197 research outputs found
Self-­‐ordering in solution of a novel polyamidoaminoacid with a chiral arginine side chain
Polyamidoaminoacids (PAAC) are a new class of bioinspired polymers obtained by polyaddition of selected aminoacids with N,N'-methylenebisacrylamide. Here we report some results obtained for the first such polymer, ARGO7, obtained with arginine stereoisomers. The CD spectra of the ARGO7 isomers showed an important and unexpected indication of a self-ordered pH-dependent secondary structure that was however nearly unaffected by temperature, ionic strength and denaturating agents. Theoretical modeling studies of L-ARGO7 carried out with classical Molecular Dynamical simulations showed that it assumed a folded structure with a transoid arrangement of the main chain reminiscent of the protein hairpin motif due to the intramolecular interactions. Torsion angles along the backbone showed a similar distribution at pH 6 and 14 consistent with the similarity of the CD spectra from pH 6 onwards
Conformations and dynamics of stars and dendrimers: the Gaussian Self-Consistent approach
Some conformational and dynamical aspects of branched polymer are reviewed.
We discuss the theoretical Gaussian Self-Consistent (GSC) approach
proposed in our group and used to study the behaviour of regular
star polymers and dendrimers in different solvent conditions. Within a single
framework, we consider the unperturbed Ξ state, as well as the goodsolvent
state in comparison with other theoretical or simulation approaches,
and with some experimental results. We also briefly report the further
results obtained for amphiphilic copolymer stars in selective solvents so as
to highlight the potentialities of the method, as well as its strengths and its
shortcomings.ΠΠ΄ΡΠΉΡΠ½Π΅Π½ΠΎ ΠΎΠ³Π»ΡΠ΄ Π΄Π΅ΡΠΊΠΈΡ
ΠΊΠΎΠ½ΡΠΎΡΠΌΠ°ΡΡΠΉΠ½ΠΈΡ
ΡΠ° Π΄ΠΈΠ½Π°ΠΌΡΡΠ½ΠΈΡ
Π°ΡΠΏΠ΅ΠΊΡΡΠ² Π³ΡΠ»ΠΊΠΎΠ²ΠΈΡ
ΠΏΠΎΠ»ΡΠΌΠ΅ΡΡΠ². ΠΠΈ ΠΎΠ±Π³ΠΎΠ²ΠΎΡΡΡΠΌΠΎ ΡΠ΅ΠΎΡΠ΅ΡΠΈΡΠ½ΠΈΠΉ Π³Π°ΡΡΡΠ²ΡΡΠΊΠΈΠΉ ΡΠ°ΠΌΠΎΡΠ·Π³ΠΎΠ΄ΠΆΠ΅Π½ΠΈΠΉ ΠΏΡΠ΄Ρ
ΡΠ΄, Π·Π°ΠΏΡΠΎΠΏΠΎΠ½ΠΎΠ²Π°Π½ΠΈΠΉ Ρ Π½Π°ΡΡΠΉ Π³ΡΡΠΏΡ, ΡΠΎ Π²ΠΈΠΊΠΎΡΠΈΡΡΠΎΠ²ΡΡΡΡΡΡ Π΄Π»Ρ Π²ΠΈΠ²ΡΠ΅Π½Π½Ρ ΠΏΠΎΠ²Π΅Π΄ΡΠ½ΠΊΠΈ ΡΠ΅Π³ΡΠ»ΡΡΠ½ΠΈΡ
Π·ΡΡΠΊΠΎΠ²ΠΈΡ
ΠΏΠΎΠ»ΡΠΌΠ΅ΡΡΠ² ΡΠ° Π΄Π΅Π½Π΄ΡΠΈΠΌΠ΅ΡΡΠ² Π² ΡΠΌΠΎΠ²Π°Ρ
ΡΡΠ·Π½ΠΈΡ
ΡΠΎΠ·ΡΠΈΠ½Π½ΠΈΠΊΡΠ². ΠΠΈ ΡΠΎΠ·Π³Π»ΡΠ΄Π°ΡΠΌΠΎ Π² ΡΠ΄ΠΈΠ½ΠΈΡ
ΡΠ°ΠΌΠΊΠ°Ρ
ΡΠΊ Π½Π΅Π·Π±ΡΡΠ΅Π½ΠΈΠΉ Ξ -ΡΡΠ°Π½, ΡΠ°ΠΊ Ρ ΡΡΠ°Π½ Π΄ΠΎΠ±ΡΠΎΠ³ΠΎ ΡΠΎΠ·ΡΠΈΠ½Π΅Π½Π½Ρ, Π² ΠΏΠΎΡΡΠ²Π½ΡΠ½Π½Ρ Π·
ΡΠ½ΡΠΈΠΌΠΈ ΡΠ΅ΠΎΡΠ΅ΡΠΈΡΠ½ΠΈΠΌΠΈ ΡΠΈ ΡΠΈΠΌΡΠ»ΡΡΡΠΉΠ½ΠΈΠΌΠΈ ΠΏΡΠ΄Ρ
ΠΎΠ΄Π°ΠΌΠΈ, Π° ΡΠ°ΠΊΠΎΠΆ Π· Π΄Π΅ΡΠΊΠΈΠΌΠΈ Π΅ΠΊΡΠΏΠ΅ΡΠΈΠΌΠ΅Π½ΡΠ°Π»ΡΠ½ΠΈΠΌΠΈ ΡΠ΅Π·ΡΠ»ΡΡΠ°ΡΠ°ΠΌΠΈ. ΠΠΈ ΠΊΠΎΡΠΎΡΠΊΠΎ ΠΏΡΠ΅Π΄ΡΡΠ°Π²Π»ΡΡΠΌΠΎ
ΡΠ°ΠΊΠΎΠΆ ΠΏΠΎΠ΄Π°Π»ΡΡΡ ΡΠ΅Π·ΡΠ»ΡΡΠ°ΡΠΈ, ΠΎΡΡΠΈΠΌΠ°Π½Ρ Π΄Π»Ρ Π°ΠΌΡΡΡΡΠ»ΡΠ½ΠΈΡ
ΠΊΠΎΠΏΠΎΠ»ΡΠΌΠ΅ΡΠ½ΠΈΡ
Π·ΡΡΠΎΠΊ Ρ Π²ΡΠ΄Π±ΡΡΠΊΠΎΠ²ΠΈΡ
ΡΠΎΠ·ΡΠΈΠ½Π½ΠΈΠΊΠ°Ρ
, Π· ΠΌΠ΅ΡΠΎΡ ΠΏΠΎΠΊΠ°Π·Π°ΡΠΈ ΡΠΊ ΠΌΠΎΠΆΠ»ΠΈΠ²ΠΎΡΡΡ ΠΌΠ΅ΡΠΎΠ΄Ρ, ΡΠ°ΠΊ Ρ ΠΉΠΎΠ³ΠΎ Π½Π΅Π΄ΠΎΠ»ΡΠΊΠΈ
Organic Inhibitors to Prevent Chloride-Induced Corrosion inΒ Concrete: Atomistic Simulations of Triethylenetetramine-Based Inhibitor Film
Inhibitors are largely used to prevent chloride-induced corrosion in reinforced concrete structures thanks to both a barrier effect on chloride penetration and a competition with the adsorption of the inhibitor. The interaction mechanisms between passive film on carbon steel, the inhibitor molecule, and chlorides still require deeper understanding. Theoretical studies based on molecular mechanics (MM) and molecular dynamics (MD) methods can be useful to better understand the passive film formation and its interaction with chlorides. In this work, the interaction between a triethylenetetramine (TETA) inhibitor film on Ξ³-FeOOH surface and chlorides is studied using MD methods. After MM optimization in the initial adsorption stage, some chlorides are close to protective TETA film. After MD run at room temperature effectively, chlorides remain close to the protective film. In order to have an effective barrier on chloride attack, the metal oxide must remain wholly covered by the protective film. The TETA film well covers the lepidocrocite surface but cannot kinetically efficiently prevent the chloride-induced corrosion compared to other organic films exposing COOβ groups because it does not exert any repulsion to chlorides
Self-Structuring in Water of Polyamidoamino Acids with Hydrophobic Side Chains Deriving from Natural α-Amino Acids
This paper reports on synthesis, acid-base properties and self-structuring in water of chiral polyamidoamino acids (PAACs) obtained by polyaddition of N,N'-methylenebisacrylamide with L-alanine, L-valine and L-leucine (M-L-Ala, M-L-Val, M-L-Leu) with potential for selective interactions with biomolecules. The polymers maintained the acid-base properties of amino acids. In water, the circular dichroism spectra of PAACs revealed pH-dependent structuring in the range 3-11 and in the wavelength interval 200-280 nm. Taking as reference the values at pH 3, the differential molar ellipticities were plotted in the pH interval 3-11. Sigmoidal curves were obtained presenting inflection points at pH 8.1, 6.8 and 7.3 for M-L-Ala, M-L-Val and M-L-Leu, respectively, corresponding to the amine half-ionization. Theoretical modeling showed that PAACs assumed stable folded conformations. Intramolecular interactions led to transoid arrangements of the main chain reminiscent of protein hairpin motif. Oligomers with ten repeat units had simulated gyration radii consistent with the hydrodynamic radii obtained by dynamic light scattering
Bulk and Interfacial Shear Thinning of Immiscible Polymers
Nonequilibrium molecular dynamics simulations are used to study the shear
thinning behavior of immiscible symmetric polymer blends. The phase separated
polymers are subjected to a simple shear flow imposed by moving a wall parallel
to the fluid-fluid interface. The viscosity begins to shear thin at much lower
rates in the bulk than at the interface. The entire shear rate dependence of
the interfacial viscosity is consistent with a shorter effective chain length
that also describes the width of the interface. This is independent
of chain length and is a function only of the degree of immiscibility of
the two polymers. Changes in polymer conformation are studied as a function of
position and shear rate.Shear thinning correlates more closely with a decrease
in the component of the radius of gyration along the velocity gradient than
with elongation along the flow. At the interface, this contraction of chains is
independent of and consistent with the bulk behavior for chains of length
. The distribution of conformational changes along chains is also studied.
Central regions begin to stretch at a shear rate that decreases with increasing
, while shear induced changes at the ends of chains are independent of .Comment: 8 pages, 8 figure
Effect of chain length and topological constraints on segmental relaxation in cyclic PDMS
We present a detailed investigation of local dynamics of linear and cyclic poly(dimethylsiloxane) (PDMS) covering a wide range of molar masses. To aid interpretation of the experimental data, QENS measurements in the time scale from 2 to 200 ps and at Q = 0.3 to 1.8 Γ
β1 are complemented by theoretical calculations. These make use of a methodology developed by us elsewhere applicable to both simple chain models and real chains and applied here, for the first time, to cyclic PDMS. Analysis of the incoherent dynamic structure factor at T < Tm shows that the rotational motion of the methyl groups is unaffected by polymer topology. At higher temperatures, the QENS data are described by a model that consists of two dynamic contributions: methyl group rotation and segmental motion, the latter described by a stretched exponential function. Relaxation times of both linear and cyclic PDMS increase with increasing molar mass. Several features predicted by theory are also reproduced by the experimental data. We show, unambiguously, that rings have higher relaxation times for the segmental motion compared to linear chains of the same number of monomer units. Theoretical calculations support the idea that such slowing down of local dynamics is due to the topological constraint imposed by the ring closure, a constraint which becomes negligible for very large molar masses. Our calculations suggest that due to its albeit small conformational rigidity, cyclic PDMS undergoes an additional constraint which further increases the relaxation time, producing a shallow maximum for N β 50 repeat units. A similar feature is also observed in the experimental QENS data. Values of activation energy, Ea, are derived from analysis of the temperature dependence of the quasi-elastic broadening and are found to be in agreement with viscosity measurements reported in the literature. Although the pronounced molar mass dependence of Ea for linear PDMS is certainly linked to the presence of mobile chain ends, for the cyclic polymers the behavior appears to be more complex than anticipated
Dynamics of heteropolymers in dilute solution: effective equation of motion and relaxation spectrum
The dynamics of a heteropolymer chain in solution is studied in the limit of
long chain length. Using functional integral representation we derive an
effective equation of motion, in which the heterogeneity of the chain manifests
itself as a time-dependent excluded volume effect. At the mean field level, the
heteropolymer chain is therefore dynamically equivalent to a homopolymer chain
with both time-independent and time-dependent excluded volume effects. The
perturbed relaxation spectrum is also calculated. We find that heterogeneity
also renormalizes the relaxation spectrum. However, we find, to the lowest
order in heterogeneity, that the relaxation spectrum does not exhibit any
dynamic freezing, at the point when static (equilibrium) ``freezing''
transition occurs in heteropolymer. Namely, the breaking of
fluctuation-dissipation theorem (FDT) proposed for spin glass dynamics does not
have dynamic effect in heteropolymer, as far as relaxation spectrum is
concerned. The implication of this result is discussed
Early Stages of Homopolymer Collapse
Interest in the protein folding problem has motivated a wide range of
theoretical and experimental studies of the kinetics of the collapse of
flexible homopolymers. In this Paper a phenomenological model is proposed for
the kinetics of the early stages of homopolymer collapse following a quench
from temperatures above to below the theta temperature. In the first stage,
nascent droplets of the dense phase are formed, with little effect on the
configurations of the bridges that join them. The droplets then grow by
accreting monomers from the bridges, thus causing the bridges to stretch.
During these two stages the overall dimensions of the chain decrease only
weakly. Further growth of the droplets is accomplished by the shortening of the
bridges, which causes the shrinking of the overall dimensions of the chain. The
characteristic times of the three stages respectively scale as the zeroth, 1/5
and 6/5 power of the the degree of polymerization of the chain.Comment: 11 pages, 3 figure
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