A model for the helix–coil transition of DNA

The helix–coil phase transition of DNA is studied through a model equivalent to the one used in the theory of superconductivity. A distribution function for the excitations analogous to the usual Fermi function is obtained which depends on the parameter T₀, the maximum temperature at which all pairs of bases are bonded. The expression for the number of unbonded pairs of bases depends only on the two parameters, T₀ and T_c (critical temperature).     

Non-newtonian viscosity of polypeptides in the helix–coil transition region

The non-Newtonian intrinsic viscosities [η] of poly(γ-methyl L-glutamate) were measured in the helix–coil transition region under various conditions in this work. The helix content f_H, which represents the degree of conformational transition, was obtained by using a polarimeter. Our experimental results show that the non-Newtonian behavior of the polypeptide is markedly affected by its conformation; i.e., the non-Newtonian effect becomes larger as f_H increases. The effect of external pressure ΔP on [η] was studied carefully; [η] increases with f_H when ΔP < 1.5 psi, but it decreases when ΔP > 1.5 psi and f_H > 0.8. The reason for this result is considered in the text.     

A database of lipid phase transition temperatures and enthalpy changes

The systematic study of the mesomorphic phase properties of synthetic and biologically derived lipids began some 30 years ago. In the past decade, interest in this area has grown enormously. As a result, there exists a wealth of information on lipid phase behavior, but unfortunately, these data have, until now, been scattered throughout the literature in a variety of books, proceedings, and journals. The data have recently been compiled in a centralized database with a view to providing ready access to the same and to the appropriate literature. The compilation facilitates review of what has thus far been accomplished and highlights what remains to be done in this active research area. As such, it represents a convenient summary of the existing data which, when evaluated, will enable us to identify where deficits exist in the data, to reveal the fundamental physicochemical principles upon which lipid phase behavior is based, and to understand more completely lipid phase relations in biological, reconstituted, and formulated systems. The compilation consists of a tabulation of all known mesomorphic and polymorphic phase transition temperatures and enthalpy changes for synthetic and biologically derived lipids in the dry and in the partially and fully hydrated states. Also included is the effect on these thermodynamic values of pH, and of salt and metal ion concentration and other additives such as proteins, drugs, etc. The methods used in making the measurements and the experimental conditions are reported. Bibliographic information includes complete literature referencing and list of authors. As of this writing, the database is current through June 1990 and contains 9500 records. Each record contains 28 fields. Here, we describe how the database originated, its scope and contents, data abstraction procedures, and issues relating to mesophase and lipid nomenclature, data analysis, and evaluation, and database maintenance and distribution.     

Age related changes in the thermal transition of Turkey leg flexor tendon collagen

A differential scanning calorimetric, thermogravimetric and electron microscopic investigation has been carried out on the uncalcified areas of turkey leg flexor tendon as a function of age. Rehydrated samples exhibit an increase of thermal stability with age. The H_D values drop from about 11 cal·g^–1 in the first weeks of life down to 7 cal·g^–1 after the 11th week.At about 11 weeks, the collagen fibril diameter distribution passes from unimodal to multimodal. The DSC curves as well as the TG-DTG curves recorded from dried samples do not show any appreciable difference with ageing. The variations in thermal behaviour of rehydrated samples and fibril diameter distribution could be related to modifications in water binding with ageing.The Authors are grateful to Dr. G. Fabris for discussion and help in the selection of the samples. They also wish to thank Mr. G. Pizzuto for excellent technical assistance. The financial support by Consiglio Nazionale delle Ricerche is gratefully acknowledged.     

Integral enthalpy of mixing of the liquid ternary Au–Cu–Sn system

The integral enthalpy of mixing of the ternary Au–Cu–Sn has been determined with a Calvet type calorimeter at 6 different cross sections at 1273 K. The substitutional solution model of Redlich–Kister–Muggianu was used for a least square fit of the experimental data in order to get an analytical expression for the integral enthalpy of mixing. The ternary extrapolation models of Kohler, Muggianu and Toop were used to calculate the integral enthalpy of mixing and to compare measured and extrapolated values. Additional calculations of the integral enthalpy of mixing using the Chou model have been performed. With the calculated data, the iso-enthalpy lines have been determined using the Redlich–Kister–Muggianu model. A comparison of the data has been made.     

Length dependence of the coil <--> beta-sheet transition in a membrane environment

The most abundant structural element in protein aggregates is the beta-sheet. Designed peptides that fold into a beta-sheet structure upon binding to lipid membranes are useful models to elucidate the thermodynamic characteristics of the random coil <-->beta-structure transition. Here, we examine the effect of strand length on the random coil <--> beta-sheet transition of the (KIGAKI)n peptide with the total chain length varying between 7 and 30 amino acids. The beta-sheet content of the peptides in the presence and absence of membranes was measured with circular dichroism spectroscopy. The peptides were titrated with small unilamellar lipid vesicles, and the thermodynamic binding parameters were determined with isothermal titration calorimetry (ITC). Membrane binding includes at least two processes, namely (i) the transfer of the peptide from the aqueous phase to the lipid surface and (ii) the conformational change from a random coil conformation to a beta-sheet structure. CD spectroscopy and ITC analysis demonstrate that beta-sheet formation depends cooperatively on the peptide chain length with a distinct increase in beta-structure for n > 10-12. Binding to the lipid membrane is an entropy-driven process as the binding enthalpy is always endothermic. The contribution of the beta-sheet folding reaction to the overall process was determined with analogues of the KIGAKI repeat where two adjacent amino acids were replaced by their D-enantiomers. The folding reaction for peptides with n >or= 12 is characterized by a negative free folding energy of DeltaG(degree)beta approximately equal -0.15 kcal/mol per amino acid residue. The folding step proper is exothermic with DeltaH(degree)(beta) approximately equal -0.2 to -0.6 kcal/mol per residue and counteracted by a negative entropy term TDeltaS(degree)(beta) = -0.1 to -0.5 kcal/mol per residue, depending on the chain length (18 相似文献   

Semiempirical calculations of binding enthalpy for protein–ligand complexes

A systematic semiempirical quantum mechanical study of the interactions between proteins and ligands has been performed to determine the ability of this approach for the accurate estimation of the enthalpic contribution to the binding free energy of the protein–ligand systems. This approach has been applied for eight test protein–ligand complexes with experimentally known binding enthalpies. The calculations were performed using the semiempirical PM3 approach incorporated in the MOPAC 97, ZAVA originally elaborated in Algodign, and MOPAC 2002 with MOZYME facility packages. Special attention was paid to take into account structural water molecules, which were located in the protein–ligand binding site. It was shown that the results of binding enthalpy calculations fit experimental data within ～2 kcal/mol in the presented approach. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Spontaneous formation of narrowly‐distributed self‐assembly from polyamidoamine dendron‐poly(L‐lysine) block copolymers through helix‐coil transition of poly(L‐lysine) block

The self‐assembly of head‐tail type block copolymers composed of polyamidoamine dendron head block and poly(L ‐lysine) (PLL) tail block was studied using a light scattering technique and transmission electron microscopy. A PLL tail block in a head‐tail type block copolymer exhibits a coil‐to‐helix transition as a result of the change in solvent quality from water to methanol. When the PLL tail block takes a helical conformation in high methanol content, the resulting head‐tail type block copolymer has a defined three‐dimensional structure like that of a protein molecule. Self‐assemblies of such block copolymers having a totally fixed molecular shape spontaneously form polymersome‐like self‐assemblies with an extremely narrow size distribution through converging to a thermodynamically stable assembling state. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1217–1223, 2009     

Helix–coil kinetics of two 14-residue peptides

The helix–coil transition kinetics of two 14-residue helical peptides of different stability were studied by time-resolved infrared (IR) spectroscopy coupled with laser-induced temperature-jump (T-jump) technique. The T-jump induced relaxation kinetics of both peptides show strong dependence on the final temperature, implying the existence of an enthalpic barrier for the nucleation process. In addition, the peptide with end-capping groups, which is more stable, folds faster. Together, these results suggest that the overall helical stability plays an important role in controlling the kinetics of the helix–coil transition, in agreement with results of early theoretical studies.     

Quantum analogue of the Kolmogorov–Arnold–Moser transition in different quantum anharmonic oscillators

The results of numerical computations are presented for the Bohmian trajectories of the family of different one‐ and two‐dimensional anharmonic oscillators, which exhibit regular or chaotic motion in both classical and quantum domains, depending on the values of the parameters appearing in the respective Hamiltonians. Quantum signatures of the Kolmogorov–Arnold–Moser (KAM) transition from the regular to chaotic classical dynamics of these oscillators are studied using a quantum theory of motion (QTM) as developed by de Broglie and Bohm. A phase space distance function between two initially close Bohmian trajectories, the associated Kolmogorov–Sinai–Lyapunov (KSL) entropy, the phase space volume, the autocorrelation function, the associated power spectrum, and the nearest‐neighbor spacing distribution, clearly differentiate the quantum analogues of the corresponding regular and chaotic motions in the classical domain. These quantum anharmonic oscillators are known to be useful in several diverse branches of science. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Dynamic heat capacity and reaction enthalpy during the two-stage network growth in diepoxide–diamine compositions

The real and imaginary components of the dynamic heat capacity, C_p′ and C_p″, respectively, have been measured for a fixed frequency of 5 mHz during the polymerization of various compositions of a diepoxide–diamine, molecular liquid mixture to a network structure. The heat evolved during the polymerization was measured simultaneously. C_p′ decreased in two steps as the covalent bonds formed and the network structure grew. The steps became more separated when the amount of the already excess diepoxide was further increased. C_p″ showed a peak in its plot against the polymerization time, but only in the region where C_p′ showed a second step. This is attributed to the increase in the relaxation time leading to vitrification of the liquid. For the diepoxide-rich compositions, the enthalpy release also occurred in two steps and it was more for the second stage of the network's growth than for the first. Combined measurements of the exothermic effects and C_p′ and C_p″ thus delineated two stages of the network's growth by two chemical reactions. The nature of the second-stage network growth that ultimately vitrifies the stoichiometric liquid mixture is discussed. It is concluded that the second-stage growth is mass-controlled and occurs by an etherification reaction whose thermodynamic consequences have been elusive in past studies.     

Upper liquid–liquid transition in unsaturated polyesters

The upper liquid–liquid transition (T_lρ) was detected and investigated in unsaturated polyesters. This less known transition is caused by a stepwise decrease of intramolecular short‐range local order that remains above the glass and lower liquid–liquid transitions. The local order is based on secondary valent interactions and is enhanced by hydrogen bonding, if possible, as in the polyesters under consideration. The T_lρ was detected as a change in the temperature dependence of the viscosity and electrical conductivity and differential scanning calorimetry thermograms revealed an endothermic change, above which the activation energy of crosslinking decreased. The T_lρ temperature was lower in a polyester containing a diole with more flexible structure. The changes in electrical conductivity and ¹H NMR spectra indicated that the breaking of hydrogen bonding caused the T_lρ. Its intramolecular nature resulted in low sensitivity to crosslinking and addition of solvents. Near‐IR spectroscopy was chosen for further investigation because of its greater sensitivity to structural changes in polymers than mid‐IR. The spectra were recorded at selected temperatures. Significant intensity changes and wavelength shifts of hydroxyl and carbonyl absorptions at 1430 and between 1900 and 2100 nm, respectively, occurred at the T_lρ, while carbon–hydrogen absorptions showed only minor changes. This confirmed that breaking of the hydrogen bonding was responsible for disruption of the short‐range local order. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 129–145, 2001     

Ordered nanostructures from self‐assembly of H‐shaped coil–rod–coil molecules

A new class of π‐conjugated, skewed H‐shaped oligomers, consisting of biphenyl, phenylene vinylene, and phenylene ethynylene units as the rigid segment, were synthesized via Sonogashira coupling and Wittig reactions. The coil segments of these molecules were composed of poly(ethylene oxide) (PEO) or PEO with lateral methyl groups between the rod and coil segment, respectively. The experimental results revealed that the lateral methyl groups attached to the surface of the rod and coil segments dramatically influenced the self‐assembling behavior of the molecules in the crystalline phase. H‐shaped rod–coil molecules containing a lateral methyl group at the surface of the rod and PEO coil segments self‐assemble into a two‐dimensional columnar or a three‐dimensional body‐centered tetragonal nanostructures in the crystalline phase, whereas molecules lacking a lateral methyl group based on the PEO coil chain self‐organize into lamellar or hexagonal perforated lamellar nanostructures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 85–92     

Effect of high pressure on the nematic–isotropic transition in aerosil–liquid crystal composites

We report the first high pressure investigations of the nematic–isotropic transition in the composites of a liquid crystal compound with hydrophilic aerosil particles. The low concentrations of the aerosil particles used create soft gels of the composites. As expected T_N–Iso, the nematic–isotropic transition at room pressure exhibits a non-monotonic variation with increasing aerosil concentration. This non-monotonic behaviour is seen in the isobaric scans over the wide range of pressures studied, and its “magnitude” is dependent on the pressure applied. The surprising result of the present investigations on these nanocolloidal systems is that the slope of the pressure–temperature boundary also exhibits a non-monotonic dependence with the aerosil concentration, which qualitatively is similar to that of the transition temperature variation. Employing the transition enthalpy values determined at room pressure using differential scanning calorimetric scans collected at low heating rates, we calculate the transition volume dependence on the aerosil concentration. The study adds a new dimension, namely, the influence of pressure on liquid crystalline transitions in restricted geometries.     

Effect of crosslink length on the enthalpy relaxation of fully cured epoxy–diamine resins

Enthalpy relaxation of epoxy–diamine thermosets of different crosslink lengths (CLL) has been studied by DSC. The epoxy resins based on diglycidyl ether of bisphenol A were cured with ethylenediamine (FEDA), and diamines of polyoxypropylene of 2.6 and 5.6 oxypropylene units, named FJ230 and FJ400, respectively. As was expected, increasing the CLL decreases the glass transition temperature T_g from 121°C (FEDA) to 47°C (FJ400). Aging experiments at T_g − 20 K for each resin permit the determination of the enthalpy loss, the relaxation rate per decade (β_H), and the nonlinearity parameter, x. The apparent activation energy, Δh*, and the nonexponentiality parameter β are found for each resin from intrinsic cycles in which the sample is heated at 10 K min⁻¹ following cooling at various rates through the glass transition region. An increase of CLL is related to an increase of β_H, and of the nonlinearity parameter. In agreement with the general trend for thermoplastic polymers, the increase of the parameter x is correlated with a decrease of Δh* and with an increase in the nonexponentiality parameter. Application of the Adam–Gibbs (AG) theory reveals that the parameters B and T_f/T₂ increase with CLL, corresponding to a decrease of the nonlinear behavior of the glassy epoxies. However, the T₂ values calculated in this way appear unrealistic, and the alternative assumption that T₂ = T_g −51.6 K, making use of the “universal” WLF constant, leads to a much smaller variation of B, which nevertheless still increases with CLL. From a consideration of the minimum number of configurations required for a cooperative rearrangement, it is argued that the elementary activation energy Δμ increases, and the minimum size of the cooperatively rearranging region decreases as CLL increases. This is consistent with the relaxation process becoming more cooperative as the CLL decreases, as is suggested by the decrease in the value of β. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 456–468, 2000     

Hydration-dependent structural deformation of guanine in the electronic singlet excited state

Theoretical study was performed to investigate how the degree of hydration affects the structures and properties of the canonical form (keto-N9H) of guanine in the ground and lowest singlet pipi* excited state. This work is the continuation of our earlier work where we have studied the hydration of guanine in the first solvation shell with one, three, five, and six water molecules. In the present investigation, we have considered 7-13 water molecules in hydrating guanine. Ground-state geometries were optimized at the Hartree-Fock level, whereas the configuration interaction-singles (CIS) method was used for the excited-state geometry optimization. The 6-311G(d,p) basis set was used in all calculations. The harmonic vibrational frequency analysis was used to determine the nature of the optimized ground- and excited-state potential energy surfaces; all geometries were found to be minima at the respective potential surfaces. It was found that the degree of hydration has a significant influence on the excited-state structural nonplanarity of guanine. It is expected that excited-state dynamics of guanine will depend on the degree of hydration. Ground- and excited-state geometries of selected hydrated species were also optimized in the bulk water solution using the polarizable continuum model (PCM). It was found that bulk water solution generally does not have significant influence on the structure of the hydrated species. Effects of hydration on different stretching vibrations in the ground and excited states are also discussed.     

Nature of melt rheological transition in a styrene–butadiene–styrene block copolymer

Our laboratory previously reported the observation of a high temperature, melt rheological transition in a styrene–butadiene–styrene (S:7 × 10³ and B:43 × 10³) block copolymer from the highly elastic, nonlinear viscous behavior typical of a multiphase structure to linear viscous behavior with insignificant elasticity typical of a single-phase structure. We have investigated the precise nature of this melt rheological transition in the 7S-43B-7S sample by measuring the dynamic viscoelastic properties at more than 11 temperatures, including several in the transition region. A new procedure was developed for accurately measuring the sample temperature in a Weissenberg rheogoniometer. The transition is found to start at about 140°C and proceed over a narrow transition region from 140 to about 150°C. Data at all temperatures superimpose onto a single master curve only at high reduced frequencies. At low reduced frequencies, two characteristic branches of the master curve are formed. The data at temperatures below the transition region superimpose onto the upper branch where the dynamic viscosity η′(ω) is a strong function of ω, whereas the data at temperatures above the transition region superimpose onto the lower branch where η′(ω) is independent of ω. The data at temperatures within the transition region fall between the upper and lower branches, ordered according to their temperature positions. The apparent flow activation energy is found to be constant at about 22.8 kcal/mole below the transition region, but appears to decrease to about 17.4 kcal/mole above the transition region. The narrowness of the rheological transition far above the glass transition temperature of the polystyrene domains and the limiting linear viscoelastic behavior at low frequencies above the transition suggest an accompanying morphological transition rather than a gradual weakening of the polystyrene domains.     

Macromolecule–metal complexes exhibiting phase transition

A copolymer gel prepared from N-isopropyl-acrylamide (NIPA) and vinylferrocene (VF) exhibits volume phase transition. The phase transition can be controlled electrochemically, and electrochemical behavior of the gel can be controlled thermally. A copolymer of NIPA and VF, which is not crosslinked, also possesses similar characteristics. Those polymer and gel can be applied to enzyme electrodes. © 1997 John Wiley & Sons, Ltd.     

Liquid–liquid transition in polymers and glass-forming liquids

It is shown that many simple glass-forming liquids exhibit a phenomenon known in the area of polymer science as the liquid–liquid transition. The phenomenon manifests itself as a third-order transition in the equilibrium liquid-specific heat data around approximately 1.2 T_g and also as a bifurcation of the liquid relaxation into primary and secondry processes. It is stressed that the above phenomenon is due to a smooth changeover of the liquid from one dynamic regime to the other and hence is not due to any real phase transition. It is suggested that a liquid cluster kind of picture for the supercooled liquid regime, is capable of explaining the above phenomenon and is consistent with observation made on polymers and monomeric liquids. © 1993 John Wiley & Sons, Inc.