Steric information

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

I certainly think that the formation of the trigonal bipyramid is the most important step in most cases. If you look at the steric information, when you block fifth or sixth position you do not get a factor of 2 when you block one position you get a retardation factor such as 30 or 50, indicating that moving the leaving group down into the leaving position is very important. 

Triple Helicates. The steric information contained in the oligo-bipy strands based on bipy units connected in the 6,6 positions is designed to yield double helices on complexation of metal ions undergoing tetrahedral coordination. Steric effects due to the 6,6 -disubstitution hinder the binding of metal ions of octahedral coordination geometry, which would be expected to lead to triple helical complexes. 

Steric information is most easily summarised in a conformational energy map. Such calculations have been made by a number of workers but they require semi-empirical potentials or the effective van der Waals radii which are subject to error and dispute. We have carried out calculations of this sort for polyethylene units using different potentials and find that although the relative energies of the minima vary considerably, their locations are not particularly sensitive to the different potentials. Thus the conformations for which the scattering will be calculated may be restricted to those of low energy without loss of generality. 

Maynard, A. J., HypoGenRefme and Hi-pHopRefme pharmacophore refinement using steric information from inactive compounds. Presented at the ACS National Meeting, Spring, 2004. 

As with any new field, misconceptions abound. The literature contains statements of the form chaperones fold proteins, which suggests to the uninitiated that chaperones possess steric information essential for protein folding such grammatical forms should be avoided. Molecular chaperones and stress proteins are sometimes used as though they were interchangeable... 

Electronic information is combined with shape and steric information to characterize molecules in - charged partial surface area descriptors. Other approaches, different from those closely related to quantum-chemistry, refer to electronic descriptors, such as - electronic substituent constants, - electrotopological state indices, -> topological charge indices. - Reactivity indices and - resonance indices are also related to electronic descriptors. 

Like other carbonium ions, it is planar. The incoming nucleophile may then attack with equal ease from either above or below the carbonium ion, and so form equal amounts of each enantiomer of the product. Thus, the stereo-chemical consequence will be a complete loss of the steric information that was contained in the original compound, i.e. there will be complete racemisation. 

Conclusion. We have provided experimental evidence that the chaperonins meet the first criterion expected by their definition. They are molecules that, by their transient association with nascent, stress-destabilized or translocated proteins, prevent improper" aggregation. Chaperonins are not able to rescue aggregates once they are formed, nor do they appear to carry specific steric information, capable of directing a protein to assume a structure different from the one dictated by polypeptide primary sequence. 

In living organisms the molecules in supramolecular structures assemble spontaneously. Biomolecules are able to self-assemble because of the steric information they contain. 

The state of the art of regression analysis is illuminated by these two regression equations, which give correlations of equal excellence Hopfinger s by incorporating all possible steric information and the Californians by the careful exclusion of this property. 

Molecular connectivity indices have the advanta that they can be readily and rapidly calculated from a minimal description of chemical structure. As might be expected from thdr method of calculation they contain primarily steric information, although it is claimed that certain indices, particularly valence connectivities, also contain electronic information. Molecular connectivity has been shown to correlate with chranical properties such as water solubility, boiling point, partition coefficient, and Van der Waals volume. They have also been used to describe a variety of biological properties including toxidty, and have a number of environmental applications. 

The major benefits of H NMR spectroscopy are easy quantification and determination of steric arrangements. Disadvantages are the low sensitivity of the NMR detection and the fact that inorganic counterions and parts of the molecule without attached protons cannot be detected. Mass spectrometry, in contrast, is a very sensitive and selective detection technique steric information, however, cannot readily be obtained. Therefore, these two detection methods are mutually complementary for unambiguous structure elucidation purposes, a combined evaluation of both sets of data is highly desirable. 

As can be seen, TPT2 does not satisfy this limit showing that TPT2 does not include full steric information between the two association sites. This deficiency results from the way in which reducible graphs are aeated from irreducible graphs through elimination of the density parameters and <7. ... 

To include full steric information it is necessary to consider perturbation to infinite order. Unfortunately, we know very little of correlation functions beyond the triplet case. To proceed further we note that the TPT2 contribution given in Eq. (78) is split into a purely geometric contribution and a density dependent contribution which depends on knowledge of the triplet correlation function. A simplification of Eq. (78) would be to assume that the average (y (123)) was approximately unity giving... 

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