Problematic thiols

Although the structure of a protein may encourage the use of one type of immobilization chemistry in preference to others, the optimal strategy must be empirically determined. Efficient coupUng, while important, must not be at the expense of activity. In this section, some immobilization chemistry options for potentially problematic interactants such as acidic proteins, are discussed. Secondly, novel ways to immobiUze membrane proteins are presented and finally, some recently developed methods of thiol couphng are addressed. 

Silylation is the most widely used derivatization technique for GC. Some functional groups are problematic for GC as a result of acidic or basic properties that promote intermolecular hydrogen bond formation and thus involatility thus hydroxyl, carboxylic acid, amine, thiol, even phosphate ester groups, can be derivatized by... 

Special chemical derivatives free thiol functions of cysteine can be problematic because of post-synthetic uncontrolled oxidation. To avoid this, you can replace Cys by serine (Ser), alanine (Ala), or u-aminobutyric acid (Abu). Alternatively, choose the hydrophilic Cys(Acm) and leave protected. For the simultaneous preparation of peptides of different size with free amino terminus, couple the terminal amino acid as /V-Boc derivatives so that they will not become acetylated during the normal elongation cycle. Boc is removed during the final side chain deprotection procedure (Protocol 4). Special labels can be attached to the N-termini by spotting respective derivatives in an additional coupling cycle. We have succes lly added biotin via the in situ formed HOBt-ester (normal activation procedure) or fluorescein via the isothiocyanate (FITC) dissolved in NMP. 

The 1,10-phenanthroline-copper complex is usually generated in situ where it exhibits maximal cleavage efficiency at a copper to phenanthroline ratio of 1 4. This results from the smaller binding constant of the second phenanthroline in the complex. At high thiol concentrations where there is competitive binding between phenanthroline and the thiol this can be problematic. By covalently attaching two phenanthrolines using a short... 

Thiols. 5-Acetyl derivatives can be prepared by the reaction of acetic anhydride and a thiol in the presence of potassium bicarbonate. Several disadvantages to the 5-acetyl group in peptide synthesis include /3-elimination upon base-catalyzed hydrolysis. Also, sulfur to nitrogen acyl migration may be problematic. 

Note that we used thioacetate (sometimes referred to as thiolacetate) end groups since these could be selectively deprotected in solution, to afford the free thiol, using NH4OH or acid during the deposition process. " Alkali metal salts can be avoided since they tend to disrupt electronics measurements. Use of the free thiols, rather than the thioacetates, proved to be somewhat problematic since they were prone to very rapid oxidative disulfide formation. But if a quality inert atmosphere box is used, use of the free thiol can be effective a N2-flush bag is not adequate to prevent the aromatic disulfide formation. Furthermore, the disulfides formed can self-assemble on gold, but the assembly is approximately KXX) times slower than with the thiols. When using the a,(i)-dithiols, oxidative polymerization ensues which rapidly results in insoluble material. Hence, in situ removal of the acetates has proven to be quite effective although not essential if strict exclusion of air is maintained. 

The application of papain in peptide synthesis is well established [23-25]. Papain can be used for fhe preparation of di- and tripepfides in an aqueous medium wifh cosolvent addition (up to 40%) and at high pH to promote synthetic activity. The enzyme is a sulfhydryl protease with no homology to the trypsin or subtilase families of hydrolases. Since the catalytic nucleophile is a cysteine and because thioesters are relatively more prone to aminolysis than oxo-esters, the enzyme could be very attractive for synfhesis. However, unlike the case with the thiol variants of some serine hydrolases, fhe proteolytic activity is still high, and the broad substrate range of proteolysis makes peptide substrate and product hydrolysis more problematic than trypsin or chymotrypsin. Extensive enzyme engineering studies on papain are lacking, probably due to the laborious procedure for isolation of active papain from inclusion bodies formed in E. coli. 

The nitroso byproduct has also proved problematic for spectroscopic analyses of ONE reactions due to its reactivity with some substrates and with proteins. This problem was circumvented by conducting the photolysis in the presence of dithiothreitol, a hydrophilic thiol and an excellent nucleophile that readily sequesters the nitroso byproduct. 

Related Reactions. p-Keto ester formation from ketones may be achieved directly with dialkyl carbonates and dialkyl dicarbonates, or indirectly with dialkyl oxalates, methyl magnesium carbonate, and ethyl diethoxyphosphinyl formate. Re-giocontrol is problematical, however, and is more reliably effected by trapping enolates with carbon dioxide, carbon disulfide, or carbon oxysulfide followed by methylation. In the latter cases, the dithio and thiol esters are converted into the parent carboxy... 

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