Collapse of a poly（N-isopropylacrylamide） （PNIPAM） chain upon heating and phase diagrams of aqueous PNIPAM solutions with very fiat LCST phase separation line are theoretically studied on the basis of cooperative ...Collapse of a poly（N-isopropylacrylamide） （PNIPAM） chain upon heating and phase diagrams of aqueous PNIPAM solutions with very fiat 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. Reentrant coil-globule-coil transition in mixed solvent of water and methanol is 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 mole fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydophobically-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 USANS with theoretical modeling of the scattering function.展开更多
Investigating average thermodynamic quantities is not sufficient to understand conformational transitions of a finite-size polymer. We propose that such transitions are better described in terms of the probability dis...Investigating average thermodynamic quantities is not sufficient to understand conformational transitions of a finite-size polymer. We propose that such transitions are better described in terms of the probability distribution of some finite-size order parameter, and the evolution of this distribution as a control parameter varies. We demonstrate this claim for the coil-globule transition of a linear polymer and its mapping onto a two-state model. In a biological context, polymer models delineate the physical constraints experienced by the genome at different levels of organization,from DNA to chromatin to chromosome. We apply our finite-size approach to the formation of plectonemes in a DNA segment submitted to an applied torque and the ensuing helix-coil transition that can be numerically observed, with a coexistence of the helix and coil states in a range of parameters. Polymer models are also essential to analyze recent in vivo experiments providing the frequency of pairwise contacts between genomic loci. The probability distribution of these contacts yields quantitative information on the conformational fluctuations of chromosome regions. The changes observed in the shape of the distribution when the cell type or the physiological conditions vary may reveal an epigenetic modulation of the conformational constraints experienced by the chromosomes.展开更多
基金supported by a Grant-in-Aid for Scientific Research on Priority Areas"Soft Matter Physics"from the Ministry of Education,Culture,Sports,Science and Technology of Japan,and partly supported by a Grant-in-Aid for Scientific Research(B) from the Japan Society for the Promotion of Science under grant number 19350057
文摘Collapse of a poly（N-isopropylacrylamide） （PNIPAM） chain upon heating and phase diagrams of aqueous PNIPAM solutions with very fiat 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. Reentrant coil-globule-coil transition in mixed solvent of water and methanol is 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 mole fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydophobically-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 USANS with theoretical modeling of the scattering function.
基金the program "Small Systems Nonequilibrium Fluctuations,Dynamics and Stochastics,and Anomalous Behavior" of the Kavli Institute for Theoretical Physics China at the Chinese Academy of Sciences(KITPC),held in Bejing in July 2013funded by the French Institut de la Recherche M'edicale,under Grant MICROMEGAS PC201104+1 种基金the French Institut National du Cancer,under Grant INCa-5960the French Agence Nationale de la Recherche under Grant No.ANR-13-BSV5-0010-03.UPMC belongs to Sorbonne Universit'es
文摘Investigating average thermodynamic quantities is not sufficient to understand conformational transitions of a finite-size polymer. We propose that such transitions are better described in terms of the probability distribution of some finite-size order parameter, and the evolution of this distribution as a control parameter varies. We demonstrate this claim for the coil-globule transition of a linear polymer and its mapping onto a two-state model. In a biological context, polymer models delineate the physical constraints experienced by the genome at different levels of organization,from DNA to chromatin to chromosome. We apply our finite-size approach to the formation of plectonemes in a DNA segment submitted to an applied torque and the ensuing helix-coil transition that can be numerically observed, with a coexistence of the helix and coil states in a range of parameters. Polymer models are also essential to analyze recent in vivo experiments providing the frequency of pairwise contacts between genomic loci. The probability distribution of these contacts yields quantitative information on the conformational fluctuations of chromosome regions. The changes observed in the shape of the distribution when the cell type or the physiological conditions vary may reveal an epigenetic modulation of the conformational constraints experienced by the chromosomes.
