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Abstract: Theoretical investigation on the intramolecular electron transfer between the side chains of tryptophan and tyrosine in the cyclopeptide has been performed. After geometric optimization and the double-well potential construction, reorganization energy and reaction energy difference of electron transfer are obtained. Furthermore, a corrected new two-sphere model for solvent reorganization energy, which differs from the conventional one, is deduced. For the double-well potential construction, the linear reaction coordinate approximation has been adopted. The electron transfer rate constant for the model system after deprotonation is estimated, k ET =9.1×108 s−1 and it is in agreement with the experimental observations. Also, the dependence of the rate constant on pH is simply discussed. [Copyright &y& Elsevier]

Abstract: We present an overview of the current status of the theories of nonequilibrium solvation and their generalizations in modern chemistry. Based on the continuous medium theory, the nonequilibrium electrostatic free energy has been reformulated. We adopt three different methods, the multi-step charging approach, the field superposition principle, and the interaction energy method, to achieve the consistent expression of electrostatic free energy of nonequilibrium. The misunderstandings on the interaction energy between the polarization field and its source (free) charge have been made clear. Derivations reveal a significant feature that this interaction energy is quite different from that between two independent fields. In the present work, the solvation free energy is found entirely contributed from the interaction between the solvent polarization and its source charge, whereas the self-energy of the polarization field is deduced to zero. The spectral shifts for light absorption and emission, and solvent reorganization energy for electron transfer in solution, have been derived to new forms differing from the conventional ones. The newly developed two-sphere model for electron transfer has been employed to investigation of the intramolecular electron transfer in Closs–Miller system. [Copyright &y& Elsevier]

In the present article, the electronic structure of 4-nitroquinoline-1-oxide (4NQO) has been theoretically investigated at the level of HFSCF/6-31G**. Conformation of monomeric 4NQO has been found to be the most stable when the nitro group twists from the quinoline plane by about 24°. The absorption spectra of the twisted 4NQO have been calculated by using complete active space self-consistent field method. The hydrogen bonded complexes of 4NQO and tryptophan have been intensively studied. The absorption spectra of the most stable complex have been calculated to investigate the photoinduced electron transfer between 4NQO and tryptophan. The first and second excited singlet states are found to be charge-separated. Population analysis shows that the low-lying triplet states are charge-separated as well, except for the lowest one that presents the characters of a locally excited state. A solvent effect on transition energies has been considered based on the Ooshika–Lippert–Martaga equation. Theoretical study shows that the optical excitation can lead to charge separation in the 4NQO–tryptophan complex. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Based on continuous medium theory, a new expression for electrostatic free energy of non-equilibrium state is deduced. Considering the change of the dielectric constant of medium during the establishment of the non-equilibrium state, a less familiar but more rational formula is employed in calculating the difference of electrostatic free energy. Both the two-step method and the field superposition approach give a consistent result for non-equilibrium, that is, G2non=(1/2)∫lower limit V dVρ2Φ2non. The direct calculation of the electrostatic free energy has been performed for the polarization field, which can represent the non-equilibrium state in charge recombination process. The result of zero coincides with that judged by using the new expression. In the present form of G2non, no quantity of intermediate equilibrium is explicitly involved, hence it satisfies the request of the thermodynamics. Starting from the electrostatic free energy, generalized formulations for solvent reorganization energy of electron transfer (ET) have been deduced. Application to the two-sphere case yields new formulas estimating this quantity. A significant feature of the present two-sphere model is the dependency of solvent reorganization energy on the direction of reaction and the charges initially brought by the donor and acceptor spheres. Results in this work reveal that the previous theory seems to overestimate the solvent reorganization energy of ET by a factor of about two. A qualitative analysis indicates that the considerable discrepancies between the theoretical predication and the experimental observation in the literatures will be solved fairly well, if the new formulations are applied. [Copyright &y& Elsevier]

In this article, ab initio calculation has been performed to evaluate the transition energy of electronic excitation in tryptophan and tyrosine by using semiempirical molecular orbital method AM1 and complete active space self-consistent field method. The solvent effect has been considered by means of the conductor-like screening model. After geometric optimizations of isolated tryptophan and tyrosine, and their corresponding radicals and cations, reaction heat of these electron transfer reactions have been obtained by the means of complete active space self-consistent field method. The transition energies from the ground state, respectively, to the lowest excited state and to the lowest triplet state of these two amino acids are also calculated and compared with the experimentally observed values. The ionization potential and electron affinity are also calculated for tryptophan and tyrosine employing Koopmans’ theorem and ab initio calculation. Compared with the experimental measurements, the theoretical results are found satisfactory. Theoretical results give good explanations on the experimental phenomena that N3⋅ can preferably oxide the side chain of tryptophan residue and then the electron transfer from tyrosine residue to tryptophan residue follows in peptides involving tryptophan and tyrosine. [Copyright &y& Elsevier]

Abstract: Based on the basic postulation of nonequilibrium solvation, we present in detail the formulations for the solvation energy and the solvent reorganization energy approximated by a revolvable ellipsoidal cavity. Applying modified formulations deduced here, we investigated the solvent reorganization energies of the complex [(Ru(NH3)5)2b]5+ in various types of solvent. Through comparison, it was found that our computational results are quite close to the observed values if the volume of the simulated cavity is of the same scale as the molecular volume. [Copyright &y& Elsevier]