Fragmentation occurrence

Empirical descriptors can be a few user-defined values for discriminating among special molecular fragments or counting local specific atom/fragment occurrences within a molecule. 

Arif SM, Holiday JD, Willett P. Analysis and use of fragment-occurrence data in similarity-based virtual screening. J Comput Aided Mol Des 2009 23 6655-668. 

Assume that a screen set is to be created that contains N screens. Partition the fragment dictionary into N partitions, each of which contains approximately the same number of fragment occurrences. The range of values encompassed within each such partition then corresponds to one of the screens that are available for assignment to database structures or to query substructures. 

The occurrence of predissociation opens up a new family of observable quantities. It is possible to measure not only linewidths or lifetimes, but also the internal state distributions of the fragments. All these quantities are sensitive to the intennolecular potential and can be used to test or refine proposed potential surfaces. 

Several models have been published where the fragments are defined on a purely atomic level. This simplifies both the recognition of fragments and the calculation, as correction substructures are not applied (see Eq. (10)). N is the occurrence of the ith atom type. 

The group contribution method allows the approximate calculation of solubility by summing up fragmental values associated with substmctural units of the compounds (see Section 7.1). In a group contribution model, the aqueous solubility values are computed by Eq. (12), where log S is the logarithm of solubility, C is the number of occurrences of a substmctural group, i, in a molecule, and is the relative contribution of the fragment i. 

The occurrence of the elements carbon, nitrogen, and oxygen manifests itself in the isotope patterns occurring for all molecular or fragment ions. For small numbers of carbon atoms, the... 

Collapse Breccia Pipe Deposits. The primary occurrence of coUapse breccia pipe deposits is in circular, vertical pipes fiUed with down-dropped fragments. Uranium is concentrated in the permeable breccia matrix and in the accurate fracture zones enclosing the pipe. An example of... 

In the discussion of some mass spectra of nitrones (41), intermediate isomerization to oxaziridines was concluded from the occurrence of aldehyde fragments. 

Distribution. The first indication of the occurrence of palytoxin in fish was presented in 1969 (16). The filefish Altera scripta belonging to family Monacathidae was traditionally known in Okinawa, Japan, to contain a toxic substance in the gut and, thus, to kill pigs when fed to them. The presence of fragments of Palythoa sp. in the guts and the resemblance in solubility between the fish toxin and palytoxin led the authors to a conclusion that the toxic principle in the filefish viscera was palytoxin. Incidence of human intoxication due to eating the filefish was not confirm. ... 

Hj- numerical factor indicating the occurrence of a fragment in a structure fj+ average number of bound protons Na- Avogadro number... 

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