Fumihiko Tanaka

Elastic theory of supercoiled DNA

This paper estimates the configurational energies of a closed DNA molecule in highly supercoiled states. Calculation is based on an elastic rod model characterized by the bending and torsional stiffness constants. We show that the global constraint due to topological restriction leads to highly branched metastable structures. We also clarify the conditions for formation of supercoils by linear stability theory and obtain low‐frequency vibrational spectra of relatively simple supercoiled structures. It is shown that even a small number of duplex unwindings can lead to the instability of a circle for a wide range of elastic constants.

Loss of heterozygosity (LOH) at 17q and 14q in human lung cancers

Our recent linkage study of urethane-induced pulmonary adenomas in SMXA RI strains of mouse revealed two host resistance genes, Par1 (chromosome 11) and Par3 (chromosome 12). The map positions of Par1 and Par3 correspond to human 17q11-23 and 14q11-24, based on synteny between mouse and human. In this study, we examined the loss of heterozygosity (LOH) in these two homologous human chromosomal regions in 30 primary lung adenocarcinoma samples with matched normal DNA. Using 15 highly polymorphic markers, two commonly deleted regions were identified on human chromosomes 14 and 17, respectively. At 17q21, nine (53%) of 17 informative tumors showed LOH between D17S588 and D17S518. On the other hand, at 14q11-12, seven (32%) of 22 informative tumors showed LOH at loci between D14S261 and D14S80. Subsequently, we examined 25 squamous cell carcinomas (SQ) and 24 small cell carcinomas (SCC). At 14q11-12, six (38%) of 16 informative SQ and five (42%) of 12 informative SCC showed LOH. In contrast, at 17q11-23, one (7%) of 15 informative SQ and two (14%) of 14 SCC showed LOH. Therefore, the gene on 17q seemed to affect selectively adenocarcinomas, whereas the other gene on 14q, all three types of lung carcinomas. These observations indicate that a comparative genetic analysis provides a promising approach to survey genes involved in multifactorial process of human lung carcinogenesis.

Theory of solvation‐induced reentrant coil–globule transition of an isolated polymer chain

This paper discusses the conformational phase change of an isolated polymer chain which is capable of forming physical bonds with solvent molecules. On the basis of the mean‐field theory of Flory type, we derive a formula for the temperature dependence of the expansion factor of the chain. Our theory takes account of the extra mixing entropy change induced by the solvation. We predict that the physical‐bond formation between polymer and solvent molecules causes a reentrant conformational change between coiled and globular state when temperature is varied. We also show that the polymer chain exhibits a sharp collapse near the Θ temperature as the temperature is increased to the lower critical solution temperature of the polymer solution. This behavior can be interpreted in terms of the breakup of the polymer–solvents complex. The result is compared with the observed coil–globule transition on poly (N‐isopropylacrylalmide) in water.

New insights into the effects of molecular weight and end group on the temperature-induced phase transition of poly(N-isopropylacrylamide) in water

In an attempt to clarify issues related to the molecular weight dependence of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in water, we prepared a library of PNIPAM samples of well-controlled molecular weight (7000 to 45000 g/mol) bearing identical groups on each chain end. The polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) with a bifunctional chain tranfer agent and further end group modification. The effects of the end group chemical structure, hydroxyethyl (HE), propargyl (Pr), chloroethyl (CE), n-butyl (nBu), n-hexyl (nHe), and isobutylsulfanylthiosulfanyl (IBS) on the phase transition temperature of aqueous PNIPAM solutions were investigated by high-sensitivity differential scanning calorimetry (HS-DSC), yielding the enthalpy ΔH and the endotherm maximum temperature (TM), and turbidimetry, providing the cloud point (TCP) of each solution. The TCP and TM of the PNIPAM sample of lowest molar mass (Mn 7,000 g/mol, 0.5 g/L) ranged, respectively, from 38.8 to 22.5 °C and 42.2 to 26.0 °C, depending on the structure of the end-group, whereas ΔH showed no strong end-group dependence. The phase transition of all polymers, except α,ω-di(n-butyl-PNIPAM), exhibited a marked dependence on the polymer molar mass.

Osmotic pressure of ring‐polymer solutions

For a pair of ring polymers in a solution, the number of mutual entanglements is invariant and, hence, a frozen variable. Such restriction on thermally accessible configurations causes a new type of intermolecular interaction. This paper theoretically studies the nature of molecular interaction caused by topological constraints. Observed depression of the theta temperature by 6 °C in the solution of ring polystyrene compared with that of linear polymer is explained by the existence of repulsive force, which acts between disjoint rings and originates in topological constraints. By the use of Gauss’ linking number for the topological invariant, the second and third virial coefficients are calculated as a function of molecular weight and compared with the experimental observation on ring polystyrene in cyclohexane.

Intramolecular and intermolecular association in thermoreversible gelation of hydrophobically modified associating polymers

Abstract In the thermoreversible gelation of hydrophobically modified water-soluble associating polymers, intramolecular association competes with intermolecular association. The former leads to intramolecular micelles with hydrophobic cores decorated by loops of hydrophilic chains. Loop formation prevents an intermolecular cross-linking. This paper theoretically details the effect of small loops on the sol/gel transition with multiple cross-link junctions in associating polymer solutions. In the case of telechelic associating polymers, the problem is shown to be mathematically equivalent to the mixture of monofunctional and bifunctional polymers. The gelation concentration changes with a parameter describing the probability of a single loop formation. Relative populations of the six fundamental chain categories are derived as a function of the total polymer concentration with special attention to the formation of flower micelles. For polymers carrying many hydrophobes, Monte Carlo computer simulation is carried out by using model chains to see how intramolecular micellization competes with intermolecular cross-linking.

Hydration, phase separation and nonlinear rheology of temperature-sensitive water-soluble polymers.

The collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating and the phase diagrams of aqueous PNIPAM solutions with a very flat lower critical solution temperature (LCST) phase separation line are theoretically studied on the basis of cooperative dehydration (simultaneous dissociation of bound water molecules in a group of correlated sequence), and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes sharper with the cooperativity parameter σ of hydration. The reentrant coil-globule-coil transition and cononsolvency in a mixed solvent of water and methanol are also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves (LCST cononsolvency) with the mol fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydrophobically modified PNIPAM carrying short alkyl chains at both chain ends (telechelic PNIPAM) are theoretically and experimentally studied. The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation), and separate from the coil-globule transition line. Associated structures in the solution, such as flower micelles, mesoglobules, and higher fractal assembly, are studied by ultra small-angle neutron scattering with theoretical modeling of the scattering function. Dynamic-mechanical modulus, nonlinear stationary viscosity, and stress build-up in start-up shear flows of the associated networks are studied on the basis of the affine and non-affine transient network theory. The molecular conditions for thickening, strain hardening, and stress overshoot are found in terms of the nonlinear amplitude A of the chain tension and the tension-dissociation coupling constant g.

Phase formation of associating polymers: gelation, phase separation and microphase formation☆

Abstract Polymers exhibit a variety of condensed phases when some of their segments are capable of forming weak bonds which can be created and destroyed by thermal motion. Transition from one phase to another caused by such ‘ segment association ’ is reversible by the change of the temperature and the concentration, so that it is called ‘ reversible phase transition ’. What types of reversible phases are possible for a given associative interaction? How does molecular association interfere with phase separation? This paper surveys, as typical examples of reversible phases, macroscopic phase separation, microphase formation, solvation, and gelation from a unified point of view. It explores the possibility of new condensed phases and critical phenomena driven by the mutual interference of different phase transitions.

Stress Buildup under Start-Up Shear Flows in Self-Assembled Transient Networks of Telechelic Associating Polymers†

The nonaffine transient network theory is used to study the time development of the shear and normal stresses under start-up shear flows in networks formed by self-assembled telechelic, hydrophobically modified water-soluble polymers. The initial slope, strain hardening, and overshoot of the shear stress are studied in detail in relation to the nonlinear tension-elongation curve of the elastically active chains in the network. The condition for the occurrence of strain hardening (upward deviation of the stress from the reference curve defined by the linear moduli) is found to be gamma > gammac(A), where gamma is the shear rate, gamma(c) is its critical value for strain hardening, and A is the amplitude of the nonlinear term in the tension of a chain. The critical shear rate gamma(c) is calculated as a function of A. It is approximately 6.3 (in the time unit of the reciprocal thermal dissociation rate) for a nonlinear chain with A = 10. The overshoot time t(max) when the stress reaches a maximum and the total deformation gamma(max) = gamma(t max) accumulated before the peak time are obtained in terms of the molecular parameters of the polymer chain. The maximum deformation gamma(max) turns out to depend weakly upon the shear rate gamma. The first and second normal stress differences are also studied on the basis of the exact numerical integration of the theoretical model by paying special attention to their overshoot, undershoot, and sign change as a function of the shear rate. These theoretical results are compared with recent rheological experiments of the solutions of telechelic hydrophobically modified poly(ethylene oxide)s carrying short branched alkyl chains (2-decyl-tetradecyl) at both ends.

Reentrant volume phase transition of cross-linked poly(N-isopropylacrylamide) gels in mixed solvents of water/methanol

The reentrant volume phase transition of cross-linked poly(N-isopropylacrylamide) gels in mixed water/methanol solvents is shown to be induced by the competition between polymer–water hydrogen bonds and polymer–methanol hydrogen bonds by means of the theoretical analysis of the experimental data. On the basis of a statistical-mechanical model for the competitive hydrogen bonds with cooperativity in forming sequences of hydrogen bonds on a polymer chain, the swelling ratio Q, the degree of hydration θw, the degree of methanol binding θm, the total coverage of polymer chains by the bound solvent molecules, the excess (selective) adsorption ΔξE of methanol, the spinodal region, are theoretically calculated as functions of temperature and the volume fraction ξ of methanol outside the gel. The results are compared with the experimental data. At room temperature T = 26.1 °C, the gel exhibits a discontinuous collapse (Q ≈ 0.15) at ξ ≈ 0.18 by the cooperative dehydration of the polymer chains. For higher ξ, the gel approaches the swollen state (Q ≈ 1.5), while in other temperature regions the coverage changes continuously.