Redox properties structural studies

A synthesis of the fused system 35 was achieved by conversion of the aryl bromide 36 to the sulfide 37, which in turn was brominated to 38, followed by metalation and a final oxidative intramolecular coupling. A study of the crystal structure of 35, as well as its electronic properties, has also been conducted <04OL4179>. The structurally similar tetra-tert-butyldicyclopenta[6 rf]thieno[l,2,3-cc 5,6,7-c <7 ]diphenalene system has also been prepared, and its redox properties were studied <04AG(E)6474>. 

Computer simulations of electron transfer proteins often entail a variety of calculation techniques electronic structure calculations, molecular mechanics, and electrostatic calculations. In this section, general considerations for calculations of metalloproteins are outlined in subsequent sections, details for studying specific redox properties are given. Quantum chemistry electronic structure calculations of the redox site are important in the calculation of the energetics of the redox site and in obtaining parameters and are discussed in Sections III.A and III.B. Both molecular mechanics and electrostatic calculations of the protein are important in understanding the outer shell energetics and are discussed in Section III.C, with a focus on molecular mechanics. 

The concept of ligand complex was nicely demonstrated in the structures of complexes (495) (Cu-Cu 3.1172A 2/=-164cm-1)408 and (496) (Cu-Cu 3.097 A, 4.984A, and 5.108A).409 For both the complexes, spectroscopic and redox properties were also studied. 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

Use of the tripodal ligand Saltren creates a tetranuclear complex [Mn402(Sal-tren)2][MnCl4] 2CH3CN (172) having a fused open cubane structure without a carboxylate group. At 300 K the value of the effective magnetic moment for (172) is 5.1 /Ub, which decreases to 2.0 /iB at 4.2 K. The redox properties of (172) in propylene carbonate have been studied by cyclic voltammetry. Two oxidation responses are observed at = —0.32V and 0.43 V vs. SCE. 

The characterization, redox properties, and pulse radiolysis study of manganese(III) complexes of type [MnLCy (where L = cyclam, meso-, and rac-5,7,7,12,14,14-hexamethylcyclam (tet a and tet b, respectively)) have been reported." An X-ray crystal structure of the meso-5,l,l, 2, A, A-hexamethyl-l,4,8,ll-tetraazacyclotetradecane complex shows that the coordination geometry of the tet a complex is close to octahedral with the macrocycle coordinated equatorially and the chlorides occupying irons axial sites. 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

CV investigations of 6-mercaptopurine and 8-mercaptoquinoline SAMs on pc-Au electrodes have been presented by Madueno et al. [186] and He etal. [187], respectively. Several model electrode reactions involving various redox probes were studied using such modified electrodes. Baunach and Kolb etal. [188] have deposited copper on disordered benzyl mercaptan film on Au(lll) surfaces. They have also studied the behavior of benzyl mercaptan SAM on Au(lll) in H2SO4 solution using CV and STM. Structural and electrical properties of SAMs based on tetrathiafulvalene derivatives on Au(lll) were investigated. These mono-layers were disordered, or at least loosely... 

The most prominent feature of the chemistry of flavin is its redox properties. These properties make flavin especially suitable for its broad involvement in biological reactions. In the following the pH-dependent species formed in one- and two-electron reductions will be dealt with first, including their visible absorption and fluorescence properties. These physical properties form the basis of many kinetical and analytical studies. In Scheme 3 the structures refer to the free and protein-bound prosthetic groups (cf. Scheme 1). To study the physical properties of the flavocoenzymes often N(3)-alkylated lumiflavin (R = CH3) is used which is better soluble in a variety of solvents. Other physical and chemical properties of these species will be discussed subsequently. 

The ongoing research into the structure and mechanism of flavoenzymes has been the subject of several recent excellent reviews The proceedings of six symposia held at intervals over the past 16 years provide an overall perspective on the progress of flavoenzyme research over this time period. The intent of this article will be to focus directly on the chemical and physical properties of the semiquinone form of flavin coenzymes to the extent that current knowledge permits, from the point of view of both model system studies and from existing knowledge of their properties in flavoenzyme systems. For an in-depth treatment of flavin and flavoenzyme redox properties in which the oxidized and hydroquinone forme as well as the semiquinone form are discussed as related to their biological function, the reader is refered to the article by F. Muller in this volume. 

In summary, the studies reviewed here use diverse strategies to take advantage of the redox properties of two classes of catechol-quinone compounds present in nature to design new compounds of pharmaceutical interest. In a third class of naturally occurring compounds of complex structure, simplification and removal of the redox-sensitive elements may be key to providing target structures with a novel antiviral character. 

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

In a more quantitative study the redox properties of the phthalocyanine complex and the structurally similar tetraphenylporphyrins were studied by electrochemical methods in solution M). It was found that the oxidation/reduction may go by two distinctly different mechanisms ... 

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