Electron mediated activation

Figure 16.11. Energy ranges and applicable precursor ion polarity and charge state for various electron-mediated activation methods in tandem mass spectrometry.

Electron-Mediated Activation Methods 609 hybrid FT-ICR mass spectrometer utilizing a Cl source mounted above the hexapole for... 

Electron-Mediated Activation Methods 613 Direction of radical attack —X C-terminus... 

Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4]. The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

Recently, electron-mediated, scalar couplings which are active between magnetic nuclei on both sides of the hydrogen bridge have been discovered in nucleic acids [28-41], proteins [42-54] and their complexes (Tabs. 9.1-9.3) [54—56]. These couplings are closely related to similar inter- and intramolecular couplings across H-bonds in smaller chemical compounds [57-60]. It is well established [31, 58, 61-74] that such trans H-bond scalar couplings follow the same electron-mediated polarization mechanism as any covalent... 

Flowever, some associated materials might be perceived as toxic. For example, complexes of osmium find frequent use as electron mediators, because of their rich chemistry, stability, and redox activity. Osmium metal and most compounds are considered nontoxic, but the neat tetroxide of osmium is a strong oxidizer and is considered highly toxic in the U.S. and very toxic by the European Union. On the other hand, the aqueous solution, osmic acid, has been injected at 1% concentration in several European clinical trials, starting in the 1970s, for treatment of arthritis and hemophilia. - No toxic effects were observed. Thus, osmium toxicity might be a question not of in vivo chemistry, but of manufacture, where a concentrated form of the oxide might need to be handled. ... 

An alternative containment scheme is immobilization of active species on a surface" " or within a tethered polymer brush or network. ° Surface immobilization can achieve high surface utilization by locating mediators and biocatalysts within nanometers of conducting surfaces. Immobilization on polymer networks allows for dense packing of enzymes within electrode volumes at the expense of long-distance electron mediation between the enzyme active center and a conductive surface. Such mediation often represents the rate-limiting step in the overall electrode reaction. 

Another technique used is a cyclic voltammetry (CV) in the presence of electron mediator. In the GOx-catalyzed oxidation reaction of glucose, cofactor FAD, which is contained at the active center of GOx, oxidizes glucose to gluconolactone and resultant FADHj is converted back to the active FAD form by Oj. In the conventional type of enzyme sensors, the H2O2 generated from O2 is oxidized at the electrode surface. 

Willner and coworkers have extended this approach to electron relay systems where core-based materials facilitate the electron transfer from redox enzymes in the bulk solution to the electrode.56 Enzymes usually lack direct electrical communication with electrodes due to the fact that the active centers of enzymes are surrounded by a thick insulating protein shell that blocks electron transfer. Metallic NPs act as electron mediators or wires that enhance electrical communication between enzyme and electrode due to their inherent conductive properties.47 Bridging redox enzymes with electrodes by electron relay systems provides enzyme electrode hybrid systems that have bioelectronic applications, such as biosensors and biofuel cell elements.57... 

The PPS yield depends on the acidity of the mixture. The oxidative polymerization does not proceed in the absence of acids. Strong acids, such as triflu-oromethanesulfonic acid and trifluoroacetic acid, are required for the VO-cata-lyzed polymerization. The oxidative polymerization of diphenyl disulfide is facilitated by the activated VO(acac)2 produced by the acid. Diphenyl disulfide is not oxidized by 02 only or an equimolar amount of VO(acac)2 in the absence of acid. The VO catalyst is estimated to be an excellent electron mediator, through activation by acids, to promote the electron transfer between diphenyl disulfide and molecular oxygen. 

Loss of Mediator Activity and Leaching Many in vivo electrochemical glucose sensors utilize a mediator to shuttle the electron to the electrode and decrease the interference by electro-oxidizing species.28 When small mediators are used, the mediators tend to diffuse out of the membrane and into the body. If the mediator enters the body, it may result in interference with biological reactions.28-30 The method of attachment for the mediator is very important because if the mediator leaches out or loses activity, the sensor may fail. The mediator may lose activity if it undergoes a detrimental reaction or denatures, causing the potential to shift or the electron mediator ability to be lost. 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

RODS Poly(A-isopropylacrylamide) (PIPAAm) NIH-3T3 fibroblasts Electron beam activated polymerization of 2-carboxy-iV-isopropylacrylamide through a mask on PIPAAm (or vice versa), subsequent NHS mediated binding of peptide to carboxyl functional groups 2007 [173]... 

Electron mediators are usually organic molecules that are redox active, such as ferrocene derivatives, benzoquinone, N-methylphenazium, and 2,6-dichlorophenolindophenol (DCPIP). Ferricyanide has also been used as an electron mediator. They offer the advantages of non-... 

---