Cleavable linkers

The power of the HR-MAS method for on-resin analysis has been further underscored in the development of new linkers. Without this method, only indirect analytical data after removal from the resin was available. Direct assessment of the resin-bound linker greatly facilitated the introduction of a 4,5-dibromo octane- 1,8-diol linker that was converted into an octane-1,8-diol linker cleavable by olefin metathesis at the end of the synthesis.6 The disappearance and reappearance of the olefinic protons as well as the growing oligosaccharide chain was clearly visible in the H spectrum (Fig. 8.7).7... 

Many other linkers besides those listed above have been developed for two-phase synthesis of oligosaccharides on insoluble supports, and it can be expected that at least some of them will be tested on soluble supports. It should be kept in mind that MPEG-supported syntheses can be easily scaled up therefore, any relationship between both types of polymer supports will be cooperative rather than mutually exclusive. Such linkers will most probably include dialkyl- or diaryl-silyl linkers,10,41 3 and linkers cleavable by photolysis such as the o-nitrobenzyl group and its modifications.44 16... 

Table 3.7. Linkers cleavable by palladium(0)-catalyzed allylic nucleophilic substitution.

Primary and secondary aliphatic amines can be linked to polymeric supports by acid-labile linkers or by linkers sensitive to nucleophiles. Linkers cleavable by light or by transition metal catalysis have also been described. The main types of linker for amines are sketched in Figure 3.24. 

Resin-bound selenium has been used as a linker for alkenes in two ways (a) as an oxidant-sensitive linker (selenoxides readily undergo [5-elimination at room temperature Entries 6-8, Table 3.43 [767-773]), or (b) as a linker cleavable by tin radicals (Figure 3.37 Entries 9 and 10, Table 3.43). The main advantages of selenides as linkers are their stability under a broad variety of (non-oxidizing) reaction conditions, including high temperatures and treatment with acids or bases, and the mild conditions required for their cleavage. 

The first solid-phase peptide synthesis reported by Merrifield had some disadvantages, which were corrected in later versions of this synthesis. The main improvements were the replacement of benzyloxycarbonyl protective groups by the TFA-labile Boc protection, and the use of TFA-resistant linkers cleavable by HF. 

The answer to the former problem came once again from peptide synthesis, when Bayer and coworkers [309] reported the first use of polyethylene glycol) (PEG) as a soluble support for peptide synthesis. The idea was immediately picked up by Koster, who prepared a PEG-20000 terminating in a dimethoxytrityl linker (Figure 19.16) and demonstrated the attachment of thymidine [310]. Later, Bayer and coworkers [311] assembled up to a pentanucleotide by the phosphodiester method on a PEG-NH2 support with a phosphoramidate linker cleavable by isoamyl... 

Scheme 12. Enzymatic synthesis of JV-acetyllactosamine using a water-soluble glycopolymer with a linker cleavable by hydrogenolysis. Reagents i) CH2=CH CO-NH2, TEMED, APS, DMSO-H2O, room temp., 24 h, 61% for 37, 83% for 38 ii) 1. HEPES buifer pH 6, 10 mM MnCl2, UDP-Gal (1 eq.), GT, 37 °C, 24 h 2. gel filtration on Sephadex G-50, q.y. for 40, 30% for 41 iii) H2, Pd/ C, H20-Me0H, 95%.

Linkers cleavable by nucleophiles include the safety-catch linker [13, 14], hydrazino benzoyl linker [15, 16] and 4-hydroxymethylbenzoic acid (HMBA) [14, 17] (Table 1). All three linkers can provide fully protected peptides, and as the HMBA and hydrazinobenzoyl linkers are acid-stable, the peptide side-chains can be deprotected prior to release of the peptide. The HMBA linker is a versatile but less often used linker for Fmoc-SPPS. Varying the nucleophile employed in the cleavage cocktail a wide range of C-terminal modified peptides can be obtained primary amide (NH3 in methanol) [18,19], hydrazides (5 % NH2NH2 in DMF) [20], acids (0.1 M aqueous NaOH) [21], and methyl esters (5 % DIEA, 5 % MeOH in DMF at 50 °C) [22] (Fig. 1). 

Rodebaugh R, Fraser-Reid B, Geysen HM. A new onitrobenzyl photo-cleavable linker for solid phase synthesis. Tetrahedron Lett 1997 38 7653-7656. 

Millington CR, Quarrell R, Lowe G. Aryl hydrazides as linkers for solid phase synthesis which are cleavable under mild oxidative conditions. Tetrahedron Lett 1998 39 7201-7204. 

A-chain immunotoxins, however, may not be quite as cytotoxic as conjugates formed from intact toxin molecules (Manske et al., 1989). In an alternative approach to A chain use, the intact toxin of two-subunit proteins is directly conjugated to a monoclonal without isolation of the A chain. Conjugation of an antibody with intact A-B chain toxins can be done without a cleavable linker, as long as the A chain can still separate from the B chain once it is internalized. Therefore, it is important to avoid intramolecular crosslinking during the conjugation process which can prevent release of the A-B complex. In addition, since the B chain... 

Petrotchenko, E.V., Olkhovik, V.K., and Borchers, C.H. (2005) Isotopically coded cleavable cross-linker for studying protein-protein interaction and protein complexes. Mol. Cell. Proteom. 4(8), 1167-1179. 

In a recent study, this so-called SPOT synthesis was applied for the preparation of pyrimidines [45]. The group of Blackwell described primarily the appropriate support modification of commercially available cellulose sheets (Scheme 7.28). The initial introduction of the amine spacer was achieved within 15 min utilizing micro-wave irradiation, as compared to 6 h by conventional heating. The acid-cleavable Wang-type linker was attached by classical methods at ambient temperature. 

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