Intra- and inter-molecular electron transfer in cytochrome c and myoglobin observed by the muon spin relaxation method

Abstract

Experiments on the positive muon relaxation in cytochrome c and myoglobin were conducted using an intense pulsed beam of positive muon of 4 MeV at the RIKEN-RAL (The Institute of Physical and Chemical Research-Rutherford Appleton Laboratory) Muon Facility. The mu(+) SR (positive muon Spin Relaxation) measurements were carried out for a number of longitudinal magnetic fields between 0 T and 0.4 T in the temperature range between 5 K and 300 K. All measurements were conducted on the powder sample. At each measurement temperature, the relaxation parameter (Gamma) was found to have 1/B(sub ext) dependence in the cases of cytochrome c with Fe(3+) and myoglobin. The similar measurements of both lysozyme and cytochrome c with Fe(2) did not show any such dependence. On closer inspection of the B(sub ext) dependence of Gamma, there are two components: (1) a region of weak field dependence (lower field); and (2) a B(sub ext)(exp -1) dependent region (higher field). The latter region exhibits the characteristic positive muon spin relaxation behavior due to linear motion of a paramagnetic electron. All of these results lead to the microscopic picture of the electron transfer process for the muon injected electron in cytochrome c and myoglobin. The obtained D (inter molecular diffusion rate) in cytochrome c is dominated by two different processes as seen in the temperature dependence, while D in myoglobin takes one-component, reflecting the different protein dynamics of the molecule. The mu(+) SR measurement is easily able to be extended to cytochrome c in various chemical and biological environments; e.g. in solution with different values of pH (hydrogen ion concentration), and etc.

チトクロムcとミオグロビンの正ミューオン緩和の実験をRIKEN-RAL(理研-Rutherford Appleton研究所)ミューオン施設での4MeV正ミューオンの強パルスビームを用いて行った。μ(+)SR(正ミューオンスピン緩和)測定は5-300Kの温度範囲で0-0.4Tの多数の縦磁場に対して行った。すべての測定は粉末試料で行った。各測定温度で、緩和パラメータ(Γ)はFe(3+)チトクロムcとミオグロビンでは1/B(sub ext)依存性を示した。リゾチームとFe(2)チトクロムcで行った同じ様な測定はそのような依存性を示さなかった。ΓのB(sub ext)依存性をさらに調べると、(1)弱い磁場依存性の領域(低磁場)と(2)B(sub ext)(exp -1)依存性領域(高磁場)の2つの成分がある。後者の領域は常磁性電子の線形運動による特性正ミューオンスピン緩和挙動を示す。これらの結果はチトクロムcとミオグロビン中にミューオンを注入した電子に対する電子移動過程の微視的な様相を導く。チトクロムcで得た分子間拡散速度(D)は温度依存性に見られるように2つの異なる過程により支配されており、ミオグロビンにおけるDは異なる蛋白質の分子動力学を反映して1成分である。μ(+)SR測定はいろいろなpH(水素イオン濃度)値の溶液などの各種化合物や生物学的環境におけるチトクロムcに容易に拡張できる。