| Abstract
| - The activation volumes (ΔV≠) 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 theredox potentials and donor−acceptor distance (D−A distance) for the ET reaction. Three Ru−ZnMbs, inwhich 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 (ΔV≠f) were −1.6 (His83Mb), +3.7(His81Mb), and +6.5 cm3 mol-1 (His48Mb). We also measured the pressure dependence of the back ETreactions (from Ru2+ complex to ZnP+), showing that the back ET reactions exhibited negative activationvolumes (ΔV≠b) for all of the Ru−ZnMbs: −11, −5.3, and −6.2 cm3 mol-1 for His83Mb, His81Mb, andHis48Mb, respectively. On the basis of these activation volumes, the pressure dependence of the redoxpotentials, (∂ΔG°/∂P)T, was estimated as about 2.94 × 10-4 eV MPa-1, regardless of the position of the Rucomplex. Since (∂ΔG°/∂P)T in the present study is close to that of Ru(NH3)62+/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 reactionand the contribution of ZnP to the pressure dependence of the redox potential on the ET reactions would besmall. In sharp contrast to (∂ΔG°/∂P)T, the pressure dependence of the D−A distance, (∂d/∂P)T, highly dependson the ET pathway and microenvironments of the redox centers. The linear compressibility, (−1/d0)(∂d/∂P)T,was (2.2 ± 0.1) × 10-10, (5.1 ± 0.5) × 10-11, and (−2.6 ± 3.2) × 10-11 m2 N-1 for His83Mb, His81Mb,and His48Mb, respectively. The different linear compressibility for the three ET reaction systems suggeststhat the structural fluctuation in proteins is not unique in protein structure and site specific local fluctuationswould be one of the factors regulating the protein ET reactions.
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