Low Molecular Weight Model Research Articles

Toward chiral polyhydroxylated dendrimers. Preparation and chiroptical properties

Four dendrimers (1b-4b) containing chiral vicinal diol-based subunits were prepared from their acetonide-protected precursors (1a-4a). The optical activity of these chiral dendritic structures was successfully modeled using structurally similar, low molecular weight model compounds. Using the [Phi](d) values of the low molecular weight model compounds 5b-7b, we calculated [Phi](d) values for dendrimers 1b-4b that agree to within 4.5% of the observed values. Agreement between the optical activity of the model compounds and that of the dendrimers leads to the conclusion that the conformational equilibria of the dendrimer subunits are not perturbed relative to those of the model compounds.

Optically active polymethacrylates with side-chain l-lactic acid residues connected to push–pull azobenzene chromophores

Novel optically active methacrylic homopolymers with side-chain l-lactic acid moieties interposed through ester bonds between the main chain and trans-4-azobenzene chromophores bearing in the 4′-position electron withdrawing groups, such as the cyano and nitro residues, have been synthesized and structurally characterized. Investigation by circular dichroism of the chiroptical and conformational properties of the polymers in solution indicates a much higher optical activity of these latter with respect to that of the corresponding low molecular weight model compounds, purposely prepared, due to the presence of dissymmetric conformations of one prevailing handedness. The trans to cis photoisomerization kinetics of the azoaromatic double bond has been measured in solution for polymers and models and the resulting data compared with those of previously reported similar photochromic derivatives. The influence of the nature of the electron withdrawing group in the 4′-position of the azobenzene chromophore on the chiroptical properties of the polymers is also discussed.

Amino-p-benzoquinone Adducts and Polymers as Adhesion Promoters for Steel

Phenolic and quinonoid compounds are widely studied in biological sciences because of their ability to chelate heavy metals like iron and copper and recently have found new applications in synthetic macromolecules. Amino-p-benzoquinone polymers, poly[(2,5-hexamethylenediamino)-1,4-benzoquinone] and poly{[2,5-(2,2′-bistrifluoromethyl)-4,4′-biphenylenediamino]1,4-benzoquinone} were synthesized and evaluated as adhesion promoters for steel/epoxy joints. An improvement in the torsional shear strength of these joints was observed when these polymers were used as adhesion promoters. The durability of the adhesive bond was also improved after boiling water treatment, relative to untreated and silane-treated joints. The improvement in adhesion could be attributed to the formation of a chelate between the polyaminoquinone (PAQ) and the iron surface and a chemical reaction between the PAQ and the epoxy resin. A low molecular weight model compound, bis[2,5-(4-methylanilido)]-1,4-benzoquinone, was also used to study coupling between the epoxy adhesive and the steel surface. Electron spin resonance (ESR), atomic absorption spectroscopy and infrared spectroscopy were used to document the epoxy-coupling agent reaction and the chelate formation.

Synthesis and chiroptical properties of optically active methacrylic polymers bearing the (S)- and/or (R)-2-hydroxysuccinimide moiety linked to thetrans-azobenzene group in the side chain

The synthesis of novel optically active monomers, namely (S)-(+)- and (R)-(-)-2 methacryloyloxy-N-(4-azobenzene) succinimide, is described. The monomers have been radically polymerized to afford the corresponding optically active homopolymers, characterized by the presence of an optically active rigid succinimide ring in the side chain linked to the trans-azoaromatic system through the imide nitrogen atom. A series of copolymers containing both the enantiomeric monomers in different molar amounts, have also been prepared in order to observe possible effects on the optical activity of the resulting deritatives. The chiroptical and conformational properties of the polymers in solution have been investigated by circular dichroism measurements and compared with those of low molecular weight model compounds, purposely synthesized, with the aim to verify the existence of cooperative interactions between chromophoric groups disposed along the macromolecular chain according to dissymmetric conformations.

New self-crosslinkable copolymers based onN-methyl-N-vinylbenzylpyrrolidinium halide and methyl methacrylate

Thermally-induced self-crosslinking behaviour has been found in copolymers containing N-methyl-N-vinylbenzylpyrrolidinium chloride (MVBPC) and methyl methacrylate (MMA). NMR, IR and low molecular weight model reactions demonstrate that this crosslinking reaction occurs between the methyl ester groups of the MMA units and the quaternary ammonium salts, with the resulting benzyl esters forming chemical links between the MVBPC and MMA units with the formation of N,N-dimethylpyrrolidinium chloride. Similar crosslinking behaviour has also been found when the Cl− anion is replaced by Br− and I−, but not in the case of BF as counter anion. © 2000 Society of Chemical Industry

Dinuclear metallo-phosphodiesterase models: application of calix[4]arenes as molecular scaffolds

An important goal in supramolecular chemistry is the synthesis of molecules that exhibit catalytic activity analogous to the activity of enzymes. In this respect, studies toward the biomimetic catalysis of phosphate diester cleavage have received particular attention. In nature this process is catalyzed by enzymes that possess often two or three divalent metal ions in their active sites. In order to mimic the active sites of these metallo-phosphodiesterases chemists generally attempt to connect ligated metal ions by a molecular spacer in such a way that the metal–metal distance matches with the anionic pentacoordinate phosphorus transition state. However, in contrast to enzymes, which bind the transition state by multiple contacts via an induced fit mechanism, many of the low molecular weight model systems exhibit only minor catalytic activity due to lack of catalytic groups and too much rigidity or flexibility. Our approach is to use calix[4]arenes as a molecular scaffold for the dynamic preorganization of multiple catalytic groups. In this review models for dinuclear metallo-phosphodiesterases and the use of calix[4]arenes in such models are described.

Sterically hindered phenol–dextran conjugates: radical scavenging activity in water and water–organic media

The radical scavenging activity investigation of sterically hindered phenols–dextran conjugates was carried out in water and water–dioxane mixtures with use of sodium salt of 2,2-diphenyl-1-picryl-hydrazyl sulfonic acid (DPPH-SO 3Na). The suggestion was made that the reaction mechanism includes the one-electron transition stage. It was found that the reaction rate constants of DPPH-SO 3Na with the conjugates are higher than the ones with low-molecular weight model. One of possible reasons for obtained data could be the fact that dextran hydration shell stabilizes the cation-radical intermediate in case of conjugates better than bulk water.

Formation of Advanced Glycation Endproducts from Low Molecular Weight Model Compounds

The process of glycation was studied in 12 model systems containing carbohydrates (Glc, Fru) and peptides (Gly-Gly, Gly-Phe, Phe-Gly, Gly-Lys) or acetylated amino acids (Ac-Lys, Ac-Arg) in order to clarify the role of different structural elements of the reacting components. The course of reaction was followed by the changes of the UV spectra of the reaction systems. The results show that the reactivity of the NH2 group correlates with its pKa value. The presence of benzene ring in the amino component accelerates glycation. Strong correlation between the intensity of the fluorescence and the absorption at 325 nm was found for all reaction systems.

Synthesis and chiroptical properties of optically active photochromic methacrylic polymers bearing in the side chain the (S)-3-hydroxypyrrolidinyl group conjugated with thetrans-azoaromatic chromophore

Novel optically active polymethacrylates, namely poly[(S)-3-methacryloyloxy-1-(4-azobenzene)pyrrolidine] and poly[(S)-3-methacryloyloxy-1-(4′-nitro-4-azobenzene)pyrrolidine], have been synthesized by radical polymerization of the corresponding monomers, prepared in turn through a synthetic route preserving the asymmetric center by any racemization reaction. These homopolymers are characterized by the presence in the side chain of an optically active pyrrolidinyl ring linked to the trans-azoaromatic system through the nitrogen atom. The optical activity of the polymers in solution appears much higher than that observed with the low molecular weight models, purposely synthesized for comparison. Circular dichroism spectra of the synthesized products demonstrated that, in solution, the macromolecules assume highly homogeneous conformations with a prevailing chirality to a larger extent with respect to analogous systems previously investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3257–3268, 1999

Synthesis and chiroptical properties of optically active photochromic polymers with side-chainL-lactic residues connected totrans-azobenzene moieties bearing a reactive formyl electron-withdrawing group

The synthesis of a novel optically active monomer containing a photochromic chromophore, trans-(S)-4-(2-methacryloyloxypropanoyloxy)-4'-formylazobenzene, is described. The monomer was radically homopolymerized and copolymerized with methyl methacrylate to afford the corresponding optically active polymers. which were fully characterized. The chirooptical and conformational properties in solution of the resulting polymers were investigated by circular dichroism and solvatochromism measurements and compared with those of the corresponding low molecular weight model compound, trans-(S)-4-(2-pivaloyloxypropanoyloxy)-4'-formylazobenzene, purposely synthesized.

Synthesis, chiroptical properties and photoresponsive behaviour of optically active poly[(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene

A novel optically active monomer containing a photochromic chromophore, such as trans-(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene and its low molecular weight model compound trans-(S)-4-(2-pivaloyloxypropanoyloxy)azobenzene were prepared. Free radically initiated homopolymerization of the methacrylic monomer afforded in fairly high yield a rather high molecular weight polymer which was molecularly and structurally characterized. The molecular mobility of the polymer in all trans configuration, as well as chiroptical properties, photoisomerization kinetics and photochromic behaviour were investigated and compared with those of the low and high molecular weight related compounds. The polymer photoresponsive properties were evaluated by circular dichroism measurements at various extents of photoisomerization. All the results are discussed in terms of structural requirements of the macromolecules.

The application of some polymeric type-I photoinitiators based on α-hydroxymethylbenzoin and α-hydroxymethylbenzoin methyl ether

Some polymeric photoinitiators based on α-hydroxymethylbenzoin and its methyl ether are used to initiate the polymerization of methyl methacrylate in solution and the crosslinking of an epoxyacrylate formulation in thin films. In general the polymeric photoinitiators perform better than corresponding low molecular weight model compounds. Moreover, it is found that polymeric photoinitiators based on α-hydroxymethylbenzoin methyl ether perform better than those based on α-hydroxymethylbenzoin. Explanations for these observations are advanced.

Aminolysis kinetics of model and polymer-bound anhydride moieties in low- and high-viscosity media

This study examines the legitimacy of using the reaction kinetics of low molecular weight model compounds in solution to predict the chemical kinetics of polymer-bound species in a homogeneous melt. The reaction under study takes place between an aliphatic secondary amine, diisooctadecylamine (DiOA), and a 5-membered anhydride ring, saturated maleic anhydride ( MA), forming an amic acid product. The MA species was present as a pendant graft on either a model compound, dodecane-g-(maleic anhydride)(dodecane-g-MA), or a polymer chain, linear low-density polyethyl-ene-g-(maleic anhydride) (LLDPE-g-MA). Pseudo-second-order kinetics of the anhydride consumption are followed by infrared spectroscopy, either in situ in dodecane solution or by scanning frozen film samples taken from a linear low-density polyethylene melt. It was found that the LLDPE-g-MA/DiOA system reacted at a slightly slower rate than the dodecane-g-MA/DiOA system in the low-viscosity solution at 140°C. In the melt, the dodecane-g-MA/DiOA system experienced a small decrease in the overall reaction rate compared to the same reaction carried out in dodecane. However, the LLDPE-g-MA/DiOA system underwent a 65% decrease in the observed second-order rate constant on going from a solution to the melt. To explain these phenomena, the effects of diffusion, miscibility, and chain entanglements in the melt are examined here.

Modeling of the phase behavior of binary and ternary blends involving copolymers of styrene, methyl methacrylate and cyclohexyl methacrylate

Modeling of the lower critical solution temperature, LCST, phase boundary of binary blends of poly(styrene) (PS) and random poly(cyclohexyl methacrylate-co-methyl methacrylate) copolymers was carried out incorporating temperature-dependent segmental interaction parameters, χij(T), into the classical mean field binary interaction model. By using literature data of the temperature dependence of the PS/poly(methyl methacrylate) (PMMA) interaction parameter, the temperature variation of the interaction energy of both PS/poly(cyclohexyl methacrylate) (PCHMA) and PMMA/PCHMA blends was obtained. The magnitude of the interaction energy between PS and PCHMA from the model was in good agreement with those estimated from experimental heat of demixing data of PS/PCHMA blends and from heats of mixing of low molecular weight model compounds. The segmental interaction parameters obtained were further employed to predict the phase behavior of binary and ternary blends involving copolymers of S, MMA and CHMA. The agreement between prediction and experiment was satisfactory.

G protein-bound conformation of mastoparan-X: heteronuclear multidimensional transferred nuclear overhauser effect analysis of peptide uniformly enriched with 13C and 15N.

Mastoparans, a family of tetradecapeptides from wasp venom, have been used as convenient low molecular weight models of receptors coupled to GTP-binding regulatory proteins (G proteins) for the understanding of the interaction between G proteins and receptors. Sukumar and Higashijima have analyzed the conformation of mastoparan-X (MP-X) bound to the G protein alpha-subunit using proton two-dimensional transferred nuclear Overhauser effect (TRNOE) spectroscopy [Sukumar, M., and Higashijima, T. (1992) J. Biol. Chem., 267, 21421-21424]. The resultant structure, however, was not well-defined due to severe overlap of peptide proton resonances. To determine the G protein-bound conformation of MP-X in detail, we have analyzed this interaction by heteronuclear multidimensional TRNOE experiments of MP-X uniformly enriched with 15N and/or 13C. By solving the overlap problem, we were able to determine the precise conformation of MP-X bound to Gi1alpha: the peptide adopts an amphiphilic alpha-helix from Trp3 to C-terminal Leu14, and the atomic root-mean-square deviation (rmsd) values in this portion about the averaged coordinates were 0.27 +/- 0.07 A for the backbone atoms (N, Calpha, C') and 0.84 +/- 0.16 A for all heavy atoms. These values are much smaller than the corresponding rmsd values of the structures obtained from the proton 2D TRNOE spectrum alone: 1.70 +/- 0.41 A for the backbone atoms (N, Calpha, C') and 2.84 +/- 0.51 A for all heavy atoms. Our results indicate that the heteronuclear multidimensional TRNOE experiments of peptides uniformly enriched with stable isotopes are a very powerful tool for analyzing the conformation of short peptides bound to large proteins. We will also discuss the structure-activity relationships of mastoparans in activating G proteins on the basis of the precise structure of MP-X bound to Gi1alpha.

Catalytic hydrogenation of alkadienes and alkynes by palladium catalysts supported on heterocyclic polyamides

The catalytic behavior of palladium supported on polyamides having a pyridine moiety has been studied in the liquid phase hydrogenation of alkadienes and alkynes at 25°C and 1 atm pressure. The palladium centre was introduced into the polyamides of formula with n=2 and 6, by ligand exchange using PdCl 2(PhCN) 2. The resulting precursor catalysts, which were activated in an hydrogen atmosphere, selectively reduced alkadienes and alkynes to monoenes and alkenes, respectively. The reaction stopped after 1 molar equivalent of hydrogen had been consumed. The effects of solvent polarity and of support structure has also been investigated. Several techniques have been used to characterize these palladium catalytic systems. The type of interaction between palladium and pyridine modified polyamides was studied by using low molecular weight model compound and IR spectroscopy. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) were used to characterize the palladium catalysts before and after use. It can be concluded from these results that both matrix functional groups i.e. N py and O CO act as coordination sites for the palladium centre and mixed valence state species are formed, giving a selective palladium catalyst. Evaluation of the stability of the supported palladium catalyst was tested in eleven successive runs involving 4300 catalytic cycles: neither loss of activity nor selectivity was found.

A DSC-Study of the Charge-Transfer Complexation of Donor Poly(3,6-Carboxy-Methylene)s

The complexation behaviour of donor poly(3,6-carbazolylmethylene)s with fluorinated and/or nonfluorinated tapered tails and of their low molecular weight model compounds with the corresponding crystalline low molecular 3,5-dinitrobenzoic acid acceptors has been investigated by DSC. Whereas the blends resulted by charge-transfer (CT) complexation of the amorphous donor polymer containing aliphatic tapered tails with the corresponding low molecular weight acceptor are also amorphous, the CT-complexed blends of the donor liquid crystalline polymer containing the perfluoiinated tapered tails with the respective low molecular acceptor maintain the mesophasic character up to a donor/acceptor ratio 1:1. Selective CT-complexation of only the fluorinated sites of an amorphous donor copolymer containing randomly distributed both the fluorinated and aliphatic tapered tails in 1:1 proportion with the respective fluorinated 3,5-dinitrobenzoic acid acceptor induces by regioselective self-organization a liquid crystalline phase in the copolymer blend. Complexation of the nonfluorinated sites of the amorphous donor copolymer by the respective nonfluorinated 3,5-dinitrobenzoic acid acceptor, on the contrary, results in an amorphous blend. This provides a simple model of a transcriptase molecule which by regioselective binding to a DNS molecule induces a conformation change in the main chain.

Dipolar correlation coefficients in the time and frequency domains for low molecular weight models of acrylate polymers

The temperature dependence of the components of the complex relative permittivity of 2-biphenyl isobutyrate (OBPI) and bis(biphenyl-2-yl) 2,4-dimethylglutarate (OBPG), model compounds of a single repeating unit and a dimer of poly(biphenyl-2-yl acrylate), respectively, is reported in the frequency range 10–4–102 kHz. The relaxation spectra of both compounds at 0.1 Hz exhibit glass–liquid relaxations with maxima located at –61.2 and –8.8 °C, respectively, for OBPI and OBPG. Subglass absorptions are not detected in the glassy state down to –120 °C. The free volume fraction at Tg is calculated to be 0.024 and 0.028 for OBPI and OBPG, respectively. Molecular dynamics (MD) simulations give the following values for the mean-square dipole moment of OBPI: 4.01, 2.92 and 2.80 D2 at 1000, 500 and 300 K, respectively. The values of 〈µ2〉 at 500 and 300 K are in rather good agreement with the experimental result, 2.69 ± 0.13 D2, at 303 K. The time dependence of the dipolar autocorrelation coefficient ϕ(t) is calculated by MD at several temperatures. Transformation of the autocorrelation coefficient from the time to the frequency domain permits the determination of the components of the complex permittivity for these compounds. At low temperatures, the mobility of the molecules is not large enough to cover the whole conformational space within a reasonable computing time. At temperatures lying in the neighbourhood of 500 K, the molecules already visit the whole conformational space, and the values of the complex permittivity in the frequency domain obtained from the ϕ(t) of OBPI and OBPG are in good agreement with the experimental results. These results suggest that the intermolecular cross-correlation terms, although present, do not alter in a significant way the time dependence of ϕ(t).

Synthesis, photochemical and photoinitiation activity of water soluble copolymers with pendent benzil chromophores

In the present paper the synthesis of three novel water soluble copolymers with benzil pendant groups are described. The new polymers have been prepared by copolymerization of the obtained benzil acrylate, 1-phenyl-2-[4-(2-methacroyloxyethoxy)phenyl]ethandione-1,2 (MBz) with three water soluble comonomers: acrylamide, 2-acrylamido-2-methylpropane sulphonic acid and 2-acryloxyethyl trimethylamonio iodine. The absorption, luminescence, photoreduction and microsecond flash photolysis characteristics of the new copolymers were examined in water and methanol and the results compared to the corresponding low molecular weight compound. The photoinduced polymerization activities of the new products have been determined by photodilatometry in the presence of a tertiary amine. The copolymers exhibit polymerization rates and quantum yields similar to those of the low molecular weight model compounds.

Volumetric properties of nucleic acids

Volumetric studies can yield useful new information on a myriad of intra- and intermolecular interactions that stabilize nucleic acid structures. In particular, appropriately designed volumetric measurements can characterize the conformation-dependent hydration properties of nucleic acids as a function of solution conditions, including temperature, pressure, ionic strength, pH, and cosolvent concentration. We have started to accumulate a substantial database on volumetric properties of DNA and RNA, as well as on related low molecular weight model compounds. This database already has provided unique insights into the molecular origins of various nucleic acid recognition processes, including helix-to-coil and helix-to-helix conformational transitions, as well as drug-DNA interactions. In this article, we review recent progress in volumetric investigations of nucleic acids, emphasizing how these data can be used to gain insight into intra-and intermolecular interactions, including hydration properties. Throughout this review, we underscore the importance of volume and compressibility data for characterizing the hydration properties of nucleic acids and their constituents. We also describe how such volumetric data can be interpreted at the molecular level to yield a better understanding of the role that hydration can play in modulating the stability and recognition of nucleic acids. © 2000 John Wiley & Sons, Inc. Biopoly 48: 264–280, 1998