Donor coupling

As we saw in the previous section, Strategy 1 plants utilize ferric reductases, with NADPH as electron donor, coupled to proton extrusion and a specific Fe(II) transport system localized in the root plasma membrane. Saccharomyces cerevisiae also uses cell surface reductases to reduce ferric iron, and in early studies (Lesuisse et ah, 1987 ... 

The problem of the nucleophilicity of amides in glycosylation reactions is not limited to the sulfoxide method and has been shown to result in the formation of glycosyl imidates from intermolecular reaction with activated donors. It appears that this problem may be suppressed by the prior silylation of the amide [348,349]. Accordingly, it may be sufficient to operate the sulfoxide method with an excess of triflic anhydride when amides are present so as to convert all amides into O-triflyl imidates, which are then hydrolyzed on work-up. Despite these problems, several examples have been published of successful sulfoxide glycosylation reactions with acceptors carrying remote peptide bonds [344,345] and with donors coupled to resins via amide-based linkages [346,347], with no apparent problems reported. Sulfonamides and tertiary amides appear to be well tolerated by the sulfoxide method [340,350],... 

The variations of the symmetry factor, a, with the driving force are much more difficult to detect in log k vs. driving force plots derived from homogeneous experiments than in electrochemical experiments. The reason is less precision on the rate and driving force data, mostly because the self-exchange rate constant of the donor couple may vary from one donor to the other. It nevertheless proved possible with the reaction shown in Scheme 3.3.11... 

Some representative examples of hydride donor couples are triarylmethanes (3a,b), cycloheptatrienes (4a,b), a-heterosubstituted methanes (5a,b), and heterocycles such as 1,4-dihydropyridines (6a,b). 

Schematised electron transfer pathway across a Gram-negative bacterial ceU membrane (with lactate as the primary electron donor), coupled with H translocation and ATP synthesis, adopted and simplified from ref. 137. MQ and MQHj are menaquinone and -hydroquinone, respectively CymA is a tetrameric and MtrC a decameric cytochrome-c type haemoprotein. See also the text and Eqnation (4.41)). Peptidoglycan (or murein), serving a structural role in the cell membrane, is a block copolymer built up from acetylhexoses and oligopeptides.

The use of anomeric sulfoxides as glycosylating reagents was reported by Kahne and coworkers in 1989 [51]. Upon activation with triflic anhydride at low temperatures, anomeric sulfoxides are transformed into extremely reactive glycosyl donors, which can glycosylate very hindered acceptors. The excellent reactivity of the sulfoxide donors coupled with the selectivity achieved by use of a participating group in the C2 position prompted their use in the synthesis of oligosaccharides on the solid support [52]. 

With alkenes in polar solvents 2,4-dicyanopyrldlne reacts similarly to give products of the type (97) however if the reaction is performed in less polar solvents then proton transfer in the initially produced radical ion pair apparently competes to produce an allyl radical and pyridyl radical. These couple and eventually give (98). With euaine donors coupling of the radical ion pair results in the isolation of (99) and (100)... 

The concept of two-photon excitation is based on the idea that two photons of comparably approximately half the energy necessary to excite the molecule also excite a fluorophore in one quantum event. NO donors coupled with the two-photon... 

On Figures 3 and 4 are represoitated the variaticms of the rate constants with the standard potoitial of the electrcm donor couple. 

When SOD was added (upper trace, right) the rise of the pH. -indicating signal was accelerated. It showed a fast and slower phase. Obviously one part of the alkalinization of the external phase was rate limited by the activity of SOD. We inhibited PSII by DCMU and restored PSI activity by addition of the donor couple diaminodurene (DAD) and dithiothreitol (DTT). 

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