HMBA linker

Figure 18.2. H spectra (region of amide protons) of different amounts of a polystyrene/ 1 % divinylbenzene resin (0.5 mmol/g in dr DMF) carrying the telrapeptide Gly-Asn-Leu-Ile through an hydroxymethyl benzoic acid (HMBA) linker. Acquisition was performed in 1 minute (16 scans).

Figure 18.5. H spectra of 0.15 mg of polystyrene/1 % divinylbenzene resin (0.27 mmol/g) carrying the tet-rapeptide Giy-Asn-Leu-Ile through an HMBA linker (A) in h7-DMF. (B) in h7-DMF with diffusion filter (10% d7-DMF is added as lock signal).

HMBA linker Protected peptide acids, amides, alcohols, hydrazides Chapter 3... 

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). 

Fig.1 C-Terminal functionalities derived from the HMBA linker...

Peptide Methyl Esters Using the HMBA Linker... 

HMBA resin, an aminomethyl polystyrene resin acylated with 4-(hydro-xymethyl)benzoic acid. It is completely resistant towards treatment with acids (even liquid HE), and enables on-resin side-chain deprotection. The highly versatile linker is cleaved by a variety of nucleophiles such as hydroxide ions (to give peptide acids), alcohols (to give esters), ammonia or amines (to give amides), hydrazine (to give peptide hydrazides), or UBH4 (to give peptide alcohols) [R. C. Sheppard, B. J. Williams, Int.J. Pept. Protein Res. 1982, 20, 451]. 

Most cleavage and deprotection strategies for Fmoc-SPPS are trifluoroacetic acid (TFA) based and vary in the percentage of acid, types of scavengers used, and reaction time. These methods will be described in detail. Next, some methods for the nucleophilic release of C-terminally modified peptides from HMBA and hydra-zinobenzoyl linkers are also presented. Moreover, suggestions for the solubilization, quantification, and storage of peptides will be discussed in the chapter. 

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