Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin

• Published in: The journal of physical chemistry. B 2016-05, Vol.120 (19), p.4365-4372
• Main Authors: Fuki, Masaaki, Murai, Hisao, Tachikawa, Takashi, Kobori, Yasuhiro
• Format: Article
• Language: English
• Subjects: Anthraquinones - chemistry; Anthraquinones - metabolism; Arginine - chemistry; Binding Sites; Computer simulation; Couplings; Docking; Electron Spin Resonance Spectroscopy; Electronics; Humans; Lasers; Molecular conformation; Molecular Docking Simulation; Protein Binding; Residues; Serum albumin; Serum Albumin, Human - chemistry; Serum Albumin, Human - metabolism; Spectra
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.6b01072

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S–) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S•2–-tryptophan214 radical cation (W214•+) separated by 1.2 nm. The ligand of AQ1S– has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218+) residue bound to AQ1S•2– mediates the long-range electron-transfer. The energetics of triad states of AQ1S•2––R218+–W214•+ and AQ1S––R218•–W214•+ have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.