Electrochemical Behaviour of HOPG and CVD‐Grown Graphene Electrodes Modified with Thick Anthraquinone Films by Diazonium Reduction

Highly oriented pyrolytic graphite (HOPG) and graphene grown on Ni (Ni‐Gra) or Cu (Cu‐Gra) by chemical vapour deposition were modified with thick anthraquinone (AQ) films (7?60 nm) by redox grafting of the pertinent diazonium salt. Glassy carbon (GC) electrodes were used for comparison. The AQ‐modified GC electrodes showed excellent blocking properties towards the Fe(CN)₆^3?/4? redox probe, although it was noted that in the case of Ni‐Gra and Cu‐Gra, the blocking ability depended strongly on the underlying substrate. Oxygen reduction studies revealed good electrocatalytic activity of AQ‐modified HOPG, Ni‐Gra, and Cu‐Gra, compared with the bare electrodes.     

The α-Effect and Competing Mechanisms: The Gas-Phase Reactions of Microsolvated Anions with Methyl Formate

The enhanced reactivity of α-nucleophiles, which contain an electron lone pair adjacent to the reactive site, has been demonstrated in solution and in the gas phase and, recently, for the gas-phase S_N2 reactions of the microsolvated HOO^–(H₂O) ion with methyl chloride. In the present work, we continue to explore the significance of microsolvation on the α-effect as we compare the gas-phase reactivity of the microsolvated α-nucleophile HOO^–(H₂O) with that of microsolvated normal alkoxy nucleophiles, RO^–(H₂O), in reactions with methyl formate, where three competing reactions are possible. The results reveal enhanced reactivity of HOO^–(H₂O) towards methyl formate, and clearly demonstrate the presence of an overall α-effect for the reactions of the microsolvated α-nucleophile. The association of the nucleophiles with a single water molecule significantly lowers the degree of proton abstraction and increases the S_N2 and B_AC2 reactivity compared with the unsolvated analogs. HOO^–(H₂O) reacts with methyl formate exclusively via the B_AC2 channel. While microsolvation lowers the overall reaction efficiency, it enhances the B_AC2 reaction efficiency for all anions compared with the unsolvated analogs. This may be explained by participation of the solvent water molecule in the B_AC2 reaction in a way that continuously stabilizes the negative charge throughout the reaction.

Inverse hydrogen migration in arginine-containing peptide ions upon electron transfer

Collisional electron transfer from gaseous Cs atoms was studied for singly and doubly protonated peptides Gly-Arg (GR) and Ala-Arg (AR) at 50- and 100-keV kinetic energies. Singly protonated GR and AR were discharged to radicals that in part rearranged by migration of a C_α hydrogen atom onto the guanidine group. The C_α-radical isomers formed were detected as stable anions following transfer of a second electron. In addition to the stabilizing rearrangements, the radicals underwent side-chain and backbone dissociations. The latter formed z fragments that were detected as the corresponding anions. Analysis of the (GR+H)^· radical potential energy surface using electronic structure theory in combination with Rice-Ramsperger-Kassel-Marcus calculations of rate constants indicated that the arginine C_α hydrogen atom was likely to be transferred to the arginine side-chain on the experimental timescale of ≤200 ns. Transfer of the Gly C_α-H was calculated to have a higher transition-state energy and was not kinetically competitive. Collisional electron transfer to doubly protonated GR and AR resulted in complete dissociation of (GR+2H)^+· and (AR+2H)^+· ions by loss of H, ammonia, and N-C_α bond cleavage. Electronic structure theory analysis of (GR+2H)^+· indicated the presence of multiple conformers and electronic states that differed in reactivity and steered the dissociations to distinct channels. Electron attachment to (GR+2H)²⁺ resulted in the formation of closely spaced electronic states of (GR+2H)^+· in which the electron density was delocalized over the guanidinium, ammonium, amide, and carboxyl groups. The different behavior of (GR+H)^· and (GR+2H)^+· is explained by the different timescales for dissociation and different internal energies acquired upon electron transfer.     

Attractive double-layer forces and charge regulation upon interaction between electrografted amine layers and silica

Chitosan (CS) and poly(acrylic acid) (PAA) were crosslinked by an ionic gelation method to form super absorbent polymers (SAPs). CS and PAA form amide bonds between the amino and carboxyl groups. The CS-PAA copolymers were synthetically engineered by varying the feed ratios of the prepolymer units. The copolymer materials possess tunable sorption and mucoadhesive properties with a backbone structure resembling proteinaceous materials. The sorption properties of the copolymers toward methylene blue (MB) in aqueous solution were studied using UV-Vis spectrophotometry at ambient pH and 295 K. The copolymers showed markedly varied interactions with MB, from physisorption- to chemisorption-like behavior, in accordance with their composition, surface area, and pore structure characteristics. The sorption isotherms were evaluated with the Sips model to provide estimates of the sorption properties. The sorbent surface area (271 and 943 m²/g) and the sorption capacity (Q_m = 1.03 and 3.59 mmol/g) were estimated for the CS-PAA copolymer/MB systems in aqueous solution.     

A simulation study on tissue harmonic imaging with a single-element intravascular ultrasound catheter

Recently, in vivo feasibility of tissue harmonic imaging with a mechanically rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, a combination of pulse inversion and analog filtering was used. In this paper the development of a simulation tool to investigate nonlinear IVUS beams is reported, and the influence of transducer rotation and axial catheter-to-tissue motion on the efficiency of PI signal processing is evaluated. Nonlinear beams were simulated in homogeneous tissue-mimicking media at a transmit frequency of 20 MHz, which resulted in second harmonic pressure fields at 40 MHz. The competing effects of averaging and decorrelation between neighboring rf lines on the signal-to-noise ratio (SNR) were studied for a single point scatterer. An optimal SNR was achieved when lines were combined over 3 degrees - 3.75 degrees. When the transducer was rotated with respect to point scatterers, simulating the acoustic response of tissue, the fundamental frequency suppression using PI degraded rapidly with increasing interpulse angles. The effect of axial catheter-to-tissue motion on the efficiency of pulse inversion seemed to be of less influence for realistic motion values. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

Prediction of pH-dependent aqueous solubility of druglike molecules

In the present work, the Henderson-Hasselbalch (HH) equation has been employed for the development of a tool for the prediction of pH-dependent aqueous solubility of drugs and drug candidates. A new prediction method for the intrinsic solubility was developed, based on artificial neural networks that have been trained on a druglike PHYSPROP subset of 4548 compounds. For the prediction of acid/base dissociation coefficients, the commercial tool Marvin has been used, following validation on a data set of 467 molecules from the PHYSPROP database. The best performing network for intrinsic solubility predictions has a cross-validated root mean square error (RMSE) of 0.70 log S-units, while the Marvin pKa plug-in has an RMSE of 0.71 pH-units. A data set of 27 drugs with experimentally determined pH-solubility curves was assembled from the literature for the validation of the combined pH-dependent model, giving a mean RMSE of 0.79 log S-units. Finally, the combined model has been applied on profiling the solubility space at low pH of five large vendor libraries.     

Transition metals as electron traps. I. Structures,energetics, electron capture,and electron‐transfer‐induced dissociations of ternary copper–peptide complexes in the gas phase

Electron‐induced dissociations of gas‐phase ternary copper‐2,2′‐bipyridine complexes of Gly‐Gly‐Gly and Gly‐Gly‐Leu were studied on a time scale ranging from 130 ns to several milliseconds using a combination of charge‐reversal (⁺CR^?) and electron‐capture‐induced dissociation (ECID) measured on a beam instrument and electron capture dissociation (ECD) measured in a Penning trap. Charge‐reduced intermediates were observed on the short time scale in the ⁺CR^? and ECID experiments but not in ECD. Ion dissociations following electron transfer or capture mostly occurred by competitive bpy or peptide ligand loss, whereas peptide backbone fragmentations were suppressed in the presence of the ligated metal ion. Extensive electron structure theory calculations using density functional theory and large basis sets provided optimized structures and energies for the precursor ions, charge‐reduced intermediates, and dissociation products. The Cu complexes underwent substantial structure changes upon electron capture. Cu was calculated to be pentacoordinated in the most stable singly charged complexes of the [Cu(peptide ? H)bpy]^{+ ?} type where it carried a ～+ 1 atomic charge. Cu coordination in charge‐reduced [Cu(peptide ? H)bpy] intermediates depended on the spin state. The themodynamically more stable singlet states had tricoordinated Cu, whereas triplet states had a tetracoordinated Cu. Cu was tricoordinated in stable [Cu(peptide ? H)bpy]^{? ?} products of electron transfer. [Cu(peptide)bpy]^{2 + ?} complexes contained the peptide ligand in a zwitterionic form while Cu was tetracoordinated. Upon electron capture, Cu was tri‐ or tetracoordinated in the [Cu(peptide)bpy]⁺ charge‐reduced analogs and the peptide ligands underwent prototropic isomerization to canonical forms. The role of excited singlet and triplet electronic states is assessed. Copyright © 2009 John Wiley & Sons, Ltd.     

Threshold received sound pressure levels of single 1-2 kHz and 6-7 kHz up-sweeps and down-sweeps causing startle responses in a harbor porpoise (Phocoena phocoena)

Mid-frequency and low-frequency sonar systems produce frequency-modulated sweeps which may affect harbor porpoises. To study the effect of sweeps on behavioral responses (specifically "startle" responses, which we define as sudden changes in swimming speed and/or direction), a harbor porpoise in a large pool was exposed to three pairs of sweeps: a 1-2 kHz up-sweep was compared with a 2-1 kHz down-sweep, both with and without harmonics, and a 6-7 kHz up-sweep was compared with a 7-6 kHz down-sweep without harmonics. Sweeps were presented at five spatially averaged received levels (mRLs; 6 dB steps; identical for the up-sweep and down-sweep of each pair). During sweep presentation, startle responses were recorded. There was no difference in the mRLs causing startle responses for up-sweeps and down-sweeps within frequency pairs. For 1-2 kHz sweeps without harmonics, a 50% startle response rate occurred at mRLs of 133 dB re 1 μPa; for 1-2 kHz sweeps with strong harmonics at 99 dB re 1 μPa; for 6-7 kHz sweeps without harmonics at 101 dB re 1 μPa. Low-frequency (1-2 kHz) active naval sonar systems without harmonics can therefore operate at higher source levels than mid-frequency (6-7 kHz) active sonar systems without harmonics, with similar startle effects on porpoises.