Biomedical subjects

Zhidong Li

Publications and source records attributed to Zhidong Li.

8 recordsLinked to original sources

PubMed2007

Density-functional theory for complex fluids.

Density-functional theory (DFT) and its variations provide a fruitful approach to the computational modeling of the microscopic structures and phase behavior of soft-condensed matter. The methodology takes deep root in quantum mechanics but shares a mathematical similarity with a number of classical approaches in statistical mechanics. This review discusses different strategies commonly used to formulate the free-energy functional of complex fluids for either phenomena-oriented applications or as a generic description of the thermodynamic nonideality owing to various components of intermolecular forces. We emphasize the connections among different schemes of DFT approximations, their underlying assumptions, and inherent limitations. We also address extensions of equilibrium DFT to phenomenological theories for the dynamic properties of complex fluids and for the kinetics of phase transitions. In addition, we highlight the recent literature concerning applications of DFT to diverse static and time-dependent phenomena in complex fluids.

Journal Article↗

PubMed2006

Mapping the antigenic determinants and reducing the immunogenicity of trichosanthin by site-directed mutagenesis.

Trichosanthin (TCS) is a type I ribosome-inactivating protein (RIP) possessing multiple pharmacological properties. One of its interesting properties is to inhibit human immunodeficiency virus (HIV) replication but its strong immunogenicity has limited the repeated clinical administration. To map the antigenic determinants and reduce the immunogenicity of TCS, two potential antigenic sites (YFF81-83 and KR173-174) were identified by computer modeling, and then three TCS mutants namely TCS(YFF81-83ACS), TCS(KR173-174CG), and TCS(YFF-KR) were constructed by site-directed mutagenesis. The RI activity and DNase-like activity of the three constructed TCS mutants were similar to natural TCS but with much lower immunogenicity. Results suggested that the two selected sites are all located at or near the antigenic determinants of TCS. In toxicity studies, the LD(50) of the three TCS mutants was not different from natural TCS. These findings would be useful in designing a better therapeutic agent for AIDS.

Animals↗

PubMed2006

Density functional theory for planar electric double layers: closing the gap between simple and polyelectrolytes.

We report a nonlocal density functional theory (NLDFT) for polyelectrolyte solutions within the primitive model; i.e., the solvent is represented by a continuous dielectric medium, and the small ions and polyions by single and tangentially connected charged hard spheres, respectively. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for hard-sphere repulsion, an extended first-order thermodynamic perturbation theory for chain connectivity, and a quadratic functional Taylor expansion for electrostatic correlations. With the direct and cavity correlation functions of the corresponding monomeric systems as inputs, the NLDFT predicts the segment-level microscopic structures and adsorption isotherms of polyelectrolytes at oppositely charged surfaces in good agreement with molecular simulations. In particular, it faithfully reproduces the layering structures of polyions, charge inversion, and overcharging that cannot be captured by alternative methods including the polyelectrolyte Poisson-Boltzmann equation and an earlier version of DFT. The NLDFT has also been used to investigate the influences of the small ion valence, polyion chain length, and size disparity between polyion segments and counterions on the microscopic structure, mean electrostatic potential, and overcharging in planar electric double layers containing polyelectrolytes.

Journal Article↗

PubMed2006

Density functional theory for polyelectrolytes near oppositely charged surfaces.

We report a nonlocal density functional theory of polyelectrolyte solutions that faithfully accounts for both short- and long-range correlations neglected in a typical mean-field method. It is shown that for systems with strong electrostatic interactions, the long-range correlations are subdued by direct Coulomb attractions, thereby manifesting strong local excluded-volume effects. The theory has also been used to describe the influence of the polyion chain length and small ion valence on charge inversion due to the adsorption of polyelectrolytes at an oppositely charged surface.

Electrolytes↗

PubMed2006

Preparation and characterization of four novel monoclonal antibodies specific to N51(L6)C46 polypeptide simulating fusogenic core structure of GP41 subunit of HIV-1.

We have generated monoclonal antibodies (MAbs) against the N51(L6)C46 polypeptide, which could stimulate the fusogenic core structure of the gp41 subunit of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein. Hybridomas were screened by indirect enzyme-linked immunosorbent assay (ELISA) using either N51(L6)C46 or N36(L6)C34 polypeptide. The MAb against N36(L6)C34, NC-1, was used as positive control, and N36 C34 peptides as negative controls. As a result, we generated four MAbs, three (1A2, 1A4, 1B9) against the polypeptide N51(L6)C46 monomer and one against the trimer, named 3A1. The four anti- N51(L6)C46 MAbs could bind N51(L6)C46 and N36(L6)C34 but not N36 or C34. The results of epitope analysis with competitive ELISA showed that the MAbs could recognize the similar epitopes, whereas the difference lie in that 3A1 could recognize the epitopes similar to NC-1s, whereas NC-1 could not recognize the epitopes similar to 3A1s. Taken together, these MAbs would be effective tools in selecting anti-HIV polypeptide and analyzing the characteristics of gp41 epitopes.

Animals↗

PubMed2005

Layering, condensation, and evaporation of short chains in narrow slit pores.

The phase behavior of short-chain fluids in slit pores is investigated by using a nonlocal-density-functional theory that takes into account the effects of segment size, chain connectivity, and van der Waals attractions explicitly. The layering and capillary condensation/evaporation transitions are examined at different chain length, temperature, pore width, and surface energy. It is found that longer chains are more likely to show hysteresis loops and multilayer adsorptions along with the capillary condensation and evaporation. Decreasing temperature favors the inclusion of layering transitions into the condensation/evaporation hysteresis loops. For large pores, the surface energy has relatively small effect on the pressures of the capillary condensation and evaporation but affects significantly on the layering pressures. It is also observed that all phase transitions within the pore take place at pressures lower than the corresponding bulk saturation pressure. The critical temperature of condensation/evaporation is always smaller than that of the bulk fluid. All coexistence curves for confined phase transitions are contained within the corresponding bulk vapor-liquid coexistence curve. As in the bulk phase, the longer the chain length, the higher are the critical temperatures of phase transitions in the pore.

Journal Article↗

PubMed2005

Density-functional theory and Monte Carlo simulation for the surface structure and correlation functions of freely jointed Lennard-Jones polymeric fluids.

We present a nonlocal density-functional theory of polymeric fluids consisting of freely jointed Lennard-Jones chains with explicit consideration of the segment size, van der Waals attraction, and structural correlations due to chain connectivity. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the short-ranged repulsion and the first-order thermodynamic perturbation theory for chain connectivity. The contribution of the long-ranged attraction to the Helmholtz energy functional is taken into account using a quadratic density expansion with the direct correlation function obtained from the first-order mean-spherical approximation. The numerical performance of the density-functional theory is compared well with the simulation results from this work as well as those from the literature for the segment-level density profiles and correlation functions of Lennard-Jones chains in slit pores, near isolated nanoparticles, or in bulk.

Journal Article↗

PubMed2004

Density-functional theory for the structures and thermodynamic properties of highly asymmetric electrolyte and neutral component mixtures.

Density-functional theory (DFT) is applied to investigate the structural and thermodynamic properties of concentrated electrolyte and neutral component mixtures that are highly asymmetric in terms of both size and charge mimicking a crowded cellular environment. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-sphere repulsion and a quadratic functional Taylor expansion for the electrostatic interactions. The direct correlation functions are obtained from the analytical solutions of the mean-spherical approximation. In the context of a primitive model where biomacromolecules are represented by neutral or charged hard spheres and the solvent is represented by a continuous dielectric medium, this DFT is able to take into account both the excluded-volume effects and the long-ranged electrostatic interactions quantitatively. The performance of the theoretical method has been tested with Monte Carlo simulation results from this work and from the literature for the pair correlation functions, excess internal energies, and osmotic coefficients for a wide variety of aqueous dispersions of charged and neutral particles.

Journal Article↗