Optical rotatory dispersion and circular dichroism studies of copolyaspartates with long alkyl side chain

Copoly(hexyl-l-aspartate-benzyl-l-aspartate), copoly(nonyl-l-aspartate-benzyl-l-aspartate) and copoly(dodecyl-l-aspartate-benzyl-l-aspartate) with various compositions were prepared by ester exchange reaction of poly(β-benzyl-l-aspartate). Copolyaspartates of low degree of alkylation were found to take a left handed α helix, similar to poly(β-benzyl-l-aspartate), and those of high degree of alkylation changed to a right handed α helix. Copoly(hexyl-l-aspartate-benzyl-l-aspartate) and copoly(dodecyl-l-aspartate-benzyl-l-aspartate) changed helix sense from left to right at 50% hexylation and 10% dodecylation, respectively. A small induced circular dichroism trough appeared around 230nm for these two copolyaspartates above 75% alkylation, indicative of a certain ordering of the carbonyl group due to the formation of an ordered structure of a long alkyl side chain. However, copoly(nonyl-l-aspartate-benzyl-l-aspartate) of 13% nonylation exhibited the induced circular dichroism spectrum and took a left handed α helix. Copoly(nonyl-l-aspartate-benzyl-l-aspartate) caused the reversal in helix sense above 70% nonylation according to a stabilizing effect of the ordered structure of the side chain.     

Theoretical study of temperature induced phase transitions in poly(β-benzyl-l-aspartate) and it's copolymer

In polypeptides and proteins, the phenomenon of conformational transitions has been successfully explained on the basis of Zimm and Bragg model, as a function of temperature, pressure, solvent concentration and pH etc. The Zimm and Bragg theory of helix↔coil transition has been modified to explain the temperature induced helix→helix transitions in solid films of poly(β-benzyl-l-aspartate) and in its copolymer copoly(β-stearyl-l-aspartate-β-benzyl-l-aspartate) with varying degrees of stearylation. An expression for the degree of order is obtained from the grand partition function for the entire chain in terms of nucleation parameter, which controls the transition width. At low degree of stearylation, an α→ω transition occurs with the increase in temperature, whereas, at degree higher than 40%, the reversal of α helix sense from right-handed to left-handed takes place. The theoretical transition curves are found to be in good agreement with the experimental data.     

3,3'-diethylthiatricarbocyanine iodide: a highly sensitive chiroptical reporter of DNA helicity and sequence

Using UV-vis absorption and circular dichroism (CD) spectroscopies, we explored the binding interactions of 3,3'-diethylthiatricarbocyanine iodide (Cy7) with polynucleotides of different sequences and helicity. CD showed to be a very diagnostic tool giving different spectroscopic chiroptical signatures for all explored DNA sequences upon Cy7 binding. Cy7 was able to spectroscopically discriminate between the right handed B-DNA of poly(dG-dC)(2) and its left handed Z-DNA counterpart induced by spermine or Co(III)hexamine via nearly opposite induced circular dichroic signal.     

Highly cooperative helix-sense reversal of polyaspartates. Influence of the comonomer of the opposite screw-sense preference

Poly(l-aspartic acid ester)s are known to form an α-helix in either the right(r)- or left(?)-handed screw depending on the chemical constitution of the ester group in the side chain. While poly(β-benzyl l-aspartate) (PBLA) normally remains in the ?-form in conventional helicoidal solution at all temperatures, poly(β-phenethyl l-aspartate) (PPLA) exhibits transformation from the r- to ?-form with increasing temperature. In this work, random copolymers comprising residues of the two opposite screw-sense preferences, BLA and PLA, were chosen to investigate the stability of the α-helical hydrogen-bond array in a helix-forming solvent such as chloroform or 1,1,2,2-tetrachloroethane. Sharp S-shape transitions of the helix sense were observed for copolymers in the range PLA > 50%. The minor component BLA strictly obeys the majority rule in these copolymers. ²H NMR studies of partially deuterated samples in the nematic liquid-crystalline (LC) state have proved that the thermally induced r ↔ ? helix-sense inversion takes place highly cooperatively along the α-helical backbone: i.e., the cooperativity of the transition was not much affected by the inclusion of substantial amount of weaker points along the chain. These observations unambiguously suggest a zipper-type propagation process for PPLA starting from one terminal to the other. For given copolymer samples, the transition temperatures determined in dilute isotropic solution are nearly identical to those observed in the LC state, suggesting that the transition mechanism remains also similar. In the succeeding paper, relevant experimental data will be interpreted according to the theory consistent with these observations.     

Circular dichroism of poly(β-benzyl-l-aspartate) films in the α, β and ω conformations

The conformations of poly(β-benzyl l-aspartate) (PBLA) in film were studied by means of circular dichroism (c.d.). Films of PBLA show c.d. which is related to the benzyl chromophores of this polypeptide. The magnitude and sign of the c.d. of the benzyl chromophores depend on the conformation i.e. α-helix, ω-helix or β-form. The aromatic c.d. of the left-handed α-helix or the left-handed ω-helix is negative in sign, and that of the β-form is positive in sign. The absolute magnitude of the c.d. of PBLA is much larger in the ω-helical conformation than in the α-helical or the β-conformations. In the left-handed α-helical PBLA (α-PBLA), the positive dichroism band of the $\text{n?π1}$ peptide electronic transition is observed at about 226 nm. When the α-PBLA is converted to the left-handed ω-helical PBLA (ω-PBLA), the band at 226 nm shifts to 224 nm, and its magnitude becomes much smaller. In the β-form PBLA (β-PBLA), this band is negative in sign and observed at 223 nm. The absolute magnitude of the c.d. relative to this electronic transition follows the order: β-form < ω-helix < α-helix.     

N.m.r. of random copolymers of benzyl-l-aspartate with o- and p-nitrobenzyl-l-aspartate

The p- and o-substitution of the benzene ring of poly(β-benzyl-l-aspartate) (PBLA) with a nitro group induces a reversal of the helix sense in the PBLA. Therefore, two series of random copolymers of benzyl-l-aspartate with o- and p-nitrobenzyl-^l-aspartate were prepared in order to study the left-handed→right-handed α-helix transition. The conformational behaviour has been studied by nuclear magnetic resonance spectroscopy and the left-handed→right-handed α-helix transition has been observed from the α-CH shift, from CDCl₃ solutions. Moreover, for each copolymer, the α-helix→coil transition was also studied in $\text{CDCl}{}_3\text{trifluoroacetic acid}$ mixtures. In this case, the transition can be observed from the α-CH shift and from the splitting of the β-CH₂ resonance in the α-helical form also.     

Synthesis and conformational study of poly(l-β-3,4-dihydroxyphenyl-α-alanine)

High molecular weight poly(l-β-3,4-dihydroxyphenyl-α-alanine) has been synthesized. Both optical rotatory dispersion and circular dichroism spectra of the polypeptide are anomalous and give little information about its conformation. From the total results obtained by a study of the optical rotation, nuclear magnetic resonance and infra-red absorption, poly(l-β-3,4-dihydroxyphenyl-α-alanine) is most probably right-handed helical in trimethyl phosphate, methanol or water/trimethyl phosphate ($\text{1:1}\text{v}\text{v}$) mixed solvents below pH 10.4. It is in the random coil structure in dimethyl sulphoxide or water/trimethyl phosphate mixed solvents above pH 11. The transition midpoint is pH 10.6 in water/trimethyl phosphate mixed solvents. The results were compared with those of poly(l-tyrosine).     

Helix formation of charged polypeptides in polyion complexes with weak and strong polyelectrolytes prepared in aqueous alcohols

Using circular dichroism, the helix contents of sodium poly(L -glutamate) (PLGNa) and poly(L -lysine) hydrobromide (PLLHBr) in their weak and strong polyelectrolyte complexes have been estimated in aqueous alcohol solutions. Helix (de)stabilization observed in the systems is compared with earlier results on the PLGNa/PLLHBr polyion complex.     

Temperature influence on the behavior of polysulfone‐b‐poly(alkylene oxide)‐b‐poly(dimethylsiloxane) triblock copolymers in a selective solvent

Triblock copolymers containing polysulfone, poly(alkylene oxide), and poly(dimethylsiloxane) segments were obtained by addition of preformed α,ω‐bis(hydrogensilyl) poly(dimethylsiloxane) oligomers to alyl end‐capped poly(alkylene oxide)‐b‐polysulfone. Viscometric and UV absorption measurements were carried out in dilute 1,2‐dichlorethane solutions, in the temperature range of 20–75°C. The specific interactions exhibited by the block copolymers in a selective solvent are influenced by the copolymer composition and temperature. The results point to a conformational transition phenomenon, located around 55°C, which is attributed to the transition from a segregated to a pseudo‐Gaussian conformation through a compressed‐segregated conformation. POLYM. ENG. SCI., 57:114–118, 2017. © 2016 Society of Plastics Engineers     

Sequence peptide polymers: 3. Synthesis and conformational study in solution of poly (l-leucyl-l-leucyl-ε-N-carbo-benzyloxy-l-lysine

The synthesis and characterization of poly[Leu-Leu-(ε-Z)-Lys] and its conformational study in organic solvents are reported. The sequential polypeptide has been obtained with a satisfactory optical purity by the self-condensation method of the p-nitrophenyl ester of the corresponding tripeptide derivative according to the general procedure of De Tar et al. Poly[Leu-Leu-(ε-Z)-Lys] showed a right-handed α-helical conformation in solution of helicogenic solvents, as displayed by circular dichroism measurements and ultra-violet absorption spectra. The presence of the α-helix has been also detected in the solid state by infra-red spectroscopy. The helix stability of the polypeptide in chloroform—dichloroacetic acid solution is considerably high. This property may be related to the ordered sequence of the bulky leucyl residues which give rise to a screening effect, protecting the chain backbone from the acid solvent molecules.     

Influence of the thermal history on dielectric properties of poly(vinyl chloride)

In this paper, we report dielectric permittivity and loss of poly (vinyl chloride) samples that have received three different thermal treatments: (a) as received, (b) quenched from 110°C to 20°C and (c) slow cooled at 5°C/h. There are several observations: first, the secondary (β) loss peak-is not representative of a simple mechanism of transition, in agreement with results of other authors (10), second, in the glass transition zone, there are clearly two peaks (α₁ and α₂)—α₁, is a typical peak of an amorphous glass transition; the second, α₂, has possibly a crystalline origin—and, third (and the most interesting fact), there is an increase of the loss tangent in the intermediate zone between α and β peaks showing a new relaxational peak with high activation energy (70 Kcal/mole), in agreement with dynamic mechanical results (6).     

Conformation and electrical conductivity of poly(γ-methyl-L(LD)-glutamates) reticulate-doped with 1,3-dimethylimidazolium-7,7,8,8-tetracyano-p-quinodimethane complex salt

Polymeric composites were prepared by reticulate doping of poly(γ-methyl-L-glutamate) (PMLGA) and poly(γ-methyl-LD-glutamate) (PMLDGA, 1 : 1) having different conformation forms with 1,3-dimethylimidazolium-TCNQ complex salt. The heat induced α-helix to coil transition in PMLGA and α-helix to β-form and coil transition in PMLDGA were carried out in chlorobenzene and o-dichlorobenzene at temperatures between 70 and 150°C. The conductivity of polyglutamate composites thermally treated and prepared at 70°C is higher by approximately two orders of magnitude than that of thermally not treated films cast at ambient temperature. The conductivity of the most conductive reticulate-doped PMLGA film is higher by more than four orders of magnitude than that of the 1,3-dimethylimidazolium-TCNQ complex salt in pressed pellets.     

Conformational variability of helix sense reversals in poly(methyl isocyanate)

The conformations and energies of several helix sense reversal geometries in poly(methyl isocyanate) (PMIC) have been determined using the PCFF forcefield. In an extension of previous studies, a larger conformational variability for a helix sense reversal has been investigated. In addition to the reversal geometry previously detailed by several authors that results in a relatively small angle deviation from the rod-like polyisocyanate structure, we report the discovery of reversals of similar energy with much larger angle deviations from linearity. The effect of electrostatic interactions as controlled by the value of the dielectric constant, ε, on the conformation and energy of a reversal is also shown to be important. At ε=1.0 (vacuum) the conformations of the reversals with large and small angle ‘kinks’ have similar energies. However, at ε=2.0 (non-polar organic solvent) and ε=3.5 (bulk state) the reversals corresponding to the large angle kinks have lower energies.     

A study of photooptical processes in photosensitive cholesteric azobenzene-containing polymer mixture under an action of the polarized and nonpolarized light

New liquid crystalline cholesteric mixture consisting of azobenzene-containing side-chain copolymer and the chiral low-molar-mass dopant (5%) was prepared and its properties were studied. It was shown that freshly prepared spin-coated thin films are optically isotropic. However, the films possess a noticeable circular dichroism with a maxima corresponding to the absorbance of the nonchiral azobenzene fragments. This phenomenon is an evidence for the existence of the helical supramolecular structure elements in the mixture films. An annealing of the films at temperatures above the glass transition leads to the strong increase in the circular dichroism due to formation of the perfect cholesteric phase. UV light irradiation results in the E-Z isomerization of azobenzene groups and the significant drop in circular dichroism values (almost to zero) both for the fresh and the annealed films that is associated with the transition from cholesteric to the fully isotropic state. An action of polarized UV and visible light leads to the photoorientation of azobenzene groups perpendicular to the electric vector of the polarized light. Kinetic features of the photoorientation processes in cholesteric phase in comparison with nematic systems were studied. It has been experimentally found that the helical order to some extent prevents the photoorientation of the side groups. Upon UV irradiation of the thick, planarly-oriented films of the mixture at temperatures higher than glass transition, the selective light reflection maximum is shifted to the long-wavelength spectral region. All photoinduced phenomena occurring in the mixture are thermally reversible and annealing of the films at temperatures above glass transition results in the back recovery to the cholesteric nonoriented state and to the initial helix pitch value.     

Optically active tyrosine‐containing poly(N‐propargylamides): Synthesis,helical conformation,and infrared emissivity study

Novel optically active and racemic poly(N‐propargylamides) bearing tyrosine pendants (LPT, DPT, and RPT) were synthesized and characterized. The polymers possessed moderate molecular weights and were thoroughly soluble in common organic solvents. The secondary structure of these three polymers in chloroform was studied and the results indicated that LPT and DPT could adopt helical structures with predominantly one‐handed screw sense according to their intense Cotton effect and large specific rotation values, while RPT adopted random coiled polymer chain. The amide groups could form amide‐amide hydrogen bonds to engender the twist of dihedral angle and the strong steric repulsion of phenol groups might have the opposite effect on the helix. The helical structure of the optically active polymers were conclusively constructed and stabilized by the optimization of intra‐ and intermolecular hydrogen bonding and steric repulsion of the side chains. Moreover, the infrared emissivity values of LPT and DPT at 30 °C were 0.682 and 0.671, which were much lower than RPT. LPT and DPT also showed superior heat resistance due to their unique helical conformation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44824.     

Mechanism of the screw-sense reversal of tightly hydrogen-bonded α-helical network triggered by the side-chain conformation

The paper presents theoretical treatment of a screw-sense reversal of tightly hydrogen-bonded α-helical polypeptide chains. The transition is triggered by the conformational free-energy difference (s) of the side chain flanking the chiral (right(r)- or left(?)-handed) backbone. The two-parameter scheme (s, σ) corresponding to a zipper-type transition model starting from one terminal has been constructed and applied to the thermally-induced helix-helix transition characteristic of polyaspartic acid esters observed in the helicoidal solution. The weighting parameter σ takes care of the instability associated with the ?/r junction, i.e., the unfolded site where at least three successive hydrogen-bonds should become free to change their partner. The enthalpy of transition (ca. 2 kJ/mol) of poly(β-phenethyl l-aspartate) (PPLA) (Mv = 6 × 10⁴) observed in the lyotropic liquid crystalline state in 1,1,2,2-tetrachloroethane has been reproduced with the instability parameter σ = 0.001-0.0001. Attempts were further extended to interpret the thermally-induced helix-helix transition behaviors of the PPLA homopolymers and random copolymers involving benzyl l-aspartate residues observed under various conditions.     

Structure of AB block copolymers with a polypeptide block: Effect of the chain conformation

The structure of AB copolymers with a polypeptide block has been studied by X-ray diffraction, electron microscopy, infrared spectroscopy, and circular dichroism. Copolymers with a polyvinyl block (polybutadiene or polystyrene) and a hydrophobic polypeptide block (polybenzyl-L-glutamate or polycarbobenzoxy-L-lysine) exhibit a lamellar structure in the dry state and in solution in dioxane or in different chlorinated solvents. This lamellar structure consists of plane, parallel, equidistant sheets. Each sheet results from the superposition of two layers: one formed by the polyvinyl chains in amore or less random coil conformation, the other formed by the polypeptide chains in an α-helix conformation, arranged in a hexagonal array and generally folded. Copolymers with a polyvinyl block and a hydrophilic polypeptide block (poly-Llysine or poly-L-glutamic acid) exhibit a larnellar structure in water solution and in the dry state. The difference between this lamellar structure and the preceding one consists in the conformation of the polypeptide chains: an intramolecular mixture of coiled chains, α-helices, and β chains. Copolymers with a polysaccharide block and a hydrophobic polypeptide block exhibit in DMSO solution and in the dry state a lamellar structure similar to that of copolymers with a polyvinyl block and a hydrophobic polypeptide block.     

Aggregate structure change induced by intramolecular helix-coil transition

Self-assembly behavior of poly(γ-benzyl l-glutamate)-graft-poly(ethylene glycol) (PBLG-g-PEG) in ethanol medium was studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), laser light scattering (LLS), and circular dichroism (CD). The experimental results revealed that the conformation change of the polypeptide graft copolymer exerts marked effect on its self-association behavior. Spindle-like micelles with polypeptide blocks aligned inside the cores are formed in ethanol solution without denaturant acid. When the denaturant acid is added, the rigid α-helix transforms to random coil, resulting in an aggregate structure change from the spindle-like micelle to large compound micelle. For the large compound micelles, the coiled polypeptide chains and PEG blocks pack randomly within the cores, surrounded by the PEG chains outside to stabilize the aggregates.     

The low-temperature structure of poly(di-n-hexyl germane)

The structure of poly (di-n-hexyl germane) has been investigated using X-ray and conformational energy analysis. Wide-angle X-ray diffraction analysis shows that the polymer undergoes an order/disorder transition at about 15°C. The polymer is known to exhibit thermochromism associated with this phase transition. Below the transition temperature, the backbone conformation is all-trans and the molecules are arranged in an orthorhombic unit cell. Above the transition temperature, the backbone disorders and the polymer loses long-range ordering. Fixed bonding conformational energy calculations found the lowest energy structure to be a 7/3 helix, very similar to that found for poly(di-n-hexyl silane). Most probably, the observed all-trans backbone conformation is due to intermolecular interactions which force the germanium backbone to an all-trans conformation. In order to carry out the molecular mechanics calculations, non-bonded energy parameters for germanium atoms were derived.     

Synthesis and characterization of novel optically active poly(urethane urea)s: Effect of left/right rotation conformation

Helical R/S‐poly(urethane urea)s (R/S‐HPUUs) were synthesized successfully with R/S‐1,1′‐binaphthyl‐2′,2‐diol, toluene 2,4‐diisocyanate, and 1,4‐diaminobenzene by a simple hydrogen‐transfer addition reaction. The structure and optical properties of R/S‐HPUUs were investigated systematically with Fourier transform infrared spectroscopy, ¹H‐NMR spectra, absorption and circular dichroism, thermogravimetry, differential scanning calorimetry, and X‐ray diffraction measurements. Fourier transform infrared spectra showed that all the ? NH and carbonyl groups in R/S‐HPUUs were hydrogen‐bonded. R‐HPUU and S‐HPUU were two enantiomorphs with wonderful mirror‐image symmetry according to circular dichroism spectroscopy analysis. R/S‐HPUUs exhibited optical activity, and the optical rotations were +54.7 and ?60.7°, respectively. R/S‐HPUUs showed excellent thermal stability because of the plane‐bifurcated interchain hydrogen bonds and helical stereostructures according to thermogravimetry, differential scanning calorimetry, and X‐ray diffraction analysis. Moreover, the left‐handed rotation was more stimulative for the construction of hydrogen bonds than the right‐handed rotation, so S‐HPUU showed higher thermal stability and better crystallizability. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009