Attachment of the first residue

Anchoring of the first residue to the synthesis support is one of the most critical steps in peptide synthesis an injudidous combination of linker, loading method, or amino add derivative can have a highly deleterious effects on the purity of the final product, and the use of poorly loaded resins is simply just a waste of expensive reagents. 

The properties of the various linker-derivatized resins used in Fmoc SPPS have already been discussed and were summarized in Chapter 2, Table 2. For the purpose of selecting the appropriate procedure for attachment of the first residue, these can be conveniently separated into three categories on the basis of their reactive functional groups (hydrox5methyl-, trityl chloride-, and aminomethyl-based resins) the mostly commonly used are listed in Table 1. 

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The problems involved in all of the above steps have been the subject of extensive investigations, and many significant and novel adaptations of the above basic strategy have been effected depending upon the case in hand. The use of various anchoring groups of different stability between the polymer support and the first amino acid particularly facilitated the attachment of the first residue and the final cleavage of the finished peptide U),... 

Trityl-based resins are highly acid-labile. The steric hindrance of the linker prevents diketopiperazine formation and the resins are recommended for Pro and Gly C-terminal peptides. Extremely mild acidolysis conditions enable the cleavage of protected peptide segments from the resin. These resins are commercially available as their chloride or alcohol precursors. The trityl chloride resin is extremely moisture-sensitive, so reagents and glassware should be carefully dried before use to avoid hydrolysis into the alcohol form. It is necessary to activate the trityl alcohol precursor and it is highly recommended to reactivate the chloride just before use see Note 4). After activation, attachment of the first residue occurs by reaction with the Fmoc amino acid derivative in the presence of a base. This reaction does not involve an activated species, so it is free from epimerization. Special precautions should be taken for Cys and His residues that are particularly sensitive to epimerization during activation (Table 2). 

To produce a carboxyamide peptide The peptide will be linked to the modified Rink linker via an amide bond. The attachment of the first residue can be carried out under conditions for peptide bond formation (e.g., with TBTU) by using the activation procedures described in Subheading 3.3.2,2, (Methods A-E) of this chapter. Do not forget to deprotect the linker before coupling of the amino acid. 

For the synthesis of C-terminal peptide amides, the commonly used resins are 1 to 3 (Fig. 4) (8-10). These resins are compatible with Fmoc chemistry and final TFA cleavage. For attachment of the first urethane A-protected residue, standard peptide coupling procedures (Protocol 5) can be used. These resins are usually supplied Fmoc-protected and should be deprotected before incorpora-... 

The first enzymatic step involves the covalent attachment of the first carbohydrate unit to the side chain of a specific amino acid residue. Further sugar residues are added in sequential order by specific glycosyltransferases for each of the sugar residues. It appears that specific multienzyme systems are required for the biosynthesis of each type of polymer (153). Frequently the carbohydrate chain terminates with an N-acetylneuraminic acid residue. 

Figure 1.6 Complex Saccharide Structures of Glycoproteins. A large number of complex-type structures are possible and those shown in this figure illustrate some typical features. Fucose is commonly, but not always, found linked to C-6 of the first residue of A/-acetyl-glucosamine. Rarely short hetero-saccharide sequences occur linked to C-3 of either or both of the A-acetylglucosaminyl residues of the di-A-acetylchitobiosyl sequence. The attachment of A-acetylglucosamine to C-4 of the P-mannosyl residue is another variable feature. The outer chains vary in number and may be linear or branched, complete or incomplete and of various lengths. Sequences with two linear outer chains are termed biantennate , those with three are triantennate etc. (Yoshima, Furthmayr and Kobata, 1980 Yoshima, Takasaki and Kobata, 1980 Kornfeld, 1978.)...

In 1962, a new approach to peptide synthesis was introduced by R. B. Mer-rifield. The key idea was that the first amino acid residue would be attached to an insoluble polymer. In practice, the attachment was accomplished by introducing chloromethyl groups on polystyrene and alkylating the carboxyl end of the first residue ... 

Rochelmeyer, after oxidation experiments on acetylsolanidine with chromic acid, concluded that the heterocyclic residue could not be attached to the homocyclic portion of the molecule at one point only as in formula (II). The first positive evidence as to the nature of the heterocyclic residue was provided by Prelog and Szpilfogel, ° > who investigated... 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

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