Size descriptor

MW is often taken as the size descriptor of choice, while it is easy to calculate and is in the chemist s mind. However, other size and shape properties are equally simple to calculate, and may offer a better guide to estimate potential for permeability. Thus far no systematic work has been reported investigating this in detail. Cross-sectional area Ad obtained from surface activity measurements have been reported as a useful size descriptor to discriminate compounds which can access the brain (Ad<80A ) of those that are too large to cross the blood-brain barrier (BBB) [55]. Similar studies have been performed to define a cut-off for oral absorption [56]. 

This also involves flexibility and rigidity descriptors (RB and AP), a size descriptor (MW) and a polarity descriptor (log Pow). It appears that these kinds of descriptors are essential for AGfUs-... 

Descriptor median values naturally depend on the composition and size of compound databases. Whenever source databases are changed, reduced, or extended in size, descriptor medians need to be re-calculated to ensure accurate MP analysis. Relatively small changes in median values can significantly alter partitioning results. 

Other techniques, such as fight scattering and sedimentation, are also sensitive to particle shape. The direct observation of particles by a scanning electron microscope and distribution measurements by image analysis would appear to overcome many of the problems associated with the various other techniques, but problems of describing irregularly shaped particles remain. A universal particle size descriptor has not yet been developed. The technique adopted and the results obtained are most useful when empirical correlations with the end use can be made. 

The Shape Analyzer is a scientific instrument which is used to analyze the shape, size, and texture of objects. The item for analysis may be the object itself, its photograph, optical or electron micrograph, etc. The shape-size analysis is achieved by converting the profile to a set of shape and size descriptors which are complete, unequivocal and invariant. The texture analysis converts the full image of the object into a set of textural descriptors which are unequivocal, complete and invariant. These shape, size and texture descriptors can be used in research, in quality control, in process control and specifications. Additional details are given in Table 1 and the Appendix. 

Kovats retention index chromatographic descriptors Kuhn lengti) -> size descriptors... 

Meyer anchor sphere volume -> size descriptors Meyer-Richards similarity index -v similarity/diversity Meyer visual descriptor of globularHy -> shape descriptors Meylan-Howard hydrophobic model -> lipophilicity descriptors Mezey 3D shape analysis... 

The three-dimensional representation views a molecule as a rigid geometrical object in space and allows a representation not only of the nature and connectivity of the atoms, but also the overall spatial configuration of the molecule. This representation of a molecule is called geometrical representation, and molecular descriptors derived from this representation are called 3D-descriptors. Examples of 3D-descrip-tors are -> geometrical descriptors, several -> steric descriptors, and -+ size descriptors. 

These parameters are - size descriptors also depending on the molecular shape (- Petitjean shape indices), such as their topological counterpart, i.e. - topological radius and - topological diameter. 

From the geometry matrix, the usual -> graph invariants can be calculated such as -t characteristic polynomial, -> eigenvalue-based descriptors, -> path counts, - ID numbers, -> 3D-Balaban index, -> 3D-Schultz index and so forth [Randic, 1988b Nikolic et al, 1991]. It is noteworthy that all these indices are sensitive to molecular geometry. Moreover, the geometry matrix is used for the calculation of size descriptors and - 3D-MoRSE descriptors. 

Among the size descriptors, this is the simplest and most used molecular - OD-descriptor, calculated as the sum of the atomic weights. It is related to molecular size and is atom-type sensitive. It is defined as ... 

Square root molecular wmg t (MW2), defined as MW2 = MW, and cube root molecular weight (MW3), defined as MW3 = MW and corresponding to a linear dimension of size, are also used as molecular size descriptors. 

Molecular descriptors related to the dimensions of the molecule and often calculated from the - molecular geometry. Combined with molecular shape information, they are closely related to - steric descriptors. The simplest size descriptors are atom count, -> bond count, -> molecular weight, and some among the -> volume descriptors such as van der Waals volume. Other size descriptors are - Sterimol parameters and - WHIM size descriptors. 

Other size descriptors are listed below. Several of them are often used to represent geometrical characteristics of long-chain molecules such as polymers and macromolecules [Volkenstein, 1963 Hory, 1969 Arteca, 1991]. 

A size descriptor defined as the radius of the smallest sphere, centred on the centre of mass, completely enclosing all atoms of a molecule [Volkenstein, 1963] ... 

It is a size descriptor used for macromolecules, polymers, and proteins. In this case, a useful parameter is also the contour length Lq defined as ... 

A simple size descriptor for long-chain molecules defined as [Flory, 1969] ... 

A size descriptor for the distribution of atomic masses in a molecule [Tanford, 1961 Volkenstein, 1963], defined as ... 

A substituent size descriptor defined as the volume V of the portion of the substituent within a sphere centred at the link atom [Meyer, 1986b], Tlie radius of the sphere was chosen equal to 0.3 nm to comprise the substituent portion responsible for the steric effect of the substituent. It was used, together with the - ovality index calculated on the substituent, to estimate substituent steric effects for substituents with equal volume V , much larger steric effects are observed for globular substituents. 

Steric descriptors account for both the size and shape of molecules and substituents, and are thus contemporarily related to -> size descriptors and - shape descriptors. However, in several QSAR models, size descriptors are encountered as estimates of steric molecular/fragment properties. 

Steric descriptors and/or -> size descriptors representing the volume of a molecule. The volume of a molecule can be derived from experimental observation such as the volume of the unit cell in crystals or the molar volume of a solution or from theoretical calculations. In fact, analytical and numerical approaches have been proposed for the calculation of molecular volume where the measure depends directly on the definition of - molecular surface-, -> van der Waals volume and -> solvent-excluded volume are two volume descriptors based on van der Waals surface and solvent-accessible surface, respectively. 

---