Reversal In Helix Sense Research Articles

Direct determination of helix structures involved in the screw-sense reversal of poly(β-phenylpropyl l-aspartate) by synchrotron X-ray diffraction.

The helix-sense reversal of poly(β-phenylpropyl l-aspartate) (3PLA) in the solid state was studied by synchrotron wide-angle X-ray diffraction and small-angle X-ray scattering. The direct determination of the characteristic helical pitch before and after the transition revealed that the transition takes place reversibly between the two α-helices having opposite screw-sense during the heating and cooling cycle. While the hexagonal packing remains unaltered, the helix-sense inversion causes discontinuous changes in the molecular arrangement and, by extension, the crystalline dimension. In this study, another transition was detected at a higher temperature from the left-handed α-helix to the π-helix, the molecular chirality being unaffected.

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(ℓ)-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 ℓ-form in conventional helicoidal solution at all temperatures, poly(β-phenethyl l-aspartate) (PPLA) exhibits transformation from the r- to ℓ-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 ↔ ℓ 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.

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.

Photoresponsive Polypeptides

Polypeptides containing azobenzene or spiropyran units attached to the macromolecules respond to light or dark conditions giving reversible variations of their structure. In this Account we provide a short overview of current research in the field and describe the most significant experimental examples of photoresponse effects. They include photoinduced random coil/alpha-helix transitions, helix-sense reversal, photostimulated aggregation/disaggregation processes, and photomechanical effects. These fascinating properties suggest that photoresponsive polypeptides may become suitable materials for designing sensors and devices that can be photomodulated. Findings also demonstrate that it is possible to synthesize model systems which respond to light similarly to naturally occurring photoreceptors.

Optically active and highly threodiisotactic poly(triphenylmethyl crotonate) with conformational chirality

AbstractPolymerization of triphenylmethyl crotonate with a 9‐fluorenyllithium/(S,S)‐(+)‐2,3‐dimethoxy‐1,4‐bis(dimethylamino)‐butane complex in toluene at −78°C gave a highly threodiisotactic polymer. The polymer had a trimodal distribution of molecular weight (MW); the highest‐MW part showed a small negative optical rotation and the other two showed positive rotation. The middle‐MW fraction (DP=21.5, M̄w/M̄n=1.07) had an [α]D of +53.3°. The optical rotation of this fraction decreased gradually in chloroform at 60°C while the GPC curve of the fraction remained unchanged. The loss of optical activity was ascribed to the racemization through helix sense reversal of the polymer chain. The rate of the decrease in optical rotation obeyed first‐order kinetics. From the rate constants at 50 and 60°C, the activation enthalpy for the helix reversal was determined to be 28.4 kcal/mol.

Helical conformations, internal motion, and helix sense reversal in polyisocyanates and the preferred helix sense of an optically active polyisocyanate

Empirical force field studies of poly(alkyl isocyanates) are presented. A barrier of 12.5 kcal/mol for cis-trans isomerization of the polymer-backbone's conjugated partial double bonds makes the calculated energy of helix sense reversal agree best with corresponding experimental results. The alternating cis and trans torsion angles are found to be distorted, to about 170° and -55° for right-handed helices, by strong repulsions between the α-carbon of each alkylside chain and the neighboring backbone atoms of four consecutive monomer units

Haloaldehyde polymers. 51. Helix-sense reversal of isotactic chloral oligomers in solution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHaloaldehyde polymers. 51. Helix-sense reversal of isotactic chloral oligomers in solutionKoichi Ute, Katsuo Hirose, Hiroaki Kashimoto, Koichi Hatada, and Otto VoglCite this: J. Am. Chem. Soc. 1991, 113, 16, 6305–6306Publication Date (Print):July 1, 1991Publication History Published online1 May 2002Published inissue 1 July 1991https://doi.org/10.1021/ja00016a076RIGHTS & PERMISSIONSArticle Views222Altmetric-Citations82LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (277 KB) Get e-Alerts Get e-Alerts

Photoinduced conformational changes of azoaromatic polyaspartates containing octadecyl side chains

AbstractCopolymers of p‐(phenylazo)benzyl‐L‐aspartate and n‐octadecyl‐L‐aspartate exist as right‐ and left‐handed α‐helices in solution at 25°C depending on the copolymer composition: the reversal of helix sense from a right‐ to left‐handed one occurs with increasing the azobenzene content. The α‐helices of the copolymers are very sensitive to trifluoroacetic acid (TFA), and are converted into random coil below 2.0% of TFA. Among the copolymers, the copolymer containing 47% azobenzene groups is unique since it exhibits a TFA‐induced conformational change from right‐handed α‐helix to random coil via left‐handed α‐helix. Upon UV light irradiation at 25°C, the copolymers containing 68 and 89% azobenzene groups caused the reversal in helix sense from a left‐ to right‐handed one. The conformations of the copolymers were dependent on temperature, mostly right‐handed and left‐handed α‐helices at lower and higher temperatures, respectively. On this basis, the copolymer containing 47% azobenzene groups could be made to undergo a photoinduced helix reversal at high temperatures.

Temperature‐induced reversal in helix sense of polyaspartates in solid film

Die Makromolekulare Chemie, Rapid CommunicationsVolume 10, Issue 12 p. 683-686 Article Temperature-induced reversal in helix sense of polyaspartates in solid film Akihiko Ueno, Corresponding Author Akihiko Ueno Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanPharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorJunko Nakamura, Junko Nakamura Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorKayo Adachi, Kayo Adachi Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorTetsuo Osa, Tetsuo Osa Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this author Akihiko Ueno, Corresponding Author Akihiko Ueno Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanPharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorJunko Nakamura, Junko Nakamura Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorKayo Adachi, Kayo Adachi Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this authorTetsuo Osa, Tetsuo Osa Pharmaceutical Institute, Tohoku University, Aobayama, Sendai 980, JapanSearch for more papers by this author First published: December 1989 https://doi.org/10.1002/marc.1989.030101212Citations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume10, Issue12December 1989Pages 683-686 RelatedInformation

Reversal in helix sense of copoly(beta-alkyl-L-aspartate-beta-benzyl-L-aspartate).

The conformation of copoly(beta-alkyl-L-aspartate-beta-benzyl-L-aspartate), in which the alkyl group is ethyl, propyl, butyl, hexyl, nonyl, dodecyl, or stearyl, was studied in solution and the solid state by optical rotatory dispersion and circular dichroism methods. The helix sense of the copolyaspartate studied here is transformed from a left-handed to right-handed alpha-helix as the degree of alkylation increases. Reversal in helix sense occurs, i.e., the left-handed alpha-helix based on the handedness of poly(beta-benzyl-L-aspartate) is transformed into a right-handed alpha-helix with increase in alkyl groups with right-handed nature. Reversal in helix sense is also observed for copolyaspartates with an intermediate or high degree of alkylation as temperature rises. Copolyaspartates with hexyl, nonyl, or dodecyl groups exhibit an induced circular dichroism around 230-238 nm and can form an ordered side chain structure which is broken down at high temperature. One has to consider the conformation of the omega-helix and beta-form of the copolyaspartates in the solid state in addition to the reversal in helix sense. Copolyaspartates with a low degree of alkylation are in the alpha-helical conformation over the low temperature range and adopt the omega-helical conformation in the high temperature range, indicative of a thermal alpha-omega transition. A small number of alkyl groups can be incorporated into the benzene ring stacking of the omega-helix, but not a large number. All the copolyaspartates can assume the beta-form at high temperatures. The helix conformation is not significantly affected by the formation of side chain crystals of the copolyaspartate with a large number of stearyl groups, in contrast to copolyglutamate.

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.

Helix sense of copoly(ethyl- l-aspartate-benzyl- l-aspartate)

Copoly(ethyl-L-aspartate-benzyl-L-aspartate)s with various compositions were prepared by ethylation of poly (β-benzyl-L-aspartate). Helix sense of copolyaspartates in chloroform solution was examined by optical rotatory dispersion and circular dichroism methods as a function of a degree of ethylation and temperature. Copolyaspartates of ethylation less than 35% exhibited the same left handed helix as that of poly (β-benzyl-L-aspartate), while copolymer of ethylation more than 70% assumed the right handed α-helical conformation like poly (β-ethyl-L-aspartate). Copolymers with the intermediate ethyl content of 35–70% indicated the reversal in helix sense from a right handed to a left handed helix with increasing temperature from −20° to 60°C. The transition of reversal was reversibly observed and transition temperature rose with the increase of ethylation. In these intermediate copolymers, the mechanism of the reversal in helix sense was examined by circular dichroism and dielectric measurements. Three possibilities were postulated here for the conformation which should arise at the transition temperature of the reversal; First is the random conformation, second is the equimolar mixture of left and right handed α-helices in a chain, and the third is the equimolar mixture of left and right handed α helix chains. No observational circular dichroism spectra characteristic to random coil conformation and no remarkable change in residue dipole moment strongly suggested the third possibility.

Structure and properties of copolyaspartate. III. Dielectric studies of copoly(butyl-L-aspartate-benzyl-L-aspartate)

Abstract The side-chain molecular motion of the α helix, ω helix, and β form of copoly(butyl-L-aspartate-benzyl-L-aspartate) with degrees of butylation 0%, 25%, and 43% was investigated by means of dielectric relaxation. Copoly(butyl-L-aspartate-benzyl-L-aspartate) with butylation 0% and 25% has a left-handed α helix, and, on the contrary, that of the 43% copolymer is a right-handed α helix. These two a helices transformed into the left-handed ω helix, with stacking between the benzyl groups, or between benzyl and butyl groups, or between butyl groups when heated to 140°C. The dielectric constant decreased at the α-ω transition temperature, irrespective of frequency. In particular, a remarkable decrease of dielectric constant was observed to accompany the reversal in helix sense. The ω-β transformation was observed at about 180°C by further heating, accompanied by an abrupt increase in dielectric constant. The side-chain dispersion temperature was in the following order: T(ω) ≲ T(β) < T(α). Relatively free rotation of the side chain for the ω helix is possible, although the side-chain molecular motion, the side-chain dispersion, is restricted by the stacking of the side chain in the ω helix. The side-chain molecular motion of the α helix, rotation of the benzene ring, is hindered by the presence of δ O and ϵ CH2 atoms of the residue above and below the benzene ring.