Functional groups modulate the sensitivity and electron transfer kinetics of neurochemicals at carbon nanotube modified microelectrodes

The surface properties of carbon-based electrodes are critically important for the detection of biomolecules and can modulate electrostatic interactions, adsorption and electrocatalysis. Carbon nanotube (CNT) modified electrodes have previously been shown to have increased oxidative sensitivity and reduced overpotential for catecholamine neurotransmitters, but the effect of surface functionalities on these properties has not been characterized. In this study, we modified carbon-fiber microelectrodes (CFMEs) with three differently functionalized single-wall carbon nanotubes and measured their response to serotonin, dopamine, and ascorbic acid using fast-scan cyclic voltammetry. Both carboxylic acid functionalized and amide functionalized CNTs increased the oxidative current of CFMEs by approximately 2-6 fold for the cationic neurotransmitters serotonin and dopamine, but octadecylamine functionalized CNTs resulted in no significant signal change. Similarly, electron transfer was faster for both amide and carboxylic acid functionalized CNT modified electrodes but slower for octadecylamine CNT modified electrodes. Oxidation of ascorbic acid was only increased with carboxylic acid functionalized CNTs although all CNT-modified electrodes showed a trend towards increased reversibility for ascorbic acid. Carboxylic acid-CNT modified disk electrodes were then tested for detection of serotonin in the ventral nerve cord of a Drosophila melanogaster larva, and the increase in sensitivity was maintained in biological tissue. The functional groups of CNTs therefore modulate the electrochemical properties, and the increase in sensitivity from CNT modification facilitates measurements in biological samples.     

Effect of Dilution Ratio on Amount of Asphaltenes Separated from Stock Tank Oil

We have investigated the effect of n‐alkane dilution on the amount of asphaltene separated from stock ‐tank oil. Asphaltenes were produced from three different oils by mixing each oil with varying amounts of n‐alkanes, including n‐pentane, n‐hexane, and n‐heptane. The n‐alkane:oil ratio ranged from 1∶1 to 1000∶1. With increasing n‐alkane:oil ratio, the amount of separated asphaltene initially increased, passed through a maximum, then decreased gradually with further dilution. The maximum occurred at an n‐alkane:oil ratio of around 30∶1 to 40∶1 for n‐hexane and n‐heptane, and 80∶1 for n‐pentane. A two‐component thermodynamic model based on Flory‐Huggins theory was adapted to match the observed trend of produced asphaltene by assuming a polydisperse model of the asphaltene fraction.     

Perturbation Theory for Solutions to Second Order Elliptic Operators with Complex Coefficients and the L p Dirichlet Problem

We establish a Dahlberg-type perturbation theorem for second order divergence form elliptic operators with complex coefficients. In our previous paper, we showed the following result: If L_0 = div A~0(x)? + B~0(x) · ? is a p-elliptic operator satisfying the assumptions of Theorem 1.1 then the LpDirichlet problem for the operator L_0 is solvable in the upper half-space Rn+. In this paper we prove that the Lpsolvability is stable under small perturbations of L_0. That is if L_1 is another divergence form elliptic operator with complex coefficients and the coefficients of the operators L_0 and L_1 are sufficiently close in the sense of Carleson measures, then the LpDirichlet problem for the operator L_1 is solvable for the same value of p. As a corollary we obtain a new result on Lpsolvability of the Dirichlet problem for operators of the form L = div A(x)? + B(x) · ? where the matrix A satisfies weaker Carleson condition(expressed in term of oscillation of coefficients). In particular the coefficients of A need no longer be differentiable and instead satisfy a Carleson condition that controls the oscillation of the matrix A over Whitney boxes. This result in the real case has been established by Dindoˇs,Petermichl and Pipher.     

Microelectrode Sensing of Adenosine/Adenosine‐5′‐triphosphate with Fast‐Scan Cyclic Voltammetry

Adenosine and adenosine‐5′‐triphosphate (ATP) are important extracellular signaling molecules. Here, we studied adenosine and ATP using fast‐scan cyclic voltammetry at carbon‐fiber microelectrodes. Although ATP and adenosine have similar oxidation potentials, ATP oxidation current was highly dependent on buffer pH and divalent cation concentrations but adenosine current was not. Therefore, they can be distinguished by adding a divalent cation chelator or calibrating electrodes at different pH values. The enzymatic degradation of adenosine by adenosine deaminase was monitored in a mixture of adenosine and ATP in presence of EDTA (ethylenediaminetetraacetate). This sensing method is promising for enzyme kinetics or in vitro studies.     

Systematic approach to understanding macrolide-ribosome interactions: NMR and modeling studies of oleandomycin and its derivatives

The three-dimensional structures of oleandomycin (1) and its derivatives oleandomycin-9-oxime (2) and 10,11-anhydrooleandomycin (3) were determined in different solvents by the combined use of NMR and molecular modeling methods. The experimental NMR data were compared with the results of molecular modeling and known crystal structures of the related molecules. It was shown that the dominant conformation of the lactone ring is the folded-out conformation with some amounts of the folded-in one depending on the solvent and temperature, while desosamine and cladinose sugars adopt the usual chair conformations. Modeling calculations provided evidence for conformational changes in the upper lactone region as well. Saturation transfer difference (STD) NMR experiments have provided information on the binding epitopes of 1-3 in complexes with E. coli ribosomes. The obtained molecular surfaces in close contact with ribosomes were compared with recently available 3D structures of the related macrolide-ribosome complexes, and the observed differences were discussed. The knowledge gained from this study can serve as a platform for the design of novel macrolides with an improved biological profile.