Covalent capture

Reductive amination of an aldehyde with excess primary amine, using a support-bound borohy-dride, provides the desired secondary amine contaminated with the primary amine precursor. Covalent capture of the primary amine with a support-bound aldehyde provides the pure secondary amine. Treatment with excess isocyanate yields the final urea product, which is purified by reaction with a support-bound amine to remove unreacted isocyanate. For the full potential of this method to be realized, further development of support-bound reagents and scavengers for most of the important chemical transformations will be necessary. Al-... 

As well as similar demonstrations of covalent capture of hydrogen-bonded dimers (e.g., see Reference 46). 

Figure 1.11 Covalent capture of peptide macrocycle dimer (Ghadiri).

Clark TD, Ghadiri MR. Supramolecular design by covalent capture. Design of a peptide cylinder via hydrogen-bond-promoted intermolecular olefin metathesis. J Am Chem Soc 1995 117 12364-12365. 

The first report of resin capture in solution-phase chemical library synthesis involved the covalent capture of solution-phase Ugi reaction products onto a functionalized polystyrene resin.73 Excess reactants, reagents, and reagent byproducts were washed away from the resin-captured intermediates. Further manipulation and release afforded purified solution-phase products for screening. More recently the same group reported on resin capture as a technique for the preparation of tetrasubstituted olefin libraries.74 75 As illustrated in Scheme 5, m-vinyl di-boryl esters were reacted with aryl halides (R3ArX) in parallel Suzuki reactions, leading to solution-phase intermediates. Another Suzuki reaction, this time with the... 

Figure 10.1 Principle of covalent capture methods. Drug fragments typically have weak binding affinity and can therefore be difficult to detect. By introducing two reactive groups, X and Y, a fragment that binds in the vicinity of X can be captured covalently by the protein target and easily identified by mass spectrometry.

X is known, information about binding location. This can be exploited to direct the screen to a particular site on a protein. This location is typically the active site, but the method can also be used to investigate allosteric sites or protein-protein binding interfaces. In the following, we will describe various practical approaches to covalent capture and provide examples of their application in studying protein-ligand interactions and drug discovery. 

Figure 10.2 Different versions of Tethering that have been used for covalent capture.

Discovery of Peptide Ligands Using Covalent Capture... 

In addition to finding small organic molecules that bind to a protein, covalent capture methods can identify peptides that interact with proteins. Kohda and colleagues used this approach to study the mitochondrial protein Tom20, an import receptor that recognizes an epitope on proteins targeted for the mitochondria.1301 Previous work had characterized this epitope as a five-residue peptide that assumes an amphiphilic helical conformation, and coarse sequence preferences had been worked out. However, Tom20 has both low affinity... 

Figure 10.4 Examples of covalent capture methods. (A) Covalent modification of native cysteines has been shown to modulate ion-channel activity and in the case of enzymes lead to allosteric inhibition. These findings can be important for structural-functional characterization or as starting points for drug discovery. (B) Reversible covalent capture using imine chemistry.

As this area of polymer supported reagents continues to expand the complexity of the polymeric supported resins has increased [18]. Although electrophilic supported reagents like the isocyanate 5 and acid chloride 12 have been shown to be efficient reagents for the covalent capture for primary and secondary amines (Table 1), they are not without their difficulties. The isocyanate resin is particularly expensive and the loading is rather low (approximately 1 mmol NCO/gram). 

These examples serve to highlight that supramolecular self-assembly and topo-chemical diacetylene polymerizations are a perfect match. Topochemical diacetylene polymerizations are an advantageous means of covalent capture for the reasons outlined above. The required order may, on the other hand, be provided by supramolecular self-assembly, which extends the scope beyond singlecrystalline monomers. This aspect becomes particularly important in the case of functional monomers in order to address specific applications. However, in contrast to previous investigations, the targeted preparation of hierarchically structured poly (diace tylene)s with a defined, finite number of strands required the presence of equally well-defined, uniform supramolecular polymers [106] with the propensity to form predictable superstructures, instead of micellar or vesicular ID aggregates. 

There are two main ways to incorporate bioactive tags the first is to include the sequence prior to assembly (see Sections 4.1 and 4.2 above for other examples). The second is to link the sequence following assembly either by covalent capture or by chemical cross-linking. Fibers that display these tags not only have typical dimensions of ECM proteins but can interact with cells in a similar way to the ECM. 

Wagner-Meerwein shift of the central bond occurred with covalent capture of the counterion, even perchlorate (see 562 and 563). The most unusual structural features of562 have been confirmed by X-ray analysis.426 Although 564 could be obtained readily, it too proved sensitive to carbonium ion rearrangement. The conversion of lactol 560 to bis-iodoformate 565 when treated with lead tetraacetate and iodine with irradiation is also notable.427 ... 

Scheme 2. Covalent capture of a self-assembled cyclic peptide dimer using the Grubbs ruthenium catalyst.

Bradner JE, McPherson OM, Koehler AN. A method for the covalent capture and screening of diverse small molecules in a microarray format. Nat. Rotoc. 2006 1 2344-2352. 

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