Conformational preferences of proline oligopeptides

A conformational study on the terminally blocked proline oligopeptides, Ac-(Pro)(n)()-NMe(2) (n = 2-5), is carried out using the ab initio Hartree-Fock level of theory with the self-consistent reaction field method in the gas phase and in solutions (chloroform, 1-propanol, and water) to explore the preference and transition between polyproline II (PPII) and polyproline I (PPI) conformations depending on the chain length, the puckering, and the solvent. The mean differences in the free energy per proline of the up-puckered conformations relative to the down-puckered conformations for both diproline and triproline increases for the PPII-like conformations and decreases for the PPI-like conformations as the solvent polarity increases. These calculated results indicate that the PPII-like structures have preferentially all-down puckerings in solutions, whereas the PPI-like structures have partially mixed puckerings. The free energy difference per proline residue between the PPII- and PPI-like structures decreases as the proline chain becomes longer in the gas phase but increases as the proline chain becomes longer in solutions and the solvent polarity increases. In particular, our calculated results indicate that each of the proline oligopeptides can exist as an ensemble of conformations with the trans and cis peptide bonds in solutions, although the PPII-like structure with all-trans peptide bonds is dominantly preferred, which is reasonably consistent with the previously observed results. In diproline Ac-(Pro)(2)-NMe(2), the rotational barrier to the cis-to-trans isomerization for the first prolyl peptide bond increases as the solvent polarity increases, whereas the rotational barrier for the second prolyl peptide bond does not show the monotonic increase as the solvent polarity increases. When the rotational barriers for these two prolyl peptide bonds were compared, it could be deduced that the conformational transition from PPI with the cis peptide bond to PPII with the trans peptide bond is initiated at the C-terminus and proceeds to the N-terminus in water. This is consistent with the results from NMR experiments on polyproline in D(2)O but opposite to the results from enzymatic hydrolysis kinetics experiments on polyproline.     

Metabolite Profiling to Monitor Organochlorine Pesticide Exposure in HepG2 Cell Culture

Gas chromatography coupled with time of flight mass spectrometry (GC/MS-TOF) was used to profile endogenous metabolites in HepG2 cell cultures to assess the metabolic changes induced by exposure to different organochlorine pesticides, their mixtures and controls (endosulfan, lindane, DDT and aldrin). Cells were cultured in DMEM with Glutamax at 37 °C with 5 % CO₂ for 72 h and then exposed to each pesticide, pesticide mixture or DMSO (as a control) for 24 h, and finally, endogenous metabolites were extracted and analyzed using GC/MS-TOF. The experiment was repeated six times under the same cell passage and culture conditions. PCA, PLS-DA and ROC were performed to analyze the GC/MS-TOF data and identify potential biomarkers. Thirty-five explanatory metabolites were found in both PCA and PLS-DA models, where Q ² was 0.86 and R ² was 0.98. Univariate and multivariate ROC showed potential biomarkers for each treatment, suggesting a general toxic mechanism for organochlorine pesticides that is specific for each type of compound. These results confirmed the effect of OCPs in sugar and amino acid metabolism that are linked with the function of cytochrome P450 in reductive dechlorination and oxidative stress.

     

Effects of Hydrogen-Enrichment on Flame-Holding of Natural Gas Jet Flames in Crossflow at Elevated Temperature and Pressure

Flow, Turbulence and Combustion - The effect of hydrogen ( $$\mathrm {H}_{\mathrm {2}}$$ ) enrichment on the flame-holding characteristics of two natural gas jet flames in crossflow is investigated...     

Preparation and Rheological Characteristics of Solvent‐Cast Poly(ethylene oxide)/Montmorillonite Nanocomposites

The organophilic montmorillonite clay and poly(ethylene oxide) (PEO) nanocomposites were intercalated by a solvent casting method using chloroform as the cosolvent. The prepared nanocomposites were characterized by an X‐ray diffraction method to examine their microstructure. Rheological properties of both the PEO/clay nanocomposites and the immiscible PEO/clay blends were investigated via a rotational rheometer in steady shear mode with a parallel plate geometry. The shear thinning viscosity data were fitted with the Carreau model, which showed that steady shear viscosity increases with increasing clay loading. The hysteresis phenomenon is observed to be enhanced with clay loading. PEO/clay nanocomposites exhibit higher zero‐shear‐rate viscosity and sharper shear thinning behaviors than immiscible PEO/clay blends.     

Ab initio SCF calculations of the linear infinite chain of LiH according to the pseudo-lattice method

The pseudo-lattice (PL) method has been reformulated for ab initio self-consistent-field (SCF) calculations. The translational symmetries of infinite systems have been applied to the finite model chain by manipulating all the intramolecular and intermolecular Fock matrices. The nuclear repulsion energy has been corrected accordingly. The method has been tested for the linear chain of lithium hydride under the constraint of equidistance between all neighboring lithium and hydrogen atoms. The calculated results of the infinite chain have been compared with those of finite chains of lithium hydride under the same geometric constraint. The equilibrium geometries, band structures, intermolecular stabilization energies and potential curves have been studied. It is found that the infinite systems cannot be described by considering only first nearest neighbor interactions, and the intermolecular interactions must be considered at least up to third nearest neighbors in order to obtain accurate value of force constant of infinite systems. We can conclude from band structures of infinite chains that the boundary effect of the finite model chain is effectively removed by the PL method.     

Duality, triality and complementary extremum principles in non-convex parametric variational problems with applications

This paper presents a reasonably complete duality theory anda nonlinear dual transformation method for solving the fullynonlinear, non-convex parametric variational problem inf{W(u- µ) - F(u)}, and associated nonlinear boundary valueproblems, where is a nonlinear operator, W is either convexor concave functional of p = u, and µ is a given parameter.Detailed mathematical proofs are provided for the complementaryextremum principles proposed recently in finite deformationtheory. A method for obtaining truly dual variational principles(without a dual gap and involving the dual variable p* of uonly) in n-dimensional problems is proposed. It is proved thatfor convex W(p), the critical point of the associated LagrangianL_µ(u, p*) is a saddle point if and only if the so-calledcomplementary gap function is positive. In this case, the systemhas only one dual problem. However, if this gap function isnegative, the critical point of the Lagrangian is a so-calledsuper-critical point, which is equivalent to the Auchmuty'sanomalous critical point in geometrically linear systems. Wediscover that, in this case, the system may have more than oneprimal-dual set of problems. The critical point of the Lagrangianeither minimizes or maximizes both primal and dual problems.An interesting triality theorem in non-convex systems is proved,which contains a minimax complementary principle and a pairof minimum and maximum complementary principles. Applicationsin finite deformation theory are illustrated. An open problemleft by Hellinger and Reissner is solved completely and a purecomplementary energy principle is constructed. It is provedthat the dual Euler-Lagrange equation is an algebraic equation,and hence, a general analytic solution for non-convex variational-boundaryvalue problems is obtained. The connection between nonlineardifferential equations and algebraic geometry is revealed.     

Polymer blend/organoclay nanocomposite with poly(ethylene oxide) and poly(methyl methacrylate)

A series of polymer blend/organoclay nanocomposite at a fixed blending ratio was prepared using equal ratio of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) via solvent casting method. With respect to nanoscale internal structure, we found that PMMA chains have better affinity with organoclay than PEO, based on the results from X-ray diffraction. Direct visualization via transmission electron microscopy (TEM) also supported the better affinity of PMMA with organoclay by indicating the existence of hybrid structures of mainly intercalated but with some exfoliated forms. The miscible nature of the blend and homogeneous dispersion state of layered silicate in the blend system were investigated via the microscopic fractured surface morphologies. From rheological measurements (storage and loss modulus), we discovered the role of the physical network structure between polymer and organoclay to be a main factor for the enhancement of elastic properties.     

Sum rules and their application to surface tension

Sum rules for inhomogeneous fluids are derived and then related to the surface tension of a liquid—vapor interface. It is found that this surface tension can be expressed in terms of the static correlation for the component of force normal to the interface. This formula is shown to be equivalent to two previous expressions for surface tension that generally are regarded as being exact. It is demonstrated that in the high-frequency or short-time limit this definition for surface tension leads directly to the standard hydrodynamic equation of motion for the normal component of the interfacial velocity.     

Solidlike transition of melt‐intercalated biodegradable polymer/clay nanocomposites

Synthetic biodegradable aliphatic polyester (BAP) intercalated into organoclay was prepared by melt compounding, and its solidlike characteristics were investigated via several rheological test modes: steady shear rotation, oscillation, and creep testing. Structural investigations with X‐ray diffraction and transmission electron spectroscopy were also performed for a better understanding of the characteristic rheological behaviors. The creep, recovery, and stress modulus exhibited a solidlike transition of BAP/clay nanocomposites that depended on the clay content. An increase in the zero shear rate viscosity and a shifting of the crossover point (storage modulus vs loss modulus) to a lower frequency were also observed with increasing clay contents. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2052–2061, 2003