Spatial accessibility

The structures of 13,14, and 15 provide an increase in spatial access to the carbon atom when compared with the 74 electron square pyramidal analog Os5C(CO)l5, as was found for [Os5C(CO)l5I] ... 

Qualitative and quantitative aspects of the Lewis theory of acids and bases, and practical applications of Lewis acids, are discussed in a series of monographs [1,4-6,30-46] and reviews [47-49], The following aspects are taken into account (a) electronic configuration of acceptors (A = M MX are generally metal and boron salts), (b) nature of anions (usually halides), (c) peculiarities of thin structure of donors (B are generally the compounds containing N, P, As, Sb O, S, Se, Te F, Cl, Br, I atoms) their electronic structure, spatial accessibility, and mutual position of donor centers. Moreover, the nature of X, order of binding of A and B in formation of adducts of type AB , nature of solvents, and evaluation of AH or AG of the processes (1.1)—(1.5) [31,48] should also be considered. 

The reaction capacity of di- and polydentate ligands can be successfully explained by hard-soft properties [2] and different spatial accessibility of their donor centers. 

As well as predicting protein structure, it is also of interest to try to predict solvent accessibility from the protein s primary sequence of amino acid residues. Solvent accessibility concerns the area of a protein s surface that is exposed to the surrounding solvent. The importance of this concept is that these accessible regions have the potential to interact with other entities, including endogenous proteins and drugs. Similarly, if the protein of interest is an enzyme, only residues with solvent accessibility could be part of the enzyme s active site. This means that an interaction site of interest, one involved in signal transduction (see Krauss, 2003), requires spatial accessibility to the solvent (Ofiran and Rost, 2005). 

It should be clear that the selection rules apply to concerted processes between spatially accessible parts of a molecule. Antarafacial [1,3], [1,5], or [3,3] reactions involving u-orbital transfers between carbon atoms are not possible. Antarafacial migrations do become feasible when the transition state forms a cycle of at least 7 or 8 atoms Woodward and Hoffmann (1965b) cite the [1,7] example (76). 

Glycomimetic peptides, peptides that functionally and/or stmcturaUy mimic oligosaccharide structures. Especially oligosaccharides that are exposed on the surface of, e.g., tumor cells or microbial pathogens can be regarded as potential therapeutic targets due to their spatial accessibility. Despite the... 

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