Conformational isomerism definition

In this chapter, it will be described how we have utilized endogenous and engineered Zn2+ binding sites to explore the structure and molecular function of Na+/Cl -depen-dent neurotransmitter transporters. The work has not only allowed the definition of the first structural constraints in the tertiary structure of this class of transporters, but also provided new insight into both conformational changes accompanying substrate translocation and mechanisms governing conformational isomerization in the translocation cycle. In the chapter, we will also review the theoretical and practical basis for... 

Conformational isomerism, as already defined (Section 3.b), is a property of stereoisomers separated by a low barrier of energy. The separation of isomers at room temperature requires half-lives of several hours, which correspond approximately to a free energy of activation of AG > 20 kcal/mol [56]. An operational and convenient definition of conformational isomerism is thus to consider as conformers those stereoisomers which are not physically separable under ordinary conditions, in other words, which are separated by an energy barrier lower than 20 kcal/mol. Such a definition is further useful in that it sets no conditions as to the chemical process by which conformer interconversion occurs while bond rotation is the most frequently encountered interconversion process, inversion processes are also important. 

A definite barrier to conformational isomerization is readily imagined in the case of ions (255) and (263). Much smaller conformational barriers would be expected to separate the bicyclo [2.2.2] octenyl cations (270a) and (270b). Nevertheless the distribution of products obtained from the ring expansions of anti- and syn-2-nor-bornenyl-7-methylamines, (269) and (272), demonstrate the existence of a memory... 

The traditional limitation of the stereoisomer concept to isomeric compounds with different configurations stems from the idea that configurations are not interconvertible under the observations conditions. ) A definition of stereoisomers on this basis is questionable as its application is then dependent upon observation conditions. It also fails to account for the fact that many mobile interconversions of configurations (e.g. configurations involving tricoordinate Nitrogen or pentacoordinate Phosphorus) are known, as well as thermally stable conformations (e.g. the atropisomers). 

Formation of the sterically unfavourable (Z)-alkadienes on dehydrosulfonylation of a, a-dialkylated (/i)-allylic sulfones promoted by Bu OK/Bu OH has been attributed to a bvn-cffect which, by definition, stabilizes the. vy -conformation required on approach to the transition state.13 The. syn-effect of substituents at the -position of the (C)-allylic sulfones decreases in the order RO— CH3— > RS— > — CFb— > (CH3)2CH—> (CH3)3C—> C6H5— and in accord with previous observations of isomerization of (/i)-vinylie to allylic sulfones. 

At first glance this definition seems straightforward. What are some of the complications For flexible molecules McCrone would include conformational polymorphs, wherein the molecule can adopt different conformations in the different crystal structures (Corradini 1973 Panagiotopoulis et al. 1974 Bernstein and Hagler 1978 Bernstein 1987). But this is a matter of degree dynamic isomerism or tautomerism... 

The isomerism found in sugars necessitated the concept of conformation. The advent of NMR and other techniques which were able to determine the properties of rotational or other stereochemicaUy nonri d molecules raised new problems concerning the definition and limitation of the concept of isomerism. The field of chemical isomerism is an active one, as proved by the several reviews on isomerism [3,6,20], the many recent books on stereochemistry [8 19], and by the award of Nobel prizes to Barton in 1969, and Prelog in 1975. 

The actual origin of the backbone colour changes is still unclear. Chain end effects were considered as well as bond isomerization , conformational transitions and aggregation phenomena without reaching a definite conclu-... 

Definitions. Define and illustrate the following terms (a) isomerism, (b) homologous series, (c) conformation, (d) configuration, (e) geometrical isomerism, (f) optical isomerism, (g) aromaticity, (h) functional group, (i) unsaturation, (j)... 

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