Partition of aminoacyl-tRNA synthetases in two different structural classes dating back to early metabolism: Implications for the origin of the genetic code and the nature of protein sequences

We describe, on the molecular level, a possible fuzzy and primordial translation apparatus capable of synthesizing polypeptides from nucleic acids in a world containing a mixture of coevolving molecules of RNA and proteins already arranged in metabolic cycles (including cofactors). Close attention is paid to template-free systems because they are believed to be the immediate ancestors of this primordial translation apparatus. The two classes of amnoacyl-tRNA synthetases (aaRSs), as seen today, are considered as the remnants of such a simple imprecise translation apparatus and are used as guidelines for the construction of the model. Earlier theoretical work by Bedian on a related system is invoked to show how specificity and stability could have been achieved automatically and rather quickly, starting from such an imprecise system, i.e., how the encoded synthesis of proteins could have appeared. Because of the binary nature of the underlying proto-code, the first genetically encoded proteins would then have been alternating copolymers with a high degree of degeneracy, but not random. Indeed, a clear signal for alternating hydrophobic and hydrophilic residues in present-day protein sequences can be detected. Later evolution of the genetic code would have proceeded along lines already discussed by Crick. However, in the initial stages, the translation apparatus proposed here is in fact very similar to the one postulated by Woese, only here it is given a molecular framework. This hypothesis departs from the paradigm of the RNA world in that it supposes that the origin of the genetic code occurred after the apparition of some functional (statistical) proteins first. Implications for protein design are also discussed.     

Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) utilizing p‐type polymers as electron‐donors and n ‐typepolymers as electron‐acceptors have attracted a great deal of attention, and their efficiencies have been improved considerably. Here, five polymer donors with different molecular orientations are synthesized by random copolymerization of 5‐fluoro‐2,1,3‐benzothiadiazole with different relative amounts of 2,2′‐bithiophene (2T) and dithieno[3,2‐b;2′,3′‐d]thiophene (DTT). Solar cells are prepared by blending the polymer donors with a naphthalene diimide‐based polymer acceptor (PNDI) or a [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM) acceptor and their morphologies and crystallinity as well as optoelectronic, charge‐transport and photovoltaic properties are studied. Interestingly, charge generation in the solar cells is found to show higher dependence on the crystal orientation of the donor polymer for the PNDI‐based all‐PSCs than for the conventional PC₇₁BM‐based PSCs. As the population of face‐on‐oriented crystallites of the donor increased in PNDI‐based PSC, the short‐circuit current density (J_SC) and external quantum efficiency of the devices are found to significantly improve. Consequently, device efficiency was enhanced of all‐PSC from 3.11% to 6.01%. The study reveals that producing the same crystal orientation between the polymer donor and acceptor (face‐on/face‐on) is important in all‐PSCs because they provide efficient charge transfer at the donor/acceptor interface.     

Immobilization of Lipase from Mucor Miehei Onto Poly (Ethylene) Based Graft Copolymers

Lipase from Mucor miehei was immobilized covalently onto hydrolyzed poly(ethylene)-g.co-hydroxyethyl methacrylate (PE-HEMA). This hydrolysis of the copolymer was achieved using 0.1 M NaOH over different periods of time, under controlled conditions. The graft copolymers and their hydrolyzed equivalents were characterized by scanning electron microscopy (SEM) and by differential scanning calorimetry analysis (DSC). Water sorption studies were undertaken to provide a measure of relative hydrophobicity of the samples.

The lipase immobilization reaction was studied in order to assess the effects of controlling various important parameters. These include the nature of the buffering medium, the time over which the immobilization was allowed to occur, the concentration of the activating and coupling agent used (CMC) and the concentration of enzyme employed during attempts at effective immobilization. The immobilized lipase was used in the hydrolysis of triolein (glycerol trioleate). From this study, the apparent K_M, the optimum pH for hydrolysis and the optimum temperature for hydrolysis were revealed.

The suitability of hydrolyzed poly(ethylene)-g.co-HEMA as a support in the immobilization of lipase was assessed by determination of the amount of lipase coupled to the support and by assessment of the retention of activity of the immobilized lipase after its exposure to the immobilization reagents, procedure and conditions.     

High‐Performance and Uniform 1 cm2 Polymer Solar Cells with D1‐A‐D2‐A‐Type Random Terpolymers

For the commercial development of organic photovoltaics (OPVs), laboratory‐scale OPV technology must be translated to large area modules. In particular, it is important to develop high‐efficiency polymers that can form thick (>100 nm) bulk heterojunction (BHJ) films over large areas with optimal morphologies for charge generation and transport. Here, D₁‐A‐D₂‐A random terpolymers composed of 2,2′‐bithiophene with various proportions of 5,6‐difluoro‐4,7‐bis(thiophen‐2‐yl)‐2,1,3‐benzothiadiazole and 5,6‐difluoro‐2,1,3‐benzothiadiazole (FBT) are synthesized. It is found that incorporating small proportions of FBT into the polymer not only conserves the high crystallinity and favorable face‐on orientation of the D‐A copolymer FBT‐Th4 but also improves the nanoscale phase separation of the BHJ film. Consequently, the random terpolymer PDT2fBT‐BT10 exhibits a much improved solar cell efficiency of 10.31% when compared to that of the copolymer FBT‐Th4 (8.62%). Moreover, due to this polymer's excellent processability and suppressed overaggregation, OPVs with 1 cm² active area based on 351 nm thick PDT2fBT‐BT10 BHJs exhibit high photovoltaic performance of 9.42%, whereas rapid efficiency decreases arise for FBT‐Th4‐based OPVs for film thicknesses above 300 nm. It is demonstrated that this random terpolymer can be used in large area and thick BHJ OPVs, and guidelines for developing polymers that are suitable for large‐scale printing technologies are presented.     

Preparation of surfactant-free nanoparticles of methacrylic acid copolymers used for film coating

The aim of the present study was to prepare surfactant-free pseudolatexes of various methacrylic acid copolymers. These aqueous colloidal dispersions of polymeric materials for oral administration are intended for film coating of solid dosage forms or for direct manufacturing of manoparticles. Nanoparticulate dispersions were produced by an emulsification-diffusion method involving the use of partially water-miscible solvents and the mutual saturation of the aqueous and organic phases prior to the emulsification in order to reduce the initial thermodynamic instability of the emulsion. Because of the self-emulsifying properties of the methacrylic acid copolymers, it was possible to prepare aqueous dispersions of colloidal size containing up to 30% wt/vol of Eudragit RL, RS, and E using 2-butanone or methyl acetate as partially water-miscible solvents, but without any surfactant. However, in the case of the cationic Eudragit E, protonation of the tertiary amine groups by acidification of the aqueous phase was necessary to improve the emulsion stability in the absence of surfactant and subsequently to prevent droplet coalescence during evaporation. In addition, a pseudolatex of Eudragit E was used to validate the coating properties of the formulation for solid dosage forms. Film-coated tablets of quinidine sulfate showed a transparent glossy continuous film that was firmly attached to the tablet. The dissolution profile of quinidine sulfate from the tablets coated with the Eudragit E pseudolatex was comparable to that of tablets coated with an acetonic solution of Eudragit E. Furthermore, both types of coating ensured similar taste masking. The emulsification-evaporation method used was shown to be appropriate for the preparation of surfactant-free colloidal dispersions of the 3 types of preformed methacrylic acid copolymers; the dispersions can subsequently be used for film coating of solid dosage forms. Published: July 28, 2006     

Graft copolymers of xyloglucan and methyl methacrylate

Xyloglucan, a water-soluble food grade polysaccharide, was reported as a substrate for graft copolymerization of methyl methacrylate (MMA). Grafting PMMA (polymethyl methacrylate) with xyloglucan (XG) makes a new material with improved thermal stability and shelf life without affecting its hydrophilicity. XG was isolated from tamarind seed mucilage by aqueous extraction. Grafting of MMA was initiated by ceric ion in aqueous medium under N₂ atmosphere and the progress of the reaction was monitored gravimetrically by varying different reaction parameters. Grafting of MMA onto XG was confirmed by FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetric (DSC) studies, thermal gravimetric analysis (TGA) studies and scanning electron micrographs (SEMs). This material might find potential to be used in drug delivery systems.     

Single Material Solar Cells: the Next Frontier for Organic Photovoltaics?

An overview of various approaches for the realization of single‐material organic solar cells (SMOCs) is presented. Fullerene‐conjugated systems dyads, di‐block copolymers, and self‐organized donor‐acceptor molecules all represent different possible approaches towards SMOCs. Although each of them presents specific advantages and poses specific problems of design and synthesis, these different routes have witnessed significant progress in the past few years and SMOCs with efficiencies in the range of 1.50% have been realized. These performances are already higher than those of bi‐component bulk heterojunction solar cells some ten years ago, demonstrating that SMOCs can represent a credible approach towards efficient and simple organic solar cells. Possible directions for future research are discussed with the aim of stimulating further research on this exciting topic.     

Physical blends of starch graft copolymers as matrices for colon targeting drug delivery systems

Colon targeting drug delivery systems have attracted many researchers due to the distinct advantages they present such as near neutral pH, longer transit time and reduced enzymatic activity. Moreover, in recent studies, colon specific drug delivery systems are gaining importance for use in the treatment of local pathologies of the colon and also for the systemic delivery of protein and peptide drugs.In previous works, our group has developed different types of hydrophilic matrices with grafted copolymers of starch and acrylic monomers with a wide range of physicochemical properties which have demonstrated their ability in controlled drug release. Since the cost of synthesizing a new polymeric substance and testing for its safety is enormous, polymer physical blends are frequently used as excipients in controlled drug delivery systems due to their versatility. So, the aim of this work is to combine two polymers which offer different properties such as permeability for water and drugs, pH sensitivity and biodegradability in order to further enhance the release performance of various drugs. It was observed that these physical blend matrices offer good controlled release of drugs, as well as of proteins and present suitable properties for use as hydrophilic matrices for colon-specific drug delivery.