To elucidate how the protein ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved electron paramagnetic resonance spectra of photoinduced charge-separated (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet triplet electron spin polarization transfer model to determine both the geometries and the exchange couplings of the CS states of AQ1S(2-center dot) - histidine-242 radical cation (H242(+center dot)) and AQ1S(2-center dot)tryptophan-214 radical cation (W214(+center dot)). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied molecular orbitals of AQ1S(2-center dot) and H242(+center dot), the electronic coupling (5.4 cm(-1)) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge separation.