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Pure Appl. Chem., Vol. 71, No. 12, pp. 2349-2365, 1999

Glossary of Terms Used in Combinatorial Chemistry

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Omission Library: Strategy for identifying active library members by the systematic omission of building blocks from mixtures. Observation of reduced activity in a certain pool suggests that the building block which was omitted in that pool contributes to activity 66.

Orthogonality: (a) Property of protecting groups or linkers allowing removal, modification, or cleavage of one such without affecting others; (b) pooling strategy whereby library members are incorporated in more than one pool, and are mixed with a different set of other members in each pool. Thus a hit results in two or more active pools with only one member in common 67,68.

Parallel Synthesis: Strategy whereby sets of discrete compounds are prepared simultaneously in arrays of physically separate reaction vessels or microcompartments without interchange of intermediates during the assembly process. Contrast Pool/Split.

Partial Library: Partly assembled library, or portion thereof which is reserved to be completed once initial property relationships have been identified. For instance, part of an intermediate pool may not be treated with the final building block until the optimal residue at the final position is known, thus avoiding the need to prepare that pool from the starting materials 69.

Partial Release: Cleavage process designed to release compound from solid support in discrete portions e.g. by using orthogonal linkers or by controlled application of cleavage reagent or condition. Photocleavable linkers, such as that shown below, are a particularly convenient application since cleavage may be controlled by simply turning on or off a lamp 70,71. Also controlled release, tiered release.


PEG: see Poly(ethylene glycol)

Peptoid: Oligomer consisting of repeating N-substituted glycine units 72,73.

Phage Display: Use of genetically engineered phage to present peptides as segments of their native surface proteins. Peptide libraries may be produced by populations of phage with different gene sequences 74-76.

Pharmacophore: 'The ensemble of steric and electronic factors which are necessary to insure supramolecular interactions with a specific biological target structure' 77-78.

Phase Switch: Strategy for compound isolation whereby the desired material is rendered sufficiently different from reagents, side products, and other impurities that it may be separated from them by simple physical processes such as filtration or extraction. May be achieved by the attachment of a tag, such as a highly fluorinated component (see fluorous synthesis), or other sequestration-enabling reagent 43.

Photolithography: Process by which selective masking generates light patterns which direct chemical transformations to certain areas of a photosensitive surface. Coupling of different building blocks to discrete sites may give rise to spatially addressable arrays of compounds 61.

Pin: An elongated device in which the tip acts as a solid support. An array of pins may typically be held such that sets of pins may be simultaneously inserted or retracted from solvents or reagents allowing library preparation by parallel synthesis 8.

Point of Diversity: Portion of a molecule, or step in a synthetic scheme, where different building blocks may be introduced.

Poly(ethylene glycol) (PEG): Polymer which has been applied both as soluble support and (as a graft copolymer with a polystyrene matrix) as a linker for combinatorial synthesis. See TentaGel, ArgoGel. The soluble support may have hydroxyls at both termini, or one or both may be capped or modified with additional functionality 59.

Pool: (a) A sub-library; (b) process of combining and mixing library components or sub-libraries. See pool/split.

Pool/Split: (split/pool; split&mix; divide, couple, recombine; portion/mix) Strategy for assembly of a combinatorial library. The solid support is divided into portions, each of which is subjected to reaction with a single building block. Pooling of these portions results in a single batch of solid support bearing a mixture of components. Repetition of the divide, couple, recombine processes results in a library where each discrete particle of solid support carries a single library member, and the number of members is equal to the product of the number of building blocks incorporated at each step (i.e. fully combinatorial) 79,80.

Positional Scan: Strategy for identifying individual compounds of interest from a library whereby a collection of sub-libraries is prepared, equal in number to the total number of building blocks used in the entire library. In each pool one point of diversity is held constant by incorporating a single building block while the other positions use all possible building blocks 81.

Preformed Scaffold: A scaffold which is incorporated into the library as a unit. Compare in situ scaffold.

Principal Components Analysis: Computational approach to reducing the complexity of, for example, a set of descriptors, by identifying those features which provide the major contributions to observed properties, and thus reducing the dimensionality of the relevant property space 30,82.

Privileged Structure: Substructural feature which confers desirable (often drug-like) properties on compounds containing that feature. Often consists of a semi-rigid scaffold which is able to present multiple hydrophobic residues without undergoing hydrophobic collapse, e.g. diazepam (below) in which the diphenylmethane moiety prevents association of the aromatic rings 83.

Property Space: Multidimensional representation of a set of compounds in which the axes represent quantifiable properties such as molecular weight, c LogP, molar refractivity, etc., and individual compounds are represented by a vector or set of coordinates.

Pseudo-Dilution: see Site Isolation

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