M-phenylene Ethynylene Research Articles

Solution-Phase Structure of an Artificial Foldamer: X-ray Scattering Study

Foldamers provide important insights into the fundamentals of noncovalent folding, which is of primary importance for understanding biological systems and developing novel self-assembling materials. The structure of artificial foldamers in solution has been studied using a variety of indirect spectroscopic techniques and theoretical methods. X-ray scattering using a high flux synchrotron source has been recently shown to provide a powerful means of studying small organic and biological arrays that are formed because of noncovalent self-assembly in solution but has not been applied to foldamer structures. We present small angle X-ray scattering (SAXS) studies on a m-phenylene ethynylene oligomer (mPE) in acetonitrile, providing for the first time-direct structural data on a mPE foldamer in solution.

Amphiphilic Poly(phenyleneethynylene)s Can Mimic Antimicrobial Peptide Membrane Disordering Effect by Membrane Insertion

Antimicrobial peptides (AMPs) are a class of peptides that are innate to various organisms and function as a defense agent against harmful microorganisms by means of membrane disordering. Characteristic chemical and structural properties of AMPs allow selective interaction and subsequent disruption of invaders' cell membranes. Polymers based on m-phenylene ethynylenes (mPE) were designed and synthesized to mimic the amphiphilic, cationic, and rigid structure of AMPs and were found to be good mimics of AMPs in terms of their high potency toward microbes and low hemolytic activities. Using a Langmuir monolayer insertion assay, two mPEs are found to readily insert into anionic model bacterial membranes but to differ in the degree of selectivity between bacterial and mammalian erythrocyte model membranes. Comparison of grazing incidence X-ray diffraction (GIXD) data before and after the insertion of mPE clearly indicates that the insertion of mPE disrupts lipid packing, altering the tilt of the lipid tail. X-ray reflectivity (XR) measurements of the lipid/mPE system demonstrate that mPE molecules insert through the headgroup region and partially into the tail group region, thus accounting for the observed disordering of tail packing. This study demonstrates that mPEs can mimic AMP's membrane disordering.

Solid-Phase Synthesis of m-Phenylene Ethynylene Heterosequence Oligomers

Both homo- and heterosequence m-phenylene ethynylene oligomers are synthesized using a conceptually simple iterative solid-phase strategy. Oligomers are attached to Merrifield's resin through a known triazene-type linkage. The phenylene ethynylene molecular backbone is constructed through a series of palladium-mediated cross-coupling reactions. The strategy employs two types of monomers that bear orthogonal reactivity, one being a monoprotected bisethynyl arene and the other being a 3-bromo-5-iodo arene. The catalyst conditions are tailored to the requirements of each monomer type. The monoprotected bisethynyl arene is coupled to the growing chain in 2 h at room temperature using a Pd(I) dimer precatalyst ((t)Bu3P(Pd(mu-Cl)(mu-2-methyl allyl)Pd)P(t)Bu3) in conjunction with ZnBr2 and diisopropylamine. In alternate steps, the resin is deprotected in situ with TBAF and coupled to the 3-bromo-5-iodo arene using the iodo selective Pd(tri-2-furylphosphine)4 catalyst in conjunction with CuI and piperidine; this reaction is also completed in 2 h at room temperature. These cross-coupling events are alternated until an oligomer of the desired length is achieved. The oligomer is then cleaved from the resin using CH(2)I(2)/I(2) at 110 degrees C and purified using preparatory GPC. Using this method, a series of homo- and heterosequence oligomers up to 12 units in length in excellent yield and purity were synthesized on the 100 mg scale. Longer oligomers were attempted; however, deletion sequences were found in oligomers longer than 12 units.

The Chain‐Length Dependence Test

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The Chain-Length Dependence Test

Trends obtained from systematic studies based on chain-length variation have provided valuable insight and understanding into the behavior of m-phenylene ethynylene foldamers. The generalization of this experimental approach, the chain-length dependence test, is useful for studying solution conformation, packing in the solid state, specific intrachain interactions, and the contributions of end groups to a particular property.

Green–blue luminescence dichroism of cyano-containing poly[( m-phenylene ethynylene)- alt-( p-phenylene ethynylene)] aggregates dispersed in oriented polyethylene

A new highly luminescent cyano-containing poly[( m-phenylene ethynylene)- alt-( p-phenylene ethynylene)] derivative (CN-PPE) has been prepared and blended in micro/nano-sized dispersions into linear low density polyethylene (LLDPE) films by melt-processing. CN-PPE macromolecules show pronounced tendency to form molecular aggregates in the excited state when dispersed in the polymer matrix with emission characteristics depending on the structural order of the luminescent chromophores. In CN-PPE/LLDPE films, the polymer matrix orientation induces effective anisotropic arrangements of the π–π staking interactions between luminescent guest aggregates, thereby generating a pronounced green–blue dichroic emission. The capability to easily modulate the luminescent properties of polyethylene films (from green to blue reversibly) simply by varying CN-PPE concentration or by stretching the polymer matrix, suggests various technological applications in the field of smart and intelligent films from thermoplastic materials.

Foldamer dynamics expressed via Markov state models. II. State space decomposition

The structural landscape of poly-phenylacetylene (pPA), otherwise known as m-phenylene ethynylene oligomers, has been shown to consist of a very diverse set of conformations, including helices, turns, and knots. Defining a state space decomposition to classify these conformations into easily identifiable states is an important step in understanding the dynamics in relation to Markov state models. We define the state decomposition of pPA oligomers in terms of the sequence of discretized dihedral angles between adjacent phenyl rings along the oligomer backbone. Furthermore, we derive in mathematical detail an approach to further reduce the number of states by grouping symmetrically equivalent states into a single parent state. A more challenging problem requires a formal definition for knotted states in the structural landscape. Assuming that the oligomer chain can only cross the ideal helix path once, we propose a technique to define a knotted state derived from a helical state determined by the position along the helical nucleus where the chain crosses the ideal helix path. Several examples of helical states and knotted states from the pPA 12-mer illustrate the principles outlined in this article.

Sequence-Specific Binding of m-Phenylene Ethynylene Foldamers to a Piperazinium Dihydrochloride Salt

[reaction: see text] Binding properties of a series of isomeric m-phenylene ethynylene oligomers containing short amide sequences to a piperazinium dihydrochloride salt were investigated by using circular dichroism (CD) measurements. Although these isomeric oligomers exhibited similar helical conformations, high affinity was observed only for one oligomer. This behavior is presumably controlled by the orientation of amino groups of the amide sequence and the folded conformation of the oligomer.

Supramolecular Chelation Based on Folding

Crystallographic analysis revealed that pyridine-palladium complexation is a good geometric match to the m-phenylene ethynylene (mPE) repeat unit and thus could serve as a reversible linking group to join oligomer segments together. A series of pyridine-terminated mPE oligomers were then synthesized and found to coordinate with palladium dichloride to give complexes effectively twice the length of the free oligomers. A quantitative analysis of these coordination equilibria by isothermal calorimetry found the ability of the pyridine end-group to form a coordination complex corresponded with their ability to fold. Oligomers that were able to form complexes of sufficient length to fold showed positive cooperativity based on experimental determination of their association constants with a palladium ion. We suggest that the additional free energy of complexation for the folded oligomers is analogous to chelation by multidentate ligands, but here the "multidentate ligand" is held together by supramolecular rather than covalent bonds.

A helicene-containing foldamer displaying highly solvent-dependent CD spectra.

[structure: see text] A m-phenylene ethynylene oligomer containing a helicene unit was synthesized to bias the twist sense of the folded helical conformation. The CD spectra of the helicene oligomer exhibited large Cotton effects that varied greatly with the solvent composition, including three separate conformational transitions.

Supramolecular control over donor-acceptor photoinduced charge separation.

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.

Simulation Studies of a Helical m-Phenylene Ethynylene Foldamer

The folded state of an oligo(m-phenylene ethynylene) foldamer solvated in water is examined using a 30 ns NpT molecular dynamics simulation. The 18-monomer oligomer with methyl ester exterior side groups maintains a helical structure, where turns of the helix are in close contact throughout the simulation. The structure exhibits large fluctuations in both the radius of the interior cylindrical pore and the effective dihedral angle between monomers. The radius fluctuations are correlated with the number of water molecules within the helical pore. The folded state is found to be surprisingly flexible while maintaining a helical structure.

Vibronic structures in the electronic spectra of oligo(phenylene ethynylene): effect of m-phenylene to the optical properties of poly(m-phenylene ethynylene)

Vibronic structures in the electronic spectra of oligo(phenylene ethynylene): effect of m-phenylene to the optical properties of poly(m-phenylene ethynylene)

Single-site modifications and their effect on the folding stability of m-phenylene ethynylene oligomers.

[reaction: see text] The folded structure of a m-phenylene ethynylene oligomer is tolerant to single-site modifications to both the backbone sequence and end groups. The helical structure is reinforced by multiple noncovalent interactions, allowing the oligomer sequence to be customized without a significant change in stability in most cases. The small changes that are observed are consistent with the expected behavior of pi-stacked systems and demonstrate subtle control over folding through single-site modifications.

Backbone-rigidified oligo(m-phenylene ethynylenes).

Oligo(m-phenylene ethynylenes) (oligo(m-PE)) with backbones rigidified by intramolecular hydrogen bonds were found to fold into well-defined conformations. The localized intramolecular hydrogen bond involves a donor and an acceptor from two adjacent benzene rings, respectively, which enforces globally folded conformations on these oligomers. Oligomers with two to seven residues have been synthesized and characterized. The persistence of the intramolecular hydrogen bonds and the corresponding curved conformations were established by ab initio and molecular mechanics calculations, 1D and 2D (1)H NMR spectroscopy, and UV spectroscopy. Pentamer 5, hexamer 6, and heptamer 7 adopt well-defined helical conformations. Such a backbone-based conformational programming should lead to molecules whose conformations are resilient toward structural variation of the side groups. These m-PE oligomers have provided a new approach for achieving folded unnatural oligomers under conditions that are otherwise unfavorable for previously described, solvent-driven folding of m-PE foldamers. Stably folded structures based on the design principle described here can be developed and may find important applications.

Pyridine-containing m-phenylene ethynylene oligomers having tunable basicities.

Incorporation of a pyridine monomer into the backbone of a m-phenylene ethynylene oligomer allows functionalization of the interior binding cavity of the folded oligomer. The basicity of the inwardly directed pyridine moiety was modulated by changing the substituents on the pyridine ring and through oligomer folding, granting access to a pK(a) range of 5-14 in acetonitrile. [reaction: see text]

Vibronic structures in the electronic spectra of oligo(phenylene ethynylene): effect of m-phenylene to the optical properties of poly( m-phenylene ethynylene)

At the low temperature, hidden vibronic structures are successfully resolved in the absorption and emission spectra of oligo(phenylene ethynylene)s 3– 5. Identification of the hidden bands allows estimation of the vibrational energy gaps in these molecules, which appears to increase with the oligomer conjugation length. The remarkable similarity between the absorption profiles of diphenylacetylene ( 3) and 1,3-bis(phenylethynyl)benzene ( 5), especially at −198 °C, confirms the effectiveness of conjugation interruption at m-phenylene. The function of precise conjugation length control via m-phenylene is further demonstrated from poly( m-phenylene ethynylene) (PmPE) ( 6). Even though the number of recurring unit (phenylene ethynylene) increases from 2 (for 5) to 12 (for 6), the absorption and emission spectra of the latter are nearly identical to that of the former.

A water-soluble m-phenylene ethynylene foldamer.

[reaction: see text] A water-soluble m-phenylene ethynylene (mPE) foldamer was realized by appending hexaethylene glycol side chains to the backbone repeat unit. UV spectra of the oligomer in aqueous solutions were consistent with a helical conformation. The association constant of the oligomer with (-)-alpha-pinene increased dramatically with increasing water composition, peaking at 90% water by volume in acetonitrile. The rate of the host/guest system's approach to equilibrium was found to decrease considerably with increasing water content.

M-Phenylene ethynylene sequences joined by imine linkages: dynamic covalent oligomers.

Imine metathesis between m-phenylene ethynylene oligomers of various lengths was performed in acetonitrile, a solvent in which oligomers containing eight or more repeat units adopt a compact helical conformation. The equilibrium constants and corresponding free energy change for the imine metathesis reactions were estimated. The results showed that the magnitude of equilibrium shifting measured by the free energy change for the formation of imine-containing oligomers increases linearly below a critical product chain length and grows asymptotically above it. The linear region is ascribed to the constant increase in contact area between monomer units of adjacent helical turns as the product chain grows to the 12-mer. Once the ligation product is 12 units in length, full contact is made between adjacent helical turns. On the other hand, for imine metathesis between oligomers leading to products having more than 12 units, the driving force is the difference between the folding energy of products and that of reactants. The additional stabilizing energy is roughly constant, regardless of the chain length, since the contact area between adjacent helical turns is unchanged. Consistent with the notion that the imine bond only minimally destabilizes the helical conformation, the position of the imine bond in the ligation product has been observed to have no significant effect on the folding stability. The magnitudes of equilibrium shifting are similar for ligation products of the same length but having the imine at various positions along the sequence. This suggests that the imine bond is compatible with the m-phenylene ethynylene backbone, regardless of the position in the sequence. Imine metathesis of m-phenylene ethynylene oligomers could allow a quick access to an unbiased, dynamic library of oligomer sequences joined by imine linkages.

Folding-Driven Reversible Polymerization of Oligo(m-phenylene ethynylene) Imines: Solvent and Starter Sequence Studies

Bis(imino) end-functionalized oligo(m-phenylene ethynylene)s were equilibrated in a closed system under conditions that promote reversible imine metathesis. The metathesis reaction joins two oligomers and produces a small molecule byproduct. In polar solvents, equilibration gave high molecular weight polymers while equilibration in chloroform produced only low molecular weight oligomers. This polymerization is hypothesized to be driven by the free energy gained from the folding of the long polymer chains directed by the noncovalent, intramolecular aromatic stacking and solvophobic interactions. This polymerization was also conducted in a series of solvents in which m-phenylene ethynylene oligomers have previously shown varied, intermediate folding stabilities. These experiments revealed a good correlation of the product molecular weight with the stability of the m-phenylene ethynylene helix. The equilibrium state of the metathesis reaction was also demonstrated to depend on the chain length of the starter...