Flexibility compounds

However, most recentiy, mixed metal stabilizers are being introduced into this area. Whereas some mixed metal stabilizers are used in rigid apphcations, it is difficult for these newer materials to compete with lead and tin based stabilizers on a cost/efficiency basis. These materials find their largest apphcations in flexible compounds, especially those requiring good stabihty to uv exposure. 

Hahn, M. (1997) Three-dimensional shape-based searching of conformationally flexible compounds. / Chem Inf Comput Sci 37, 80-86. 

Here, we consider it appropriate to mention briefly the so-called shape-based methods for flexible compound searching. Although these are not strictly seen in any relationship with the functional pharmacophore perception, the shape and size of compounds obviously influence activity and, in some cases, may be the main factors determining biological action. 

Solid-state CD can provide information on solute-solvent interactions when compared with the solution spectra in various solvents. The effects of solvents on the rotatory power are often the results of the formation of some kind of coordination compound between the solvent and the optically active molecules concerned in solution [10,18]. This may affect the optical activity of the molecule by way of conformation alteration in the case of flexible compounds, or through vicinal effects. In contrast, in the solid state, molecules are densely packed and are under a much stronger influence of neighboring molecules. In one sense, this situation can be regarded as an extreme case of the solvent effect [11]. Thus an unusual conformation of a chiral molecule that is unstable in solution may be... 

The hexacoordinate silicon complexes discussed here are highly flexible compounds, like some other hypercoordinate silicon compounds,1 6 and undergo a variety of fluxional reorganization reactions, observable by temperature-dependent NMR spectroscopy. The dimethylamino-coordinated bis-chelates 30-38 are particularly suitable for an NMR study of ligand-site exchange processes (inter- or intramolecular), because of the pairs of diastereotopic V-methyl groups, which under certain... 

These two channels compete kinetically and lead to nonradiative TICT species. In bridged compounds, where TICT formation is not possible, the quenching is absent. The competition in the flexible compounds can be governed by tuning the electron accepting properties of the benzopyrylium system with the aid of the other (non-twisted) substituent [106,110]. Some examples are shown in Fig. 17. Remarkable is the anti-loose-bolt behavior, i.e. the compounds OO, NO and NN with two flexible groups show reduced nonradiative... 

For large molecules, however, the computer requirements become increasingly prohibitive, especially when conformationally flexible compounds are tackled. Alternative approaches to quantum-mechanical methods are known, based on potential functions and parameters derived from detailed analysis of vibrational spectra. These so-called force field methods are now joined in what is called molecular mechanics, an empirical method that considers the molecule as a collection of spheres (possibly deformable) bound by harmonic forces (eventually corrected with cubic and quartic potentials). The energy... 

The combination of several observations, using also flexible compounds, notably aspartame, whose solution structure had just been determined (Lelj et al., 1976), led to a detailed quasi-planar outline of the active site. The main features of this model can be summarized as follows (1) the active site of the receptor is a flat cavity with one side partially accessible even during the interaction with the agonist (2) the lower part of the cavity hosts the AH-B entity complementary to that of the sweet molecule (3) the upper part is hydrophobic and plays an important role in the case of very active sweeteners. This is often referred to as the "Temussi model" (Walters, 1995 Walters et al., 1986). Figure 4A shows the main contour of the active site hosting a model of aspartame in an extended conformation. 

A special case is represented by flexible compounds, such as aspartame, which can exist in several conformations. For consistency with the PrGen calculations of Bassoli et al. (2002a), the first choice for the conformation of aspartame used by Morini et al. (2005) to derive the models was the folded conformation found in the crystal structure (Hatada et al., 1985) and used as paradigmatic in one of the indirect models (Yamazaki et al., 1994). However, our indirect model (Kamphuis et al., 1992) is consistent... 

Computational Chemistry Module Computes quantum-chemical and other atomic and molecular indexes in addition, performs clustering of conform-ers for flexible compounds. 

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