W. Higemoto,∗4,∗6 K. Shimomura,∗2F. L. Pratt,∗7 and K. Nagamine∗2

∗1
∗2
∗3
∗4
∗5∗6
∗7

School of Science, Kitasato University
IMSS, KEK
Graduate Faculty of Interdisciplinary Research, University
of Yamanashi
ASRC, JAEA
RIKEN Nishina Center
School of Science Tokyo Institute of Technology
ISIS Science and Technology Facilities Council- 209 -(a)polyglycine: all R = H(b)1.00.8 (s-1)

0.6
0.4
0.2
0.0

1101001000

0.0 1

101001000(c) (s-1)

1.5
1.00.5(d) (s-1)

In order to explore the electron-transfer process inlife sciences, we have been carrying out µSR studies
using the electron labeling method on cytochrome c, a
protein which is a member of the respiratory chain in
mitochondria.1–3) In order to deepen the understanding of the µSR data, we intend to determine the muon
stopping sites and the electronic structure. In addition, our LF µSR data showed a level crossing resonance (LCR) signature for cytochrome c indicatingthat some portion of the muons have a strong energy
transfer at a specific LF to the surrounding species
at the stopping site (Fig. 1(b)). The observed LCR
data around 20 G was similar to that of polyglycine(Fig. 1(c)) (F. L. Pratt, private communication). Proteins are made of amino acids, which are linked by peptide bonds, and polyglycine is the simplest polypeptide made of aliphatic (–CH2 –) parts, peptide bonds(–CONH–), terminal –COOH (or –COO− ), and –NH2(or –NH3 + ) groups. The candidates of muon stopping sites are peptide bonds (–CONH–) and terminal–COOH or –NH2 (Fig. 1(a)).
Under such a background, we carried out µSR
measurements of triglycine, tetraglycine, and Nmethylacetamide (CH3 CONHCH3 ), which is the simplest molecule containing a peptide bond, and compared the characteristics of the µSR data with thoseof glycine, glycylglycine, and polyglycine.
The low LF µSR data of glycine were fitted well using the product of Kubo-Toyabe and Lorentzian functions. LCR was not detected around 20 G, contrary to
the cases of cytochrome c and polyglycine. On theother hand, the low LF µSR data of glycylglycine,
triglycine and tetraglycine were approximately fittedwith the Lorentzian function, and the LCR was observed (Fig. 1(d)). In addition, the missing fraction of
the initial asymmetry of oligoglycine under zero magnetic field was approximately 20% of the full asymmetry, and larger than that of glycine (approximately10%). A similar tendency was observed in the µSR
data of N-methylacetamede at 10 K, although the quality of the data was inferior because the sample melted
during the treatment (melting point: 300 K), and acavity appeared in the sample cell. Taking the resultsof theoretical calculations into account,4) muon wouldstop at –COOH (or –COO− ) moiety in glycine, and at

LF (G)

Fig. 1. Schematic structure of protein (a) and longitudinal
field dependence of relaxation rates (λ) of cytochromec (18 K) (b), polyglycine (5 K) (c) and tetraglycine(16 K) (d).

CO moiety of peptide bonds in oligoglycine.
The results indicate that the LCR data of cytochrome c originates from the muon stopping at peptide bonds.
References
1) K. Nagamine et al., Physica (Utrecht), 289–290, 631(2000).
2) K. Nagamine, E. Torikai, J. Phys. Condensed Matter
16, S4797 (2004).
3) A. D. Pant et al., JPS Conference Proceedings 8,033007-1-5 (2015).
4) A. D. Pant et al., JPS Conference Proceedings 21,011038-1-4 (2018).