Voltammetric Sensing of Amino Acids in the Presence of Cu(II) in Acidic and Alkaline Solutions

A number of amino acids have been determined at carbon film electrodes in the presence of copper. Strongly acidic, 0.1 M HCl, in the presence of 0.1 mM Cu(II), as well as alkaline, 0.1 M NaOH, solution permit successful measurement of individual amino acids, clearer separation between oxidation of Cu and Cu‐complexes occurring in alkaline solution. Electrochemical impedance showed that Cu(II) facilitates charge transfer, particularly in alkaline medium. Square wave voltammetry with preconcentration increased the response compared to linear sweep voltammetry. Protein hydrolysis rates were monitored through determination of amino acids produced by decomposition, injecting samples into alkaline electrolyte solution.     

Nonlinear analytical multi-dimensional convolution solution of the second order system

In this paper, a procedure to analytically develop an approximate solution for the prototypical nonlinear mass–spring–damper system based on multi-dimensional convolution expansion theory is offered. The nonlinearity herein is mathematically considered in quadratic and bilinear terms. A variational expansion methodology, one of the most efficient analytical Volterra techniques, is used to develop an analytical two-term Volterra series. The resultant model is given in the form of first and second kernels. This analytical solution is visualized in the time domain followed by a parametric study for understanding the influence of each nonlinear/linear term appearing in the kernel structure. An analytical nonlinear step response is also conducted to characterize the overall system response from the fundamental components. The developed analytical step response provides an illumination for the source of differences between nonlinear and linear responses such as initial departure time, settling time, and steady value. Feasibility of the proposed implementation is assessed by numerical examples. The developed kernel-based model shows the ability to predict, understand, and analyze the system behavior beyond that attainable by the linear-based model.     

Enabling Efficient Folding and High-Resolution Crystallographic Analysis of Bracelet Cyclotides

Cyclotides have attracted great interest as drug design scaffolds because of their unique cyclic cystine knotted topology. They are classified into three subfamilies, among which the bracelet subfamily represents the majority and comprises the most bioactive cyclotides, but are the most poorly utilized in drug design applications. A long-standing challenge has been the very low in vitro folding yields of bracelets, hampering efforts to characterize their structures and activities. Herein, we report substantial increases in bracelet folding yields enabled by a single point mutation of residue Ile-11 to Leu or Gly. We applied this discovery to synthesize mirror image enantiomers and used quasi-racemic crystallography to elucidate the first crystal structures of bracelet cyclotides. This study provides a facile strategy to produce bracelet cyclotides, leading to a general method to easily access their atomic resolution structures and providing a basis for development of biotechnological applications.     

Circle packings and polyhedral surfaces

Given a triangulated surface, a euclidean or hyperbolic polyhedral surface can be constructed by assigning radii to the vertices of the triangulation. We develop necessary and sufficient conditions for the existence of such a polyhedral surface having specified characteristics. The results in this paper are included in the author's doctoral dissertation [12].     

Enhancing Temporal Control and Enabling Chain‐End Modification in Photoregulated Cationic Polymerizations by Using Iridium‐Based Catalysts

Gaining temporal control over chain growth is a key challenge in the enhancement of controlled living polymerizations. Though research on photocontrolled polymerizations is still in its infancy, it has already proven useful in the development of previously inaccessible materials. Photocontrol has now been extended to cationic polymerizations using 2,4,6‐triarylpyrylium salts as photocatalysts. Despite the ability to stop polymerization for a short time, monomer conversion was observed over long dark periods. Improved catalyst systems based on Ir complexes give optimal temporal control over chain growth. The excellent stability of these complexes and the ability to tune the excited and ground state redox potentials to regulate the number of monomer additions per cation formed allows polymerization to be halted for more than 20 hours. The excellent stability of these iridium catalysts in the presence of more nucleophilic species enables chain‐end functionalization of these polymers.     

Ferricyanide Confined in a Protonated Amine-Functionalized Silica Film on Gold: Application to Electrocatalytic Sensing of Nitrite Ions

An amine-functionalized porous sol–gel silica film was shown to be an effective platform to immobilize small anionic redox mediators of high solubility on solid electrodes by electrostatic interaction. The highly soluble mediator hexacyanoferrate was used as a model. The film was grown and firmly anchored on a gold electrode surface via thiol groups of a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane. Film growth and thickness were controlled by electrochemical modulation of pH at the electrode/solution interface in a sol of a hydrolyzed solution of tetraethoxysilane and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane by the application of a negative potential to the electrode. Protonation of the amine groups made the amine-functionalized surface useful to immobilize hexacyanoferrate on gold. Thus, the immobilization is pH dependent, being highly effective in strongly acidic medium. Cyclic voltammetry and scanning electron microscopy were used to characterize the film and to optimize the experimental conditions. The stability of the film was demonstrated by applying the catalytic properties of the hexacyanoferrate containing surface for nitrite sensing using a flow injection analysis (FIA) system. Under the optimized conditions, the sensor exhibited high sensitivity, low detection limit, easy handling, and stability with a linear range from 1.0 to 40.0?µmol?L^?1 and a detection limit of 0.53?µmol?L^?1 based on a signal-to-noise ratio of 3. The sensor was successfully applied to nitrite determination in water samples using FIA with excellent recoveries.     

Sonoelectrochemical determination of nucleosides and nucleotides

We summarize our recent work on sonovoltammetric studies of adenine, guanine, thymine, cytosine and their corresponding nucleosides and nucleotides. Ultrasound is found to be an excellent tool to enhance the reliability of determinations of purine and pyrimidine derivatives and can lead to improved analytical performance characteristics and lower detection limits.     

Sonoelectrochemical analysis of trace metals

Ultrasonically-enhanced mass transport was exploited to increase preconcentration efficiency in anodic stripping voltammetry. We developed a Nafion-coated mercury thin-film working electrode which is stable under ultrasonic irradiation, making it possible to achieve very low limits of detection for relatively short preconcentration times. This allows the investigation of a variety of biological and environmental samples.