Side-chain dynamics of an alpha-helical polypeptide monitored by fluorescence

The "blob" model, developed to analyze the fluorescence decays of polymers randomly labeled with pyrene, has been applied to a series of pyrene-labeled poly(glutamic acid)s (PyPGA) in DMF and carbonated buffer solutions at pH 9. Poly(glutamic acid) (PGA) exists in the ionized form in the buffer solutions as poly(sodium glutamate) (PGNa). PGA adopts an alpha-helical conformation in DMF, whereas in aqueous solution PGNa is a random coil. Fluorescence, UV-vis absorption, and circular dichroism measurements indicate that in our studies pyrene pendants attached themselves along PGA in a clustered manner. Simulations were carried out to establish that the geometry of the PGA alpha-helix induces the high level of pyrene clustering. Since the level of pyrene clustering decreased with lower pyrene content, information about naked PGA was retrieved by extrapolating the trends obtained by fluorescence to zero pyrene content. Analysis of the fluorescence decays demonstrated that during its lifetime an excited pyrene probes a 32 amino acid section of the PGA alpha-helix. This result was supported by molecular mechanics optimizations. This study establishes that the blob model, originally used to monitor the encounters between pyrenes attached randomly onto a polymer adopting a random coil conformation, can also be applied to study the dynamics of the side chains of structured proteins. Since the blob model helps in monitoring the encounters between amino acids in the initial state (i.e., random coil) and in the final state (i.e., structured protein) of the folding pathway of a protein, it could be applicable to the study of protein folding.     

Separation of double-stranded DNA fragments by capillary electrophoresis using polyvinylpyrrolidone and poly(N,N-dimethylacrylamide) transient interpenetrating network

A noncross-linked interpenetrating polymer network (IPN), consisting of poly(N,N-dimethylacrylamide) (PDMA) and polyvinylpyrrolidone (PVP, weight-average molecular weight M(w) = 1 x 10(6) g/mol) was synthesized by polymerizing N,N-dimethylacrylamide (DMA) monomers directly in PVP buffer solution and tested as a separation medium for double-stranded (ds)DNA analysis without further purification. Due to the incompatibility of PVP and PDMA, a simple solution mixture could incur a microphase separation and showed poor performance on dsDNA separation. However, a dramatic improvement was achieved by the formation of an IPN. We attributed the high sieving ability of IPN as due to an increase in the number of entanglements by the more extended polymer chains. Apparent viscosity studies showed that the IPN had a much higher viscosity than the simple mixture containing the same amount of PDMA and PVP. In 1 x Tris-borate-EDTA (TBE) buffer, the concentration ratio of PDMA and PVP had a great effect on the DNA separation. At optimal conditions, the 22 fragments in pBR322/HaeIII DNA were successfully separated within 15 min, with a resolution of better than 1.0 for 123/124 bp.     

Electric field effects on photoinduced electron transfer processes of methylene-linked compounds of pyrene and N,N-dimethylaniline in a polymer film

Time-resolved measurements of the electric-field-induced change in fluorescence intensity have been made for methylene-linked compounds of pyrene and N,N-dimethylaniline (DMA) doped in a polymer film. The lifetime of the fluorescence emitted from the locally excited state of pyrene chromophore becomes shorter in the presence of electric field (F), when the dopant concentration is high. The lifetime of the excipelx fluorescence resulting from the photoinduced electron transfer (PIET) from DMA to the excited state of pyrene chromophore between different molecules also becomes shorter in the presence of F. Based on the simulation of the electric field effect on fluorescence decay, the mechanism of intermolecular PIET between DMA and pyrene chromophore in a polymer film is discussed.     

Clay-enhanced DNA separation in low-molecular-weight poly(N,N-dimethylacrylamide) solution by capillary electrophoresis

The effect of the separation medium in capillary electrophoresis consisting of a low-molecular-mass poly(N,N-dimethylacrylamide) (PDMA) solution on the DNA separation by adding a small amount of montmorillonite clay into the polymer matrix is presented. On the separation of the pBR322/HaeIII digest, both the resolution and the efficiency were increased by adding 2.5-5.0 x 10(-5) g/mL clay into the 5% w/v PDMA with a molecular mass of only 100 K. Moreover, there was no increase in the migration time of DNA fragments. Similar results were observed by using a C-terminated pGEM-3Zf(+) sequencing DNA sample in a sequencing buffer. Experimental data also showed that the addition of clay increased the viscosity of the polymer solution. We attribute this effect to the structural change of the polymer matrix caused by the exfoliated clay sheets, whereby the thin clay sheets function like a "dynamic cross-linking plate" for the PDMA chains and effectively increase the apparent molecular mass of PDMA.     

Effect of side-chain length on the side-chain dynamics of alpha-helical poly(L-glutamic acid) as probed by a fluorescence blob model

Two series of pyrene-labeled poly(glutamic acid) (Py-PGA) were synthesized utilizing two different linkers for pyrene attachment, namely 1-pyrenemethylamine (PMA) and 1-pyrenebutylamine (PBA). Several Py-PGAs were synthesized for each series with pyrene contents ranging from 4 to 15 mol %. Py-PGA forms a rigid alpha-helix in DMF that effectively locks the backbone in place, thus enabling only side-chain or linker motions to be monitored by time-resolved fluorescence. Time-resolved fluorescence decays were acquired for the pyrene monomer of the Py-PGA constructs and the fluorescence blob model (FBM) was used to quantify the dynamics of the different linkers connecting pyrene to the backbone. Nitromethane was used to shorten the lifetime of the pyrene monomer, in effect controlling the probing time of the pyrene group, from 50 to 155 ns for PGA-PBA and from 50 to 215 ns for PGA-PMA. The FBM analysis of the fluorescence decays led to the conclusion that excimer formation around the rigid alpha-helix backbone takes place in a compact environment. The number of glutamic acid units within a blob, N blob, decreased only slightly with decreasing probing time for both Py-PGA constructs as a result of the compact distribution of the chromophores around the alpha-helix. The PGA alpha-helix was modeled using Hyperchem software and the ability of two pyrene groups to encounter was evaluated as they were separated by increasing numbers of amino acids along the alpha-helix. The number of amino acids required for two pyrenes to lose their ability to overlap and form excimer matched closely the N blob values retrieved using the FBM.     

Dynamical scaling of single chains on adsorbing substrates: diffusion processes

We study the dynamics of tethered chains of length N on adsorbing surfaces, considering the dilute case; for this we use the bond fluctuation model and scaling concepts. In particular, we focus on the mean-square displacement of single monomers and of the center of mass of the chains. The characteristic time tau of the fluctuations of a free chain in a good solvent grows as tau approximately N(a), where the coefficient a obeys a=2nu+1. We show that the same coefficient also holds at the critical point of adsorption. At intermediate time scales single monomers show subdiffusive behavior; this concurs with the behavior calculated from scaling arguments based on the dynamical exponent a. In the adsorbed state tau(perpendicular), the time scale for the relaxation in the direction perpendicular to the surface, becomes independent of N; tau(perpendicular) is then the relaxation time of an adsorption blob. In the direction parallel to the surface the motion is similar to that of a two-dimensional chain and is controlled by a time scale given by tau(parallel) approximately N(2nu(2)+1)L(-2Delta(nu/nu)), where nu(2) is the Flory exponent in two dimensions, nu is the Flory exponent in three dimensions, and Deltanu=nu(2)-nu. For the motion parallel to the surface we find dynamical scaling over a range of about four decades in time.     

Novel fluoroalkyl end-capped amphiphilic diblock copolymers with pH/temperature response and self-assembly behavior

A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by (1)H NMR and (19)F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA(30)-b-PDEA(10) diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA(10)-b-PDMA(30) diblock copolymers can form flowerlike micelles in basic aqueous solution.     

Synthesis of double-hydrophilic double-grafted copolymers PMA-g-PEG/PDMA and their protein-resistant properties

A series of double-hydrophilic double-grafted PMA-g-PEG/PDMA copolymers, which contained poly(methacrylate) (PMA) as backbone, poly(ethylene glycol) (PEG) and poly(N,N-dimethylacrylamide) (PDMA) as side chains synthesized successfully by using reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP), were used as physical coatings for the evaluation of protein-resistant properties by capillary electrophoresis (CE). Electroosmotic flow (EOF) measurement results showed that the PMA-g-PEG/PDMA copolymer coated capillaries could suppress electroosmotic mobility in a wide pH range (pH = 2.8–9.8) and EOF mobility decreased with the increase of copolymer molecular mass and PDMA content. At the same time, protein recovery, theoretical plate number of separation and repeatability of migration time demonstrated that antifouling efficiency was improved with the increase of molecular mass and PEG content.     

A study of molecular diffusion in alcohol—water solution of poly-4-vinylpyridine by fluorescence quenching

Intra- and intermolecular diffusion processes in alcohol—water solutions of poly-4-vinylpyridine (PVPy) were studied by the fluorescence quenching method. It was prepared fluorescent-labelled PVPy (PVPy-F) by copolymerizing 4-vinylpyridine with anthrylmethylmethacrylate (F) (one chromophore group per 500 chain links in the copolymer). PVPy carrying both the chromophore and quenching chain links (quaternization of the pyridine residue by dimethylsulphate) (PVPy-FQ) and PVPy carrying only the quenchers (PVPy-Q).Measurement of the mean lifetime of the excited state of the anthracene group for the polymers and for the monomer analogue (F) with increasing quencher concentration and use of the Stern-Volmer relation permit determination of the constants of bimolecular collisions between the monomer quencher (Q) (4-ethyl-N-methyl pyridinium sulphate) and F, between the monomer quencher and PVPy-F and between Q and F incorporated in the same polymer chain (PVPy-FQ). The values of the diffusion constant for the system of PVPy-F and Q and for the system of PVPy-FQ are approximately two and four times respectively less than that for the monomer pair (F + Q). For the latter system, the finding indicates some hindrance to the diffusion of the chain links with each other within the coil. Nevertheless, their mobility remains high and probably microdiffusion will not limit chemical reactions of functional groups within coils and with monomer molecules.In studying the fluorescence quenching of the chromophore for PVPy-F in the presence of the polymer quencher, it was found that mutual interpenetration of the polymer coils is intensified markedly when the solution volume is filled entirely by the swelling coils.     

Studies on the interaction of achiral cationic pseudoisocyanine with chiral metal complexes

The effect of chiral metal complexes ([Co(en)(3)]I(3)·H(2)O, cis-[CoBr(NH(3))(en)(2)]Br(2), K[Co(edta)]·2H(2)O and [Ru(phen)(3)](PF(6))(2)) on the polymer-bound J-aggregates in aqueous mixtures of pesudoisocyanine (PIC) iodine and poly(acrylic acid, sodium)(PAAS) have been studied by UV-vis absorption, circular dichroism (CD) and fluorescence spectra. At low concentration, the PIC monomers could self-assemble to form supermolecules by binding to each of the COO(-) groups on the polymer chains through electrostatic interactions. After the addition of chiral metal complexes to the formed PIC-PAAS J-aggregates, we found that only the chiral multiple π-conjugated phenanthroline metal complexes could transfer their metal-centered chiral information to the formed J-aggregates. The chiral J-aggregates showed a characteristic induced circular dichroism (ICD) in the visible region of J-band chromophore, and the ICD signals depend on the absolute configuration, concentration of the chiral multiple π-conjugated metal complexes, as well as temperature. More interestingly, the supramolecular chirality of the polymer supported PIC J-aggregates could be memorized even after the addition of an excess opposite chiral complex enantiomers. This is in sharp contrast to the behavior in the high concentrated NaCl induced PIC-J aggregates, in which the optical rotation of a mixture of two enantiomers varies linearly with their ratio.     

Molecular interpretation for the solvation of poly(acrylamide)s. I. Solvent-dependent changes in the C=O stretching band region of poly(N,N-dialkylacrylamide)s

The solvation of poly(N,N-dimethylacrylamide) (PNdMA) and poly(N,N-diethylacrylamide) (PNdEA) in various protic and aprotic solvents has been studied by using infrared (IR) spectroscopy. Because PNdMA and PNdEA have the same polar functional group, their IR spectra show quite similar solvent effects. Unexpectedly, the solvent-dependent changes of the C=O stretching vibration (nu(C=O)) bands of the two polymers cannot be explained only by dielectric constants of the solvents. Then, infrared spectra of N,N-dimethylacetamide (NdMA) and N,N-diethylacetamide (NdEA), monomer models for PNdMA and PNdEA, respectively, in the same solvents as the polymer solutions have also been examined. Interestingly, the solvent-dependent spectra in the nu(C=O) band region of NdMA and NdEA are correlated with those of PNdMA and PNdEA, respectively, except for slight deviations, which may be ascribed to molecular mobility and/or exclusive volume. These correlations permit one to regard the solvation of the polymers as that of the corresponding monomers. As a result, we have proposed the assignments of nu(C=O) bands for the PNdMA and PNdEA solutions regarding the interactions between solvents and NdMA and NdEA as hydrogen bondings. In the IR spectra of PNdMA and PNdEA in the protic solvents, two C=O bands are mainly observed; one appears at a similar frequency to that of a C=O band observed for the monomer solution, and the other is characteristic of the polymer systems. The former band is likely to reflect the solvation behavior of PNdMA and PNdEA. The results clearly show that the solvation of a polymer can be interpreted at the molecular level using infrared spectroscopy sensitive to solvent effects.     

Atom transfer radical polymerization of a novel quinoline derivative monomer and fluorescent property

A novel functional monomer incorporating quinoline derivative moiety as the side group, 2-[4-(2,7,7-trimethyl-3-ethoxycarbonyl-5-oxo-1,4,5,6,7,8-hexhydricquinoline)phenoxyl] ethylmethacrylate (HQPEMA), was synthesized and polymerized utilizing atom transfer radical polymerization (ATRP) technique. 2-(4-Chloromethylphenyl)benzoxazole (CMPB) and CuCl/PMDETA were used as the initiator and catalyst, respectively. GPC, ¹H NMR and fluorescent emission spectroscopy were conducted for characterization of polymers. The linear increase of number average molecular weight (M_n) versus conversion and the relatively narrow molecular weight distribution (M_w/M_n) of the obtained polymers confirmed that ATRP of HQPEMA was carried out successfully. In addition, the fluorescence “structural self-quenching effect” was observed in the DMF solution of the monomer HQPEMA, which bearing both electron-donating chromophore group and electron-accepting CC bond. We also found that the fluorescence properties of the newly obtained polymers containing quinoline chromophore depended on both the monomer concentration in solution and the polarity of solvents. The emission of the polymer film showed that the emission peak maxima of the polymer film shifted 50 nm towards high wavelength with respect to the polymer in DMF solution due to the intermolecular or intramolecular interactions of the polymer chains.     

无机盐对氟烷基封端的嵌段共聚物在水相聚集行为的影响

以含氟醇钾盐(NFHO-K )作为引发剂,通过阴离子活性聚合方法合成了氟烷基封端的聚甲基丙烯酸-2-(二甲氨基)乙酯-block-聚甲基丙烯酸-2-(二乙氨基)乙酯嵌段共聚物(NFHO-PDMA-b-PDEA).该共聚物与一般的含氟嵌段共聚物相比,既具有优良的表面活性,又具有较好的溶解性.通过对溶液表面张力的测定,荧光探针法对溶液临界聚集浓度(cac)的测定和透射电子显微镜(TEM)对共聚物在溶液中胶束形态的研究,发现无机盐对共聚物在水溶液中的聚集行为有明显的影响,并可以增强共聚物的表面活性.     

4-二氰亚甲基-2,6-二甲基-4H-吡喃衍生物的合成及其 荧光效应

两种新的4-二氰亚甲基-2,6-二甲基-4H-吡喃衍生物及对应的聚酰胺酸被合成和表征。该聚酰胺酸在普通有机溶剂中具有良好的可溶性。报道了这些生色团衍生物的光强度、量子产率和发射波长,以及生色团共价键合所形成的聚酰胺酸膜具有比对应的生色团单体更强的荧光效应。     

Fast and excellent healing of hydroxypropyl guar gum/poly(N,N‐dimethyl acrylamide) hydrogels

In this article, a fast and high efficient healing hydroxypropyl guar gum (HPG)/poly(N,N‐dimethyl acrylamide) (PDMA) hydrogel is prepared by a facile synthesis method. HPG networks are formed through hydrogen‐bond interaction between the hydroxyl groups in the HPG chains, and PDMA networks are self‐crosslinked without any chemical crosslinker. The cut hydrogel could heal when nanosilica solution is chosen as the connector that is related to the adsorption of polymer to the surface of nanosilica. The fracture stress of the HPG/PDMA gels presents a fast and almost full recovery within a short time (1 min), while the recovery of fracture strain and elastic modulus is related to time in 2 h. The healing efficiency of HPG/PDMA gel is investigated as a function of healing time, HPG content, and N,N‐dimethyl acrylamide content. The microscopic healing process and healing mechanism are also discussed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 239–247     

Pulsed field gradient NMR studies of polymer adsorption on colloidal CdSe quantum dots

Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments have been used to examine ligand exchange between poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMA) (Mn = 12,000, Mw/Mn = 1.20, Nn = 78) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). We show that PFG 1H NMR can quantify the displacement of TOPO by PDMA through its ability to differentiate signals due to TOPO bound to the QDs versus those from TOPO molecules free in solution. For CdSe QDs with a band edge absorption maximum at 558 nm (diameter 2.7 nm by transmission electron microscopy), we determined that, at saturation, 8 polymer chains on average displace greater than 90% of the surface TOPO groups. At partial saturation, with an average of 6 polymer chains/QD, each TOPO displaced requires 28 DMA repeat units. Assuming that one Me2N- group binds to a surface Cd2+ for each TOPO displaced, we infer that only about 3% of the DMA units are directly bound to the surface. The remaining groups are present as loops or tails that protrude into the solvent and increase the hydrodynamic diameter of the particles.     

Synthesis, structure, and luminescent properties of microporous lanthanide metal-organic frameworks with inorganic rod-shaped building units

A series of microporous lanthanide metal-organic frameworks, Tb3(BDC)(4.5)(DMF)2(H2O)3.(DMF)(H2O) (1) and Ln3(BDC)(4.5)(DMF)2(H2O)3.(DMF)(C2H5OH)(0.5)(H2O)(0.5) [Ln = Dy (2), Ho (3), Er (4)], have been synthesized by the reaction of the lanthanide metal ion (Ln3+) with 1,4-benzenedicarboxylic acid and triethylenetetramine in a mixed solution of N,N'-dimethylformamide (DMF), water, and C(2)H(5)OH. X-ray diffraction analyses reveal that they are extremely similar in structure and crystallized in triclinic space group P. An edge-sharing metallic dimer and 4 metallic monomers assemble with 18 carboxylate groups to form discrete inorganic rod-shaped building units [Ln6(CO2)18], which link to each other through phenyl groups to lead to three-dimensional open frameworks with approximately 4 x 6 A rhombic channels along the [0,-1,1] direction. A water sorption isotherm proves that guest molecules in the framework of complex 1 can be removed to create permanent microporosity and about four water molecules per formula unit can be adsorbed into the micropores. These complexes exhibit blue fluorescence, and complex 1 shows a Tb3+ characteristic emission in the range of 450-650 nm.     

N-甲基-N-(2-甲基丙烯酰氧乙基)苯胺与其聚合物的荧光光谱

<正> 我们曾报道过甲基丙烯酸-4-N,N-二甲氨基苄酯(DMABMA)、N-(4-N′N′-二甲氨基苯基)代丙烯酰胺(DMAPMA)、8-丙烯酰氧喹啉(AQ)、N-丙烯酰-N′-苯基哌嗪(APP)等在同一分子中含有缺电子双键和给电子发色基团的单体及它们的聚合物在溶液中的荧光行为。在相同的链节克分子浓度下,这些单体的荧光强度比其聚合物的荧光强度低很多。我们将这种现象称为“结构自猝灭现象”。这种现象是由于共存在这类     

柔性羧酸配体一维铜（Ⅱ）配位聚合物的合成与晶体结构

以柔性羧酸配体4-氨基-1,2,4-三氮唑-3,5-二硫代乙酸（H2L）和氯化铜为原料,用常规溶液反应法,制备了配位聚合物[Cu（L）（DMF）（H2O）]n（DMF=N,N-二甲基甲酰胺）,并用X射线衍射分析确定了其晶体结构．结构分析表明：该配合物中每个铜（Ⅱ）为五配位,呈畸变的四方锥构型．与来自两个配体的一个N原子、两个羧基O原子和一个DMF的O原子、一个水分子的O原子配位．配体将Cu（Ⅱ）桥联起来形成沿a轴方向的一维链,链间通过氢键相互连接形成沿b轴方向的二维层,层与层间又通过S…S弱相互作用构筑成三维超分子网络．此外,元素分析、红外光谱和热分析的结果也证实了配合物的组成．     

Film formation from pure and mixed latices; transient fluorescence study

A fast transient fluorescence technique was used to study latex film formation induced by organic solvent vapor. Mixtures of pyrene (P)- and naphthalene (N)-labeled and/or pure naphthalene-labeled latex films were prepared separately from poly(methyl methacrylate) (PMMA) particles. Then these pure and mixed latex films were exposed to vapor of various chloroform-heptane mixtures in seven different experiments. In both films, fluorescence lifetimes from N were monitored during vapor-induced film formation. It was observed that N lifetimes decreased as the vapor exposure time is increased. A Stern-Volmer kinetic analysis was used for low quenching efficiencies to interpret the decrease in N lifetimes. A Prager-Tirrell model was employed to obtain back-and-forth frequencies, nu, of reptating PMMA chains during latex film formation induced by solvent vapor. In both pure and mixed latex films, nu values were found to be correlated with the chloroform content in the vapor mixture. It was observed that polymer interdiffusion obeyed a t1/2 law during film formation.