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114 4 CHIRBASE: Database Current Status and Derived Research Applications using …

sets (up to 250 000 points). The present work is preliminary, and it is intended to
illustrate one interesting issue that can be addressed with CHIRBASE.

4.6.1 Comparison of Sample Similarities within a Molecule Dataset

In these first studies, similarity measures were investigated to survey the molecular
diversity of a set of molecules resolved on a given CSP in order to compare the
extent of their application range.
Three types of CSPs were compared:

Polysaccharide-based CSPs [28]:
– Chiralcel OD: Cellulose tris(3,5-dimethylphenylcarbamate coated on amino-
propyl silica.
– Chiralpak AD: Amylose tris(3,5-dimethylphenylcarbamate) coated on amino-
propyl silica.
Pirkle-like CSPs [29]:
– Whelk-O 1: (3R,4S)-4-(3,5-Dinitrobenzamido)-3-[3-(dimethylsilyloxy)pro
pyl]-1,2,3,4-tetrahydrophenanthrene).
– Pirkle DNPG: (R)-N-3,5-Dinitrobenzoyl-phenylglycine covalently bonded to
aminopropyl silica.
Inclusion-based CSPs [28]:
– Crownpak CR(+): (S)-18-crown-6 ether coated on silica.
In the first category, we have chosen two cellulose- and amylose-based CSPs
which provide today a considerable application range in CHIRBASE. In the second
category, we have chosen to evaluate the behavior of the Whelk-O 1 CSP because of
its well-recognized ability to resolve a broader range of samples than standard
Pirkle-like CSPs. In the last category, Crownpak CR is a good example of a CSP
offering a limited field of application with a large proportion of amino acids in our
molecule library. Today, such qualitative trends can be revealed to the analyst from
the published literature, or by manual examination of the structures in CHIRBASE
through a time-consuming and biased procedure, but have not yet been clearly
determined through a rational and systematic manner.
A set of about 500 molecules was used for each CSP. Figure 4-12 illustrates some
results of dot plots. The similarity measures are displayed here according to a gray
value gradient (white for 0 to black for 100). As the way the data points are presented
to the application is dependent upon the organization of the molecules in CHIR-
BASE, dots are put in the maps at random positions by the algorithm in order to pro-
vide a good statistical repartition, and thus facilitate the ability to distinguish visu-
ally the global diversity of samples.
For each of these images, a mean value of the luminance can then be measured
directly using a standard photo-editor software.
As shown in Table 4-4, the image luminance values are found to be in good agree-
ment with the calculated mean similarity values of the molecule sets. Therefore, the
level of similarity of the molecule set can be immediately judged based on the aver-