Resolution redox potential

The peak-shaped response of differential-pulse measurements results in unproved resolution between two species with similar redox potentials, hi various situations, peaks separated by 50 mV may be measured. Such quantitation depends not only upon the corresponding peak potentials but also on the widths of the peak. The width of the peak (at half-height) is related to the electron stoichiometry ... 

Contact lithography can be used to spatially control the photosubstitution process to form laterally resolved bicomponent films with image resolution below 10 pm. Dramatic changes occur in the colors and redox potentials of such ruthenium(II) complexes upon substitution of chloride for the pyridine ligands (Scheme 1). Striped patterns of variable colors are observed on addressing such films with a sequence of potentials. 

The three isomers of [Co(dien)2]3+/2+ have, as predicted by molecular mechanics calculations (see also Table 7.1 in Chapter 7, Section 7.1 for calculated and observed isomer ratios of [Co(dien)2]3+), measurably different redox potentials1415. However, the strain energy differences between various conformers of each isomer were calculated to be too small for a measurable difference of the redox potentials, and the order of stability in both oxidation states was the same[44]. A similar problem occurred with [Co(sep)]3+/2+t44]. For [Co (S)-pn 3]3+/2+ the four redox potentials between isostructu-ral pairs lie within the predicted range of 20 mV However due to lack of resolution, a quantitative analysis was not possible. An unambiguous proof of the model is, therefore, still lacking. 

The detection of (specifically) dopamine is hindered by the presence in the extracellular fluid of several compounds having redox potentials close to that of dopamine. The technique most likely to succeed here is fast scan cyclic voltammetry (Section 8.6) because the voltamogram provides characteristics that are indicative of the individual compound being monitored. The microelectrodes used have radii of 5 pm, but even this is not small enough to be able to determine dopamine from just one cell. The reacting compounds come from several nerve endings. Nevertheless, the fast scan cyclic voltammetry technique her sufficient time and resolution to allow information to be obtained on the part played by dopamine in neurotransmission in the brain. For example, it answers such questions as does the released dopamine stay at the synapse or does it diffuse in the extracellular fluid to contact other neurons ... 

Three main tendencies have been underlined in recent studies of structure and action mechanism ofbacterial photosynthetic reaction centers. The crystallographic structure of the reaction centers from Rps. viridis and Rb. spheroids was initially determined to be 2.8 and 3 A resolutions (Michel and Deisenhofer et al., 1985 Allen et al., 1986). Resolution and refinement of these structures have been subsequently extended to 2.2, 2.3 and 2.6 A. (Rees et al., 1989 Stowell et al., 1997, Fyfe and Johns, 2000 Rutherford and Faller, 2001). Investigations of the electronic structure of donor and acceptor centers in the ground and exited states by modern physical methods with a combination ofpico-and femtosecond kinetic techniques have become more precise and elaborate. Extensive experimental and theoretical investigations on the role of orbital overlap and protein dynamics in the processes of electron and proton transfer have been done. All the above-mentioned research directions are accompanied by extensive use of methods of sit-directed mutagenesis and substitution of native pigments for artificial compounds of different redox potential. 

The X-ray crystal structure of a reaction centre from Rhodobacter sphaeroides with a mutation of tyrosine M210 to tryptophan (YM210W) has been determined to have a resolution of 2.5 A (McAuley et al., 2000). It is shown that the main effect of the introduction of the bulkier tryptophan in place of the native tyrosine is a small tilt of the macrocycle of the (Bchl)L. The effect of the redox potential of the electron acceptor (Bchl) in RC from Rb. spheroides on the initial electron transfer rate and on the P (Bchl) population was investigated (Sporlein et al., 2000). Analysis of experimental... 

Mitochondrial cytochrome c is perhaps the most widely studied of all metalloproteins with respect to its electrochemical properties. It is located in the inner-membrane space of mitochondria and transfers electrons between membrane-bound complex III and complex IV. The active site is an iron porphyrin with a redox potential (7) of -1-260 mV vs. NHE. The crystal structures of cytochrome c from tuna have been determined (8, 9) in both oxidation states at atomic resolution. It is found that the heme group is covalently linked to the protein via two thioether bridges, and part of its edge is exposed at the protein surface. Cytochrome c is a very basic protein, with an overall charge of -1-7/-l-8 at neutral pH. Furthermore, many of the excess basic lysine residues are clustered around the mouth of the heme crevice, giving rise to a pronounced charge asymmetry. 

Electrochemical detection is very sensitive for the compounds that can be oxidized or reduced at low-voltage potentials. Therefore, it could also be applied in the HPLC analysis of phenolic acids that are present in natural samples at very low concentrations. With the recent advances in electrochemical detection, multi-electrode array detection is becoming a powerful tool for detecting phenolic acids and flavonoids in a wide range of samples. The multi-channel coulometric detection system may serve as a highly sensitive way for the overall characterization of antioxidants the coulometric efficiency of each element of the array allows a complete voltametiic resolution of analytes as a function of their reaction (redox) potential. Some peaks may be resolved by the detector, even if they are unresolved when they leave the HPLC column. ... 

Fast scan cyclic voltammetry also provides only limited chemical resolution. The redox potential (E°) of a substance is insufficiently unique for molecular identification. In addition, to distinguish between chemical species that are involved in diffusion-controlled one-electron electrolysis processes, their E° s need to differ by at least 0.118 V. in aqueous solution, the potential limits are less than 2.0 V and hence, even under optimum conditions, less than 15 compounds could be resolved[45]... 

Reduction of plastocyanin by cytochrome / would presumably involve some direct interaction between them. Cyt/will be described in detail in Chapter 35, but some relevant information on the composition and structure of Cyt / is given here to provide a perspective on the PC/Cyt / interaction. Cyt /, a subunit of the Cyt complex, is a c-type cytochrome of 33 kDa molecular mass with an in situ redox potential of -370 mV. The amino-acid composition has been determined for Cyt/from several species, that for turnip being shown in Fig. 11. Turnip Cyt/, containing 252 amino-acid residues, was crystallized and its three-dimensional structure determined in 1994 by Martinez, Huang, Szczepaniak, Cramer and Smith " at 2.3 A resolution, now refined to 1.96 A (see Chapter 35). 

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