Biomedical subjects

Xiang-Yuan Li

Publications and source records attributed to Xiang-Yuan Li.

11 recordsLinked to original sources

PubMed2006

Inhibition by bis(7)-tacrine of native delayed rectifier and KV1.2 encoded potassium channels.

Bis(7)-tacrine [bis(7)-tetrahydroaminacrine] acts as an AChE inhibitor and also exerts modulatory effects on many ligand-gated ion channels and voltage-gated Ca(2+) and K(+) channels. It has been reported previously that tacrine and some other AChE inhibitors suppressed I(K(A)) in central and peripheral neurons. The present study aimed to explore whether bis(7)-tacrine could modulate the function of native delayed rectifier potassium channels in DRG neurons and K(V)1.2 encoded potassium channels expressed in oocytes. We found that both delayed rectifier potassium currents (I(K(DR))) in rat DRG neurons and the currents recorded from oocytes expressing K(V)1.2 (I(K(K(V)1.2))) were suppressed by bis(7)-tacrine, the potency of which was two orders greater than that of tacrine. The IC(50) values for bis(7)-tacrine and tacrine inhibition of I(K(KD)) in DRG neurons were 0.72+/-0.05 and 58.3+/-3.7 microM, respectively; while the two agents inhibited I(K(K(V)1.2)) in oocytes with an IC(50) of 0.24+/-0.06 and 102.1+/-21.5 microM, respectively. The possible mechanism for bis(7)-tacrine inhibition of I(K(A)) and I(K(K(V)1.2)) was identified as the suppression of their activation, inactivation.

Animals↗

PubMed2006

Single-sphere model for absorption spectrum of interfacial molecules with application to predictions of orientational angles.

In this work, a novel expression for nonequilibrium free energy is introduced within the framework of the continuum model. A simple and analytical model for the solvent effect of absorption spectrum of an interfacial molecule is deduced. This model overcomes some lacks existing in the previous theoretical treatments. Associated with the experimental data of N,N-diethyl-p-nitroaniline and 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide, the present model gives reasonable predication of orientational angles for different interfaces.

Journal Article↗

PubMed2006

Continuous medium theory for nonequilibrium solvation: IV. Solvent reorganization energy of electron transfer based on conductor-like screening model.

In this work the authors present some evidences of defects in the popular continuous medium theories for nonequilibrium solvation. Particular attention has been paid to the incorrect reversible work approach. After convincing reasoning, the nonequilibrium free energy has been formulated to an expression different from the traditional ones. In a series of recent works by the authors, new formulations and some analytical application models for ultrafast processes were developed. Here, the authors extend the new theory to the cases of discrete bound charge distributions and present the correct form of the nonequilibrium solvation energy in such cases. A numerical solution method is applied to the evaluation of solvent reorganization energy of electron transfer. The test calculation for biphenyl-cyclohexane-naphthalene anion system achieves excellent agreement with the experimental fitting. The central importance presented in this work is the very simple and a consistent form of nonequilibrium free energy for both continuous and discrete charge distributions, based on which the new models can be established.

Journal Article↗

PubMed2005

Theoretical investigation of spectral properties and tautomerization mechanism of 2-aminopurine.

In this work the spectral properties and tautomerization mechanism of 2-aminopurine are investigated using quantum chemical calculations. TD-DFT results lead to a conclusion that two tautomers of 2-aminopurine discussed in this work are fluorescent and present the pi-->pi* transition feature of vertical excitation and emission. It is predicted that the tautomerization of 2-aminopurine hardly occurs in a direct isomerization mechanism because of the high barrier. However, the explicit involvement of water molecules into the system reduces the barrier height considerably and hence makes the ground state reaction feasible. An explanation for the almost equal populations of the two tautomers in polar solvent is given through reaction mechanism analysis.

2-Aminopurine↗

PubMed2005

Quantum chemical study on excited states and electronic coupling matrix element in a catechol-bridge-dicyanoethylene system.

The excited states of a donor-bridge-acceptor (DBA) model system have been investigated using time-dependent density-functional theory (TD-DFT) in vacuo and in solution. It is found that the MPW1PW91 functional always gives higher excitation energies than those with a B3LYP functional. Results from both TD-B3LYP and TD-MPW1PW91 are found consistent with the experimental observations. The two most intense absorptions of the DBA system, one resulting from the local excitation of catechol moiety and the other from that of dicyanoethylene, possess the pipi* transition feature. It seems that the solvent polarity does not remarkably influence the positions of absorption peaks. The spectroscopic properties of isolated donor, acceptor, and bridge and the donor-bridge compound have been investigated at the TD-B3LYP/6-31+G* and TD-MPW1PW91/6-31+G* levels. Results indicate that the donor and the acceptor are weakly coupled with the bridge. Therefore, it is more likely that the electron transfer takes place through a superexchange mechanism. In addition, we calculate the electronic coupling matrix elements according to the generalized Mulliken-Hush theory, and the detailed analyses also predict that the strong absorptions are due to the local excitation of the DBA system.

Acetonitriles↗

PubMed2005

Time-dependent density functional theory study on intramolecular charge transfer and solvent effect of dimethylaminobenzophenone.

The lower singlet excited states for dimethylaminobenzophenone have been investigated as a function of the twisting motion with inclusion of solvent effects. Theoretical calculations have been performed using time-dependent density functional theory. The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) basis set have been used to compute transition energies. The solvent effects have been described within the polarizable continuum model. Ground-state geometries are optimized using density functional theory with both B3LYP and MPW1PW91 functionals combined with 6-31G(d) basis sets. Vertical absorption energy calculations characterize the lower singlet excited states both in vacuum and in different kinds of solvents. A large redshift of the absorption maximum in the polar solvent suggests an intramolecular charge transfer character of the excited state. We have constructed the potential energy curves of two possible twisting motions of the excited states both in vacuum and in the polar solvent of acetonitrile: the twisting of only the dimethylamino group and the twisting of the dimethylaminophenyl group with respect to the benzoyl group. Both twisting processes predict the formation of the twisted intramolecular charge transfer state associated with the crossing of a low barrier. The presence of the polar solvent significantly changes the shape of the energy curves. Calculated emission energies for both the isolated and the solvated systems show a large Stokes shift between the absorption and fluorescence maxima. Two possible twisting motions produce similar fluorescence spectroscopic consequences. Our results including solvent effects explain the weak "dual-fluorescence" feature of dimethylaminobenzophenone, and imply that the two possible twisting motions may occur in the excited-state relaxation dynamics, but the twisting of the dimethylamino group seems to take place easier.

Journal Article↗

PubMed2005

Continuous medium theory for nonequilibrium solvation: III. Solvation shift by monopole approximation and multipole expansion in spherical cavity.

According to the classical electrodynamics, a new and reasonable method about electrostatic energy decomposition of the solute-solvent system has been proposed in this work by introducing the concept of spring energy. This decomposition in equilibrium solvation gives the clear comprehension for different parts of total electrostatic free energy. Logically extending this cognition to nonequilibrium leads to the new formula of electrostatic free energy of nonequilibrium state. Furthermore, the general solvation shift for light absorption/emission has been reformulated and applied to the ideal sphere case with the monopole approximation and multipole expansion. Solvation shifts in vertical ionizations of atomic ions of some series of main group elements have been investigated with monopole approximation, and the variation tendency of the solvation shift versus atomic number has been discussed. Moreover, the solvation shift in photoionization of nitrate anion in glycol has been investigated by the multipole expansion method.

Journal Article↗

PubMed2004

Investigation of photoinduced electron transfer in model system of vitamin E-duroquinone by time-dependent density functional theory.

Photoinduced electron transfer of the model system composed of vitamin E and duroquinone has been investigated using time-dependent density functional theory. Calculations for the excited states tell that the photoexcitation of the model system can directly yield the charge transfer states in which the vitamin E moiety is positively charged but the duroquinone moiety is negatively charged. Our theoretical investigations indicate that the second charge transfer state of the model system can also be produced through the decay of higher locally excited state S(4). Since S(4) state in the model system corresponds to S(1) state of the isolated duroquinone used as a model for peroxyl radical, and S(2) state has the character of electron transfer from the tertiary amine group of the vitamin E moiety to the duroquinone moiety, the decay from S(4) to S(2) corresponds to the dynamic process following the photoexcitation of the duroquinone moiety of the model system, i.e., the initial stage of antioxidant reaction of vitamin E. Calculations of the kinetic parameters for the electron transfer have been carried out in the framework of the Marcus-Jortner-Levich formalism. Our calculations confirm that the electron transfer from S(4) to S(2) possesses the character of the inverted regime and the barrier is negligibly small.

Benzoquinones↗

PubMed2004

Continuous medium theory for nonequilibrium solvation: II. Interaction energy between solute charge and reaction field and single-sphere model for spectral shift.

On the basis of continuous medium theory, a model for evaluation of spectral shifts in solution has been developed in this work. The interaction energy between solute dipole and reaction field and the self-energy of the reaction field have been formulated through derivations. Applying the interaction energy expression together with the point dipole approximation to the case of spherical cavity produces new formulations of spectral shifts. The same expression of electrostatic free energy of the nonequilibrium state is achieved by integrating the change of the electrostatic free energy for a charging process. Moreover, generalized formulations evaluating spectral shifts have been established in the charge-potential notation, and the reduction of them to the point dipole case consistently leads to the same formulations of spectral shifts as those by interaction energy approach. Mutual supports provide convincing evidences for the reliability of the present results. In this work, attentions are particularly paid to the conclusion of zero self-energy of the reaction field, which is different from the previous theory. Reasoning and arguments are given on this point. From the present derivations, it is concluded that the spectral shifts of light absorption and emission were theoretically exaggerated in the past, in particular, by a factor of 2 for the spectral shift sum.

Journal Article↗

PubMed2004

Continuous medium theory for nonequilibrium solvation: I. How to correctly evaluate solvation free energy of nonequilibrium.

Considering the influences of electrostatic potential Phi upon the change of solute charge distribution deltarho and rho upon the change deltaPhi at the same time, a more reasonable integral formula of dG = (1/2) integral (V) (rhodeltaPhi + Phideltarho)dV is used to calculate the change of the electrostatic free energy in charging the solute-solvent system to a nonequilibrium state, instead of the one of dG = integral (V) PhideltarhodV used before. This modification improves the expressions of electrostatic free energy and solvation free energy, in which no quantity of the intermediate equilibrium state is explicitly involved. Detailed investigation reveals that the solvation free energy of nonequilibrium only contains the interaction energy between the field due to the solute charge in vacuum, and the dielectric polarization at the nonequilibrium state. The solvent reorganization energies of forward and backward electron transfer reactions have been redefined because the derivations lead to a remarkable feature that these quantities are direction-dependent, unlike the theoretical models developed before. The deductions are given in the electric field-displacement form. Relevant discussions on the reliability of theoretical models suggested in this work have also been presented.

Journal Article↗

PubMed2002

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes.

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.

Journal Article↗