Pressure Dependence of the Intramolecular Electron Transfer Reaction in Myoglobin Reinvestigated

The activation volumes (Δ V ≠ ) for intramolecular electron transfer (ET) reactions in Ru-modified Zn-porphyrin (ZnP) substituted myoglobins (Ru−ZnMb) have been determined to investigate the pressure effects on the redox potentials and donor−acceptor distance (D−A distance) for the ET reaction. Three Ru−ZnMbs, in which D−A distances for the ET reactions are 12.7 Å (His48Mb), 15.5 Å (His83Mb), and 19.3 Å (His81Mb), were constructed. The activation volumes for the forward ET reactions (Δ V ≠ f ) were −1.6 (His83Mb), +3.7 (His81Mb), and +6.5 cm 3 mol -1 (His48Mb). We also measured the pressure dependence of the back ET reactions (from Ru 2+ complex to ZnP + ), showing that the back ET reactions exhibited negative activation volumes ( Δ V ≠ b ) for all of the Ru−ZnMbs: −11, −5.3, and −6.2 cm 3 mol -1 for His83Mb, His81Mb, and His48Mb, respectively. On the basis of these activation volumes, the pressure dependence of the redox potentials, (∂Δ G °/∂ P ) T, was estimated as about 2.94 × 10 -4 eV MPa -1, regardless of the position of the Ru complex. Since (∂Δ G °/∂ P ) T in the present study is close to that of Ru(NH 3 ) 6 2+/3+ (2.97 × 10 -4 eV MPa -1 ), the pressure-induced redox changes of the Ru complex were primarily responsible for that of the ET reaction and the contribution of ZnP to the pressure dependence of the redox potential on the ET reactions would be small. In sharp contrast to (∂Δ G °/∂ P ) T, the pressure dependence of the D−A distance, (∂ d /∂ P ) T, highly depends on the ET pathway and microenvironments of the redox centers. The linear compressibility, (−1/ d 0 )(∂ d /∂ P ) T, was (2.2 ± 0.1) × 10 -10, (5.1 ± 0.5) × 10 -11, and (−2.6 ± 3.2) × 10 -11 m 2 N -1 for His83Mb, His81Mb, and His48Mb, respectively. The different linear compressibility for the three ET reaction systems suggests that the structural fluctuation in proteins is not unique in protein structure and site specific local fluctuations would be one of the factors regulating the protein ET reactions.