PAM resin

Palmitic acid, structure of, 1062 Palmitoleic acid, structure of, 1062 PAM resin, solid-phase peptide synthesis and, 1037 Para (m), 519 Paraffin, 91 Parallel synthesis, 586 Parent peak (mass spectrum), 410 Partial charge, 36 Pasteur, Louis, 297, 307... 

Solid-phase peptide synthesis. 1036-1038 PAM resin in. 1037 Wang resin in, 1037 Solvation, 370... 

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. 

While allowing rapid preparation of a range of polyamides, these resins install a T,A-selective y9-Ala residue at the C-terminus, which places Hmits on the DNA sites that can be targeted ]46]. The shortest tail available from these resins is a propanolamide, obtained by reductive cleavage. Polyamides prepared on Boc-Gly-PAM resin can be reductively cleaved to obtain ethanolamide tails, but it was expected that further truncation of the C-terminus would be necessary for tolerance... 

FIGURE 5.15 Synthesis of PAM (phenylacetamidomethyl) resin. (Merrifield et al., 1976). Use of PAM resin implies a Boc-amino acid anchored to oxymethylphenylacetamidomethyl-polystyrene through an ester linkage. The acetamido group renders the ester more stable to acid. 

A similar reaction was performed by Szardenings et al., however, as a solid phase Ugi reaction (Scheme 22) with the amine component on the resin 126 [44]. In this case either tentagel S-OH or PAM resin was used. The same synthetic operations were used as in previous solid phase DKP and yields were shown to be 28-91%. Previous attempts to make these compounds took six steps, resulted in single digit yields, and still contained impurities [44]. The Ugi route proves to be a much better access to those compounds. 

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. 

Quantitative sulfation is not achieved even with a larger excess of sulfating reagent (max sulfation 80%), a fact which is not understood. On the other hand, sulfation of a suitably protected CCK-8 linked to PAM resin with pyridine/S03 proceeds smoothly (Scheme 10) and upon ammonolysis, the CCK-octapeptide is isolated in 23% overall yield. 98 ... 

Table 2 pGlu Formation in H-Gln-Ala-Val-OCH2-PAM-Resin by Treatment with Various Reagents1 1... 

Thiol esters RC(0)SR have been prepared by nucleophilic cleavage of polystyrene-bound /V-acylsulfonamides with mixtures of a thiol and sodium thiophenolate [377] or LiBr [378], or by treatment of Wang or PAM resin bound carboxylic esters with ethanethiol in the presence of a Lewis acid (AlMe2Cl or AlMe3, DCM, 20 °C, 3 h [379,380]). The latter method can also give rise to the formation of orthoesters RC(SEt)3 and ketene thioacetals, which can, however, be hydrolyzed to the desired thiol esters by treatment with TFA [379]. 

Attachment of the first amino acid is usually performed using a symmetric anhydride in the presence of DMAP, but mixed anhydrides or acid fluorides can also be used (see Section 13.4.1). Alternatively, chloromethyl polystyrene can be treated with the cesium salt of the first amino acid to yield the corresponding benzyl ester (Section 13.4.2). All proteinogenic Boc-protected a-amino acids linked to hydroxymethyl polystyrene or PAM resin are commercially available [18,19] and can be stored for long periods without deterioration. 

The tetravalent lysinyl core peptide was synthesized as described previously 130 Briefly, the synthesis of the first level of carrier core to form Boc-Lys(Boc)-Ala-OCH2-PAM-resin was achieved using a fourfold excess of Boc-Lys(Boc)-OH DCHA salt with BOP in CH2C12. The second level of lysine was generated by the same protocol. After Boc deprotection, tetravalent chloroacetyl moieties were introduced to the core peptide by using a tenfold excess of chloroacetic acid via DIC/HOBt activation. The tetravalent (chloroacetyl)lysinyl core peptide was cleaved from the resin by HF/PhOMe (9 1) and finally purified by RP-HPLC. MALDI-MS m/z [M + H]+ calcd, 780.5 found, 780.5 [M+Na —H]+ calcd, 802.5 found, 802.3. 

The appropriate pam resin (tBoc-aminoacyl-(4-carboxyamidomethyl)-benzylester-poly-(styrene-l% divinylbenzene), where the aminoacyl group is the protected C-terminal amino acid corresponding to the C-terminal residue in the chemokine to be synthesized. 

Fig. 2. RP-HPLC chromatograms of human B cell chemoattractant-1 (BCA-1) (11), an 87 residue chemokine that is the ligand for CXCR5, which is the human homolog of the mouse receptor BLR-1. Its sequence is VLEVYYTSLRCRCVQESSVFIPRR FIDRIQILPRGNGCPRKEIIVWKKNKSIVCVDPQAEWIQRMMEVLR KRSSSTLPVPVFK (11). The BCA-1 polypeptide was synthesized starting with Lys-Pam resin and adding the appropriate protected amino acids until the N-terminal valine was added. The profile of the crude product (A) reveals a major broad peak that consists of the correct polypeptide and numerous closely related and closely eluting...

MBHA and/or PAM resins for r-Boc chemistry and MBHA or Wang resins for Fmoc chemistry. 

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