ac-field-induced fluid pumping in microsystems with asymmetric temperature gradients

We present two different designs of electrohydrodynamic micropumps for microfluidic systems. The micropumps have no movable parts, and their simple design allows for fabrication by microsystems technology. The pumps are operated by ac voltages from 1to60V and were tested with aqueous solutions in the conductivity range of 1–112mSm−1. The pump effect is induced by an ac electric field across a fluid medium with an inhomogeneous temperature distribution. It is constant over a wide range of the ac field frequency with a conductivity-dependent drop-off at high frequencies. The temperature-dependent conductivity and permittivity distributions in the fluid induce space charges that interact with the electric field and induce fluid motion. The temperature distribution can be generated either by Joule heating in the medium or by external heating. We present experimental results obtained with two prototypes featuring Joule heating and external heating by a heating filament. Experimental and numerical results are compared with an analytical model

Im Gedenken an den mongolischen Ornithologen Ajurzana Bold (1936-2007)

Obituary of Mongolian ornithologist Ajurzana Bold (1936-2007)

Im Gedenken an den mongolischen Ornithologen Ajurzana Bold (1936-2007)

Obituary of Mongolian ornithologist Ajurzana Bold (1936-2007)

Detection of water molecules on the radical transfer pathway of ribonucleotide reductase by 17O electron–nuclear double resonance spectroscopy

The role of water in biological proton-coupled electron transfer (PCET) is emerging as a key for understanding mechanistic details at atomic resolution. Here we demonstrate 17O high-frequency electron–nuclear double resonance (ENDOR) in conjunction with H217O-labeled protein buffer to establish the presence of ordered water molecules at three radical intermediates in an active enzyme complex, the α2β2E. coli ribonucleotide reductase. Our data give unambiguous evidence that all three, individually trapped, intermediates are hyperfine coupled to one water molecule with Tyr-O···17O distances in the range 2.8–3.1 Å. The availability of this structural information will allow for quantitative models of PCET in this prototype enzyme. The results also provide a spectroscopic signature for water H-bonded to a tyrosyl radical

Composition and Structure of the Inorganic Core of Relaxed Intermediate

Activation of the diferrous center of the β2 (R2) subunit of the class 1a Escherichia coli ribonucleotide reductases by reaction with O2 followed by one-electron reduction yields a spin-coupled, paramagnetic Fe(III)/Fe(IV) intermediate, denoted X, whose identity has been sought by multiple investigators for over a quarter of a century. To determine the composition and structure of X, the present study has applied 57Fe, 14,15N, 17O, and 1H electron nuclear double resonance (ENDOR) measurements combined with quantitative measurements of 17O and 1H electron paramagnetic resonance line-broadening studies to wild-type X, which is very short-lived, and to X prepared with the Y122F mutant, which has a lifetime of many seconds. Previous studies have established that over several seconds the as-formed X(Y122F) relaxes to an equilibrium structure. The present study focuses on the relaxed structure. It establishes that the inorganic core of relaxed X has the composition [(OH–)FeIII–O–FeIV]: there is no second inorganic oxygenic bridge, neither oxo nor hydroxo. Geometric analysis of the 14N ENDOR data, together with recent extended X-ray absorption fine structure measurements of the Fe–Fe distance (Dassama, L. M.; et al. J. Am. Chem. Soc. 2013, 135, 16758), supports the view that X contains a “diamond-core” Fe(III)/Fe(IV) center, with the irons bridged by two ligands. One bridging ligand is the oxo bridge (OBr) derived from O2 gas. Given the absence of a second inorganic oxygenic bridge, the second bridging ligand must be protein derived, and is most plausibly assigned as a carboxyl oxygen from E238.United States. National Institutes of Health (GM 111097)United States. National Institutes of Health (GM 29595

Evidence for electron transfer between graphene and non‐covalently bound π‐systems

Hybridizing graphene and molecules possess a high potential for developing materials for new applications. However, new methods to characterize such hybrids must be developed. Herein, the wet‐chemical non‐covalent functionalization of graphene with cationic π‐systems is presented and the interaction between graphene and the molecules is characterized in detail. A series of tricationic benzimidazolium salts with various steric demand and counterions was synthesized, characterized and used for the fabrication of graphene hybrids. Subsequently, the doping effects were studied. The molecules are adsorbed onto graphene and studied by Raman spectroscopy, XPS as well as ToF‐SIMS. The charged π‐systems show a p‐doping effect on the underlying graphene. Consequently, the tricationic molecules are reduced through a partial electron transfer process from graphene, a process which is accompanied by the loss of counterions. DFT calculations support this hypothesis and the strong p‐doping could be confirmed in fabricated monolayer graphene/hybrid FET devices. The results are the basis to develop sensor applications, which are based on analyte/molecule interactions and effects on doping