Adenine cation formation

One-electron oxidation of the adenine moiety of DNA and 2 -deoxyadenos-ine (dAdo) (45) gives rise to related purine radical cations 46 that may undergo either hydration to generate 8-hydroxy-7,8-dihydroadenyl radicals (47) or deprotonation to give rise to the 6-aminyl radicals 50. The formation of 8-oxo-7,8-dihydro-2 -deoxyadenosine (8-oxodAdo) (48) and 4,6-diamino-5-formamidopyrimidine (FapyAde) (49) is likely explained in terms of oxidation and reduction of 8-hydroxy-7,8-dihydroadenyl precursor radicals 47, respectively [90]. Another modified nucleoside that was found to be generated upon type I mediated one-electron oxidation of 45 by photoexcited riboflavin and menadione is 2 -deoxyinosine (51) [29]. The latter nucleoside is likely to arise from deamination of 6-aminyl radicals (50). Overall, the yield of formation of 8-oxodAdo 48 and FapyAde 49 upon one-electron oxidation of DNA is about 10-fold-lower than that of 8-oxodGuo 44 and FapyGua 43, similar to OH radical mediated reactions [91]. 

The receptors start a second messenger cascade that is initiated by activation of G-proteins in the cell. These, in turn, interact with membrane-bound adenylyl cyclase, which catalyzes the formation of cyclic adenine monophosphate (cAMP) and opening of cAMP-gated cation channels. Depolarization then brings about an action potential, which travels along the axon of the olfactory sensory neuron. Many of the molecular components of this cascade are olfactoiy specific. 

Of the two mitochondrial membranes (Fig. 1-9), the outer one is more permeable to various small anions and cations (e.g., H+), adenine nucleotides, and many other solutes than is the inner one. The inner membrane invaginates to form the mitochondrial cristae, in which the enzymes responsible for electron transfer and the accompanying ATP formation are embedded. For instance, the inner membrane system has various dehydrogenases, an ATPase, and cytochromes (discussed in Chapters 5 and 6). These proteins with enzymatic activity occur in a globular form, and they can be an integral part of the membrane (Fig. 1-8) or loosely bound... 

NADH (nicotinamide adenine dinucleotide) is a biochemical source of hydride. In the following example NADH reduces acetaldehyde to ethanol via minor pathway H t., hydride transfer to a cationic center. A Zn ion acts as a Lewis acid to polarize the acetaldehyde carbonyl (similar to protonating the carbonyl). The Lewis acid makes the carbonyl a better electron sink by increasing the partial positive charge on carbon. In fact, the electrophilic catalysis by 2+ and 3+ metal ions can accelerate additions to carbonyls by over a million times. The formation of the aromatic pyridinium ring in the NAD" product helps balance the energetics of this easily reversible reaction. 

The transient absorption spectra similar to that of the ion-pair state of indole cation radical and flavin anion radical were also observed in D-amino acid oxidase (5), although the spectra were not so clear as those of flavodoxin. In D-amino acid oxidase, the coenzyme, flavin adenine dinucleotide (FAD), is wealtly fluorescent. The fluorescence lifetime was reported to be 40 ps (16), which became drastically shorter (less than 5 ps) when benzoate, a competitive inhibitor, was combined with the enzyme at FAD binding site (17). The dissociation constant of FAD was also marlcedly decreased by the binding of benzoate (17). These results suggest that interaction between isoalloxazine and the quencher became stronger as the inhibitor combined with the enzyme. Absorbance of the transient spectra of D-amino acid oxidase-benzoate complex was remarkably decreased. In this case both rate constants of formation and decay of the CT state could become much faster than those in the case of D-amino acid oxidase free from benzoate. 

The fluorescence quantum yields of pyrene-1-carboxaldehyde in water and methanol are 0.98 and 0.07/ an effect attributed to solvent effects on 7c,n and n,n states. Cycloaddition reactions of 1-naphthonitrile to 1,2-dimethyl-cyclopentene are attributed to both and Lj, states.It is pointed out that although dual fluorescence is known, this is the first example of divergent reaction from two nearly isoenergetic singlet states. An analysis of the u.v. spectra of some acyl pyridines, including a theoretical examination of the molecular geometry, and excited states of bipyrimidine compounds have also been made. Photo tautomerism and the fluorescence of the cation of 4-amino-pyrazole[3,4-iflpyrimidine, an analogue of adenine, has been published by Wierzchowski et Intramolecular heteroexcimer formation in... 

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