N Stacking

In this case, the tertiary and electron-deficient MPM group is retained. " A very slow cleavage of an MPM protected adenosine was attributed to its reduced electron density as a result of n stacking with the adenine. Typically, these reactions are complete in < 1 h, but in this case complete cleavage required 41 h. 

Schorastein, m. chimney, stack, flue, -gas, n. stack gas, flue gas. 

FIGURE 2.6 The procarcinogen benzo[a]pyrene oriented in the CYPlAl active site (stereo view) via n- n stacking between aromatic rings on the substrate and those of the complementary amino acid side chains, such that 7,8-epoxidation can occur. The substrate is shown with pale lines in the upper structures. The position of metabolism is indicated by an arrow in the lower structure (after Lewis 1996). 

The pentafluorophenyl group imparts greater crystallinity to the complexes and as a result many complexes have been studied by X-ray crystallography. Although vith other metal centers C Fs-CaFs or CfiFs-CfiHs n-n stacking interactions are observed [21, 22], there are not many examples in gold chemistry and they have been sho vn very recently [23]. 

In order to directly probe the dynamics of CT between Et and ZG, and to understand how the intervening DNA base stack regulates CT rate constants and efficiencies, we examined this reaction on the femtosecond time scale [96]. These investigations revealed not only the unique ability of the DNA n-stack to mediate CT, but also the remarkable capacity of dynamical motions to modulate CT efficiency. Ultrafast CT between tethered, intercalated Et and ZG was observed with two time constants, 5 and 75 ps, both of which were essentially independent of distance over the 10-17 A examined. Significantly, both time constants correspond to CT reactions, as these fast decay components were not detected in analogous duplexes where the ZG was re-... 

Fig. 9 Long-range oxidative damage in DNA is regulated by the structure of the DNA n-stack. Photoexcited [Rh(phi)2(bpy )]3+ intercalated within B-DNA oxidizes guanine doublets positioned 17 A and 40 A away with approximately equal efficiency. When a bulge is introduced in the DNA oxidation at the distal guanine doublet is inhibited. As shown by the NMR structure, bulges introduce local distortions in the DNA r-stack, although the kinking of the helical actually positions the distal guanine closer to the Rh(III) photooxidant...

With the site-selective hole injection and the hole trapping device established, the efficiency of the hole transport between the hole donor and acceptor, especially with respect to the distance and sequence dependence, were examined. Our experiments showed that hole transport between two guanines was extremely inefficient when the intervening sequence consisted of more than 5 A-T base pairs [1]. Hole injection into the DNA n-stack using photoexcited dCNBPU was accompanied by the formation of dCNBPU anion radical. Therefore, hole transport would always compete with the back electron transfer (BET). To minimize the effect of BET, we opted for hole transport between G triplets, that are still lower in oxidation potential than G doublet. With this experimental system, we researched the effect of the bridging sequence between two G triplets on the efficiency of hole transport [2]. 

Figure 4 Schematic band structures for (a) a regular n-stack (b) a Peierls distorted n-stack and (c) a Peierls distorted n-stack after p-type doping...

This bistability is favoured by structures, which will adopt n—stacked motifs, i.e. lamellar molecules. In addition the tendency to adopt the planar structure is enforced by the presence of electronegative heteroatoms (currently restricted to N), which lead to a propensity of in-plane electrostatic S- N contacts between rings. These may comprise heterocyclic S- N contacts or S- N contacts to other functional groups such as the pyrazine or cyano nitrogen atoms. 

Figure 9 (Top) schematic of bistability in 1,3,2-dithiazolyl radicals arising from a solid-solid phase transition between regular and Peierls distorted n-stacks (bottom) free energy diagram of the two structural phases present...

Table 1 Phase transition temperatures for n-stacked DTA derivatives.

Only 5 derivatives have been isolated to date. While the non-fused system forms a 71 -ji dimer (<7S s 3.27 A),86 structural studies on the more delocalised derivatives indicate a reduced tendency to dimerise. However, rather than forming optimal n-stacked motifs, there is a clear tendency to adopt slipped n-stack structures which minimise orbital overlap. As a consequence although the Hubbard energy is low, conductivity measurements show poor conductivities (10 5 S cm-1) similar to those observed in either the isomeric 1,3,2-dithiazolyl (4) or 1,2,3,4-dithiadiazolylradicals (2).84... 

Compounds R2EX2 are in many case dimers or coordination polymers 3 6 99 105 the particular structures are influenced by electronic, steric and n-stacking effects of the ligands at Se or Te. In the series of compounds Me2SeX2, the... 

These structural features observed for [Fe(abpt)2(TCNQ)2] involving pronounced and extended n-n stacking interactions lead to a duality with respect to its gradual spin crossover behaviour. It has generally been accepted that extended n-n interactions may lead to the occurrence of thermal hysteresis in mononuclear Fe(II) spin crossover compounds [62-65]. Clearly, the requirements responsible for cooperative Fe(II) spin crossover behaviour are not easy to define, since obviously [Fe(abpt)2(TCNQ)2] represents an exception to this rule in spite of the pronounced TCNQ n-n stacking interactions, the Fe(II) spin crossover displays at best weak cooperativity. 

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