Rigid species

It is very hard to analyze such an experiment if it does not produce a reporter strand, either directly, as described for the three-arm and four-arm junction, or after restriction frees it from catenated partners (Li et al. 1996). To characterize the flexibility of double-crossover molecules, it is useful to examine the crossover topology of the products of an oligomerization reaction. These are shown in Figure 17 for DAE, DAO and DAE+J motifs. It is clear that both DAE and DAE+J motifs produce a reporter strand, but DAO does not. Therefore, in our initial experiments, we have analyzed only DAE and DAE+J. In both, these motifs can be shown to behave as rigid species, because they do not cyclize to any appreciable extent. 

Although F, Cl, Br and I aU possess spin active nuclei, in practice only F (100%, / = 5) is used routinely. Fluorine-19 NMR spectroscopy is a valuable tool in the elucidation of structures and reaction mechanisms of F-containing compounds see case studies 1 and 5 and the discussion of stereochemicaUy non-rigid species in Section 2.11. 

Emission from two rigid species would imply that the fraetional eontributions of eaeh fluoreseenee lifetime should be the same whatever the state of ligation, whieh is not the ease (Table 9.2). 

Cycloheptane-1,2-semidTone" is a rigid species (t>10 sec.) up to at least 90. From an analysis of the esr hyperfine interactions it can be concluded that the single populated conformation is the staggered conformation 4(5). 

For less rigid species such as clusters or linear molecules there are many more isomers, some of which differ only in conformation ( conformers ) with a low barrier between them. 

---