Biomedical subjects

Z C Ou-Yang

Publications and source records attributed to Z C Ou-Yang.

6 recordsLinked to original sources

PubMed2005

Elasticity of polymer vesicles by osmotic pressure: an intermediate theory between fluid membranes and solid shells.

The entropy of a polymer confined in a curved surface and the elastic free energy of a membrane consisting of polymers are obtained by scaling analysis. It is found that the elastic free energy of the membrane has the form of the in-plane strain energy plus Helfrich's curvature energy [Z. Naturforsch. C 28, 693 (1973)]. The elastic constants in the free energy are obtained by discussing two simplified models: one is the polymer membrane without in-plane strains and asymmetry between its two sides, which is the counterpart of quantum mechanics in a curved surface [H. Jensen and H. Koppe, Ann. Phys. (N.Y) 63, 586 (1971)]; the other is the planar rubber membrane with homogeneous in-plane strains. The equations to describe equilibrium shape and in-plane strains of the polymer vesicles by osmotic pressure are derived by taking the first-order variation of the total free energy containing the elastic free energy, the surface tension energy, and the term induced by osmotic pressure. The critical pressure above which a spherical polymer vesicle will lose its stability is obtained by taking the second-order variation of the total free energy. It is found that the in-plane mode also plays an important role in the critical pressure because it couples with the out-of-plane mode. Theoretical results reveal that polymer vesicles possess mechanical properties intermediate between those of fluid membranes and solid shells.

Computer Simulation↗

PubMed2003

Lipid membranes with free edges.

Lipid membrane with freely exposed edge is regarded as smooth surface with curved boundary. Exterior differential forms are introduced to describe the surface and the boundary curve. The total free energy is defined as the sum of Helfrich's free energy and the surface and line tension energy. The equilibrium equation and boundary conditions of the membrane are derived by taking the variation of the total free energy. These equations can also be applied to the membrane with several freely exposed edges. Analytical and numerical solutions to these equations are obtained under the axisymmetric condition. The numerical results can be used to explain recent experimental results obtained by Saitoh et al. [Proc. Natl. Acad. Sci. 95, 1026 (1998)].

Biophysical Phenomena↗

PubMed2001

Secondary-structure-favored hydrophobic-polar lattice model of protein folding.

Protein folding is studied using a two-dimensional lattice model with the Hamiltonian including both hydrophobic interactions and main chain hydrogen bond interactions of amino acids. Since compact conformations have different designabilities and only highly designable conformations can act as native structural candidates [H. Li, R. Helling, C. Tang, and N. Wingreen, Science 273, 666 (1996)], it is shown that hydrophobic interaction alone is insufficient to explain the appearance of a high proportion of regular secondary structures, especially beta sheets whose content decreases with increasing designability, but interactions of main chain hydrogen bonds can account for this. Thus the emergence of only a small number of structure types (folds) among all possible structures can be understood to some extent.

Hydrogen Bonding↗

PubMed2001

Difference between "proteinlike" and "nonproteinlike" heteropolymers.

Based on a simple two-dimensional (2D) hydrophobic-polar (H-P) lattice model, properties of amino acid chains are studied by enumeration and Monte-Carlo simulation methods. Among them some chains with large average energy gap (E(g);) are thought to be "proteinlike" while the others are "nonproteinlike." The large E(g); between the low excited conformations and the native conformation guarantees not only the thermodynamic stability of protein but also its fast-folding property. The phase transition from molten globule to the native conformation for the "proteinlike" polymer is found to be of first order, while that for the "nonproteinlike" polymer is not. Some properties of chains as a function of E(g); shows that the transition from "nonproteinlike" to "proteinlike" heteropolymers is continuous. The simulation of folding at different temperature indicates that the main reason why some polymers fold slowly to its native conformation is their low folding temperature which makes the effective energy barrier (E(b)/T(f)) much higher than "proteinlike" chains.

Amino Acid Sequence↗

PubMed2001

Stretch-induced hairpin-coil transitions in designed polynucleotide chains.

The structural property of a poly( dG-dC) or poly( dA-dT) nucleotide is investigated. At low force and room temperatures, the polymer takes on compact hairpin structures. An abrupt transition from hairpin to random coil occurs at certain critical forces, its high cooperativity is related to the unfavorable formation of hairpin and other kinds of looped structures. It is hence necessary to consider the enthalpic effects of single-stranded loops in realistic models of RNA folding. A possible new way to obtain the statistical weights of elementary nucleotide arrangements is by single-macromolecular mechanical measurements on specifically designed polynucleotides.

Base Pairing↗

PubMed2001

Stretching single-stranded DNA: interplay of electrostatic, base-pairing, and base-pair stacking interactions.

Recent single-macromolecule observations revealed that the force/extension characteristics of single-stranded DNA (ssDNA) are closely related to solution ionic concentration and DNA sequence composition. To understand this, we studied the elastic property of ssDNA through the Monte Carlo implementation of a modified freely jointed chain (FJC), with electrostatic, base-pairing, and base-pair stacking interactions all incorporated. The simulated force-extension profiles for both random and designed sequences have attained quantitative agreements with the experimental data. In low-salt solution, electrostatic interaction dominates, and at low forces, the molecule can be more easily aligned than an unmodified FJC. In high-salt solution, secondary hairpin structure appears in ssDNA by the formation of base pairs between complementary bases, and external stretching causes a hairpin-coil structural transition, which is continuous for ssDNA made of random sequences. In designed sequences such as poly(dA-dT) and poly(dG-dC), the stacking potential between base pairs encourages the aggregation of base pairs into bulk hairpins and makes the hairpin-coil transition a discontinuous (first-order) process. The sensitivity of elongation to the base-pairing rule is also investigated. The comparison of modeling calculations and the experimental data suggests that the base pairing of single-stranded polynucleotide molecules tends to form a nested and independent planar hairpin structure rather than a random intersecting pattern.

Base Pairing↗