Atomic weight functional groups

The network operates through a series of enzyme-catalyzed reactions that constitute the metabolism. Each of the consecutive steps in a metabolic pathway brings about a specific chemical change, usually the removal, transfer, or addition of a particular atom or functional group. The precursor is converted into a product through a series of metabolic intermediates called metabolites. The term intermediary metabolism is often applied to the combined activities of all the metabolic pathways that interconvert precursors, metabolites, and products of low molecular weight. 

An important group of antimetabolites are the aza analogs of pyrimidine and purine bases which are theoretically derived by a replacement of the methine group of a pyrimidine or purine nucleus with a nitrogen atom. This replacement represents a relatively minor alteration of the structure of these substances as it does not change the functional groups, practically preserves the molecular weight, and produces almost isosteric compounds. The replacement of the methine... 

The differences can also be seen in Figure 15.4, which compares cumulative distributions of these properties between the Gasteiger and Roche datasets. Many of the literature compounds are very simple, with low molecular weight, few polar atoms, and few functional groups. They have often been included in solubility datasets because they are well characterized and because accurate solubility data are available for them, rather than because they are druglike. The inclusion of many such simple compounds in a training set for a solubility prediction tool may focus the tool on an area of chemistry space that is not well populated with druglike molecules and may make the tool less useful for the prediction of the solubility... 

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