PAM linker

The PAM linker increases the acid stability of the anchored first amino acid residue by a factor of more than 100 compared to the simple benzyl ester. This approach uses an amino-methylated polystyrene support (usually referred to as PAM resin) instead of the chloro-methylated polystyrene. Most solid-phase syntheses of polypeptides are now performed using the PAM resin. 

Esters of the PAM linker are slightly more resistant towards acids than the corresponding 4-alkylbenzyl esters [5,25-27] (Table 3.1). The PAM linker is particularly well suited for solid-phase peptide synthesis using A-Boc amino acids because less than 0.02% cleavage of the peptide from the support occurs during the acidolytic deprotection steps [27], Esters of both the 4-alkylbenzyl alcohol and PAM linkers can also be cleaved by nucleophiles (see Sections 3.1.2 and 3.3.3). 

Figure 16.3 shows a representative example of the synthesis of peptides using the Boc strategy. Hydroxymethyl polystyrene can be used as the support or, if large peptides are to be prepared, polystyrene with the more acid-resistant PAM linker. Polyacrylamides [17] with a benzyl alcohol linker are also compatible with the Boc Strategy-... 

Scheme 7.13. Solid-phase synthesis with Pam linker on polystyrene resin. Note that polystyrene resin is crosslinked by incorporating a small quantity of divinylbenzene.

Spengler and coworkers [173] have used p-nitromandelic acid (Pnm) as a safety-catch linker for the synthesis of depsipeptides by Boc/Bzl chemistry. The hexafluo-roacetone (HFA)-protected Pnm is loaded on the resin with concomitant deprotection of the hydroxy unit. After the depsipeptide is constructed, reduction of the nitro group with 6M SnCl2/1.6mM HC1 gives the p-aminomandelic (Pam) linker, which can be cleaved in acidic media (5% TFA in dioxane, 50°C, lh), thereby obtaining the target depsipeptides in good yields and purities (Scheme 18.7). 

Figure 2. Flow chart of the protocol used to synthesize tetrameric bundles of defined amino add sequence. (A) The highly substituted polystyrene resin, 0.75 mmol/g, that contains t-Boc glydne (Gj attached through a PAM linker (open box). (B) The most accessible sites are selected by coupling with only 0.25 equivalents of the t-Boc-fmoc lysine. The remaining unreacted sites are blocked using an excess of acetic anhydride (Ac).

The investigation of minor groove-binding polyamides was greatly accelerated by the implementation of solid-phase synthesis [48]. Originally demonstrated on Boc-y9-Ala-PAM resin with Boc-protected monomers, it was also shown that Fmoc chemistry could be employed with suitably protected monomers and Fmoc-y9-Ala-Wang resin (Fig. 3.8) [49]. Recently, Pessi and coworkers used a sulfonamide-based safety-catch resin to prepare derivatives of hairpin polyamides [50]. Upon activation of the linker, resin-bound polyamides were readily cleaved with stoichiometric quantities of nucleophile to provide thioesters or peptide conjugates. 

In order to facilitate the generation of peptide thioesters, several groupst have developed generalized versions of the thioester linker pioneered by Hojo and Aimoto.f A 3-sulfanylpropanoic acid residue is generated on an acid-labile linker such as Boc-Leu-PAM-resin or Boc-Leu-MBHA-resin. It is important to have a one-residue spacer between the MBHA linker and the thiol for optimal acid stability of the amide bond.f All twenty Boc-protected amino acids can be coupled to this thiol on the solid support to generate the thioester.Despite the potential reactivity of the thioester to the amino terminus, the formation of a dioxopiperazine is not generally observed when using in situ neutralization protocols. However, when the sequence was Leu-Tyr-Arg-Ala-Pro, 20% of a dipeptide deletion product, Leu-Tyr-Arg, was observed. It is likely that sequences such as C-terminal Pro-Gly would also be subject to this side reaction.t ... 

PAM AMs [poly(amidoamine) dendrimers] are commercially available hyperbranched cores. T-antigen [Thomsen-Friedenreich, [3-Gal-(l,3)-oc-D-GalNAc-OR, 29] was successfully attached to PAMAM 30 via a 3-(propyl)mercaptopropionic acid linker to investigate T-antigen-protein interactions [44] (Scheme 20.8). 

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