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Direct Comparison of Electron Transfer Properties of Two Distinct Semisynthetic Triads with Non‐Protein Based Triad: Unambiguous Experimental Evidences on Protein Matrix Effects
Author(s) -
Hu YiZhen,
Takashima Hiroshi,
Tsukiji Shinya,
Shinkai Seiji,
Nagamune Teruyuki,
Oishi Shigero,
Hamachi Itaru
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20000602)6:11<1907::aid-chem1907>3.0.co;2-9
Subject(s) - chemistry , moiety , flash photolysis , electron transfer , excited state , photochemistry , heme , cytochrome c , electron donor , stereochemistry , crystallography , biochemistry , kinetics , enzyme , reaction rate constant , catalysis , mitochondrion , physics , quantum mechanics , nuclear physics
In order to understand the roles of protein matrix in electron transfer processes (ET) within biological systems, a heme‐based donor (Zn‐heme: ZnPP)‐sensitizer (Ru 2+ (bpy) 3 )‐acceptor (cyclic viologen: BXV 4+ ) triad 1 was used as a probe molecule. Two semisynthetic systems, Cyt‐b 562 ( 1 ) and Mb( 1 ), in which the triad is incorporated into cytochrome b 562 (Cyt‐b 562 ) or into myoglobin (Mb), were constructed by cofactor reconstitution. These two semisynthetic proteins were compared with the triad itself (i.e., without the protein matrix) using absorption spectroscopy, steady state emission and excitation studies, laser flash photolysis experiments, and molecular modeling. Photoexcitation of the ZnPP moiety of Cyt‐b 562 ( 1 ) or Mb( 1 ) leads to a direct ET from the triplet state of ZnPP state ( 3 ZnPP) to BXV 4+ to generate a final charge‐separated (CS) state, Cyt‐b 562 (Zn + )‐Ru 2+ ‐BXV 3+ . or Mb(Zn + )‐Ru 2+ ‐BXV 3+ . . On the other hand, direct ET from the excited ZnPP moiety to the BXV 4+ moiety is also involved in 1 in the absence of the protein matrix, but the excited state of ZnPP involved is not 3 ZnPP, but the singlet excited state ( 1 ZnPP) in this pathway. When the Ru 2+ (bpy) 3 moiety of Cyt‐b 562 ( 1 ) or Mb( 1 ) is excited, a stepwise ET relay occurs with the ion‐pair, Cyt‐b 562 (Zn)‐Ru 3+ ‐BXV 3+ . or Mb(Zn)‐Ru 3+ ‐BXV 3+ . , as an intermediate, leading to the same final CS state as that generated in the direct ET pathway. The lifetimes of the corresponding final CS states were determined to be 300 ns for 1 in the absence of the protein matrix, 600‐900 ns for Cyt‐b 562 ( 1 ) and 1.1‐18 μ s for Mb( 1 ), the values of which are greatly affected by the protein matrix. Molecular modeling study of the three systems consistently explained the differences of their photophysical behavior.