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

RIKEN Accel. Prog. Rep. 53µSR study of the electron transfer mechanism of a blue copper protein,Pseudoazurin
A. D. Pant,∗1,∗2,∗3 T. Yamaguchi,∗1 K. Ishida,∗3 and T. Kohzuma∗1Electron transfer (ET) is an important phenomenon
to understand biological processes such as photosynthesis and respiration systems. The study of ET in proteinusing positive muon was initiated by K. Nagamine’s
group with the cytochrome c protein.1) However, theelectron transfer path/mechanism is not clearly understood yet. We aim to perform a systematic study toestablish the application of muons to life science.
Since biological systems are complex in geometry
and dynamic, we started studying the ET mechanism
in a small and single active-site blue copper protein,rather than large systems. Copper proteins that containsingle- or multi-copper sites in their prosthetic group
work in a wide range of ET processes and biologicalenergy conversion cycles.2) The copper centers in these
proteins play a main role in ET. However, there remain
several obscure points in the ET pathway;3) that is,there is no clear mechanism to understand the electrontransfer. Blue copper protein (type: T1Cu) contains
only a single copper-ion binding site that serves as anelectron mediator between large proteins. Pseudoazurin(PAz), a member of T1Cu, functions in the electrontransfer processes in several microorganisms. With theapplication of the muon spin rotation and relaxation(µSR) method, we propose to probe the electron transfer mechanism in PAz, in which electron transfer occurs
only from/to the copper center. The µSR study on
PAz will provide novel insight to understand the muon
method for studies in life science.
In order to understand the ET mechanism and functions of copper proteins, we will perform µSR measurements in reduced pseudoazurin (PAzCu1+, diamagnetic) and oxidized pseudoazurin (PAzCu2+, paramagnetic) (Fig. 1) using surface muons (∼4 MeV). For
reference, a similar set of measurements will also be
performed in buﬀer solution and pure water. The samples are prepared and puriﬁed (solution, concentration∼10 mM, volume ∼700 µL) in Kohzuma laboratory,Ibaraki University, Mito. The solution is ﬁlled in asample cell (silicone rubber ring, ϕ20 mm, t ∼ 2 mm,polyvinylidene chloride ﬁlm ∼15 µm on both sides ofthe ring), which is masked by silver ring (ϕ18 mm, t0.3 mm) so that the incident muon beam can be prevented from stopping in the rubber. The blue oxidized
PAz changes to the reduced form using ascorbic acid.
Indium sealing is used in a helium glove bag environment. The measurements will be performed at a transverse ﬁeld of 2.3 G, zero ﬁeld, and a longitudinal ﬁeld
∗1
∗2
∗3

Institute of Quantum Beam Science, Ibaraki University
IMSS, KEK
Meson Science Laboratory, RIKEN Nishina CenterFig. 1. Structure of pseudoazurin (PDB: 1BQK) with thecoordinate scheme around Cu(II). Copper is an active
site for electron transfer.

at diﬀerent temperature ranges (20 K–300 K).
When a surface muon beam (∼MeV) is incident on
the sample solution, the beam energy is transferred tothe solution until the end of the radiation track. A large
fraction of incident muons will be found in the diamagnetic muon form and some in the muonium form (boundstate of muon and an electron, Mu = µ+ e− ). If lowenergy muons are stopped around the copper center of
reduced PAz, there is a possibility of electron transferfrom the copper center to the muon to form Mu. The
Mu formed by the former (in solution) and latter (byelectron transfer) cases will be distinguished by theirfrequencies. Interactions will exist between muons andelectrons as well as muons and nuclei. These behaviors
are expected to be reﬂected in the time spectra. The
relaxation of Mu depends on its interaction with the environment. For example, a small amount of oxygen willcause the fast relaxation of Mu.4) The formation of Mu
at diﬀerent sites in the sample will provide information
about the electron transfer mechanism.
References

1) K. Nagamine et al., Physica B 289-290, 631–635S(2000).
2) O. Farver, Coord. Chem. Rev. 255, 757–773 (2011).
3) S. Wherland et al., Perspect. Sci. 6, 94–105 (2015).
4) A. D. Pant et al., J. Phys. Conf. Ser. 551, 012043 (2014).

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