Missing fragments

Leo, A. J., Hoekman, D., Calculating log Poet with no missing fragments the problem of estimating new interaction parameters. Perspect. Drug Discov. Des. 2000, 18,19-38. 

Thomas, P.D. Missing fragments of British Museum Additional Manuscript 15549. Scriptorum 24 (1970) 51-53. 

Methods based on molecular properties attempt to avoid the drawbacks of the fragmental methods, such as their failure to calculate log P for structures with missing fragments. However, in some cases, models were derived from sets of compounds with moderate chemical diversity limiting the accuracy of predicted log P. 

From Equation (7) and the values in Table 1.2, it is possible to estimate the enthalpy of melting for most pharmaceutically and environmentally relevant compounds. Although the data set used to generate these group values is quite extensive (over 2200 compounds to date), additional data is needed to obtain values for missing fragments. 

Despite the many advantages of fragment descriptors they are not devoid of certain drawbacks, which deserve serious attention. Two main problems should be mentioned (i) missing fragments and (ii) modeling of stereochemically dependent properties. 

One may think that the main difference of the second group compared to fragmental methods is the absence of the missed fragments, a problem that can seriously hamper prediction ability of fragment-based methods. However,... 

Under partial acetylation, the size of Valonia cellulose crystals diminished in diameter such decrease is not homogeneous and corresponds to the loss of discrete fragments. At the beginning of the acetylation, the la phase is more susceptible to acetylation than the ip phase the latter appears more resistant. The missing fragments correspond to la domains, which are solubilized initially. These domains, which are more susceptible to acetylation, are acetylated first leaving behind exposed surfaces somewhat depleted in the la phase (Fig. 39). This confirms that in Valonia cellulose the la and Ip phases occur as discrete phases within the same microfibril. 

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