Microwave‐assisted TFA cleavage of peptides from Merrifield resin

Abstract

Microwave-assisted (MW) reactions are of special interest to the chemical community due to faster reaction times, cleaner reactions and higher product yields. The adaptation of MW to solid phase peptide synthesis resulted in spectacular syntheses of difficult peptides. In the case of Merrifield support, used frequently in synthesis of special peptides, the conditions used in product cleavage are not compatible with off-resin monitoring of the reaction progress. The application of MW irradiation in product removal from Merrifield resin using trifluoroacetic acid (TFA) was investigated using model tetrapeptides and the effects were compared with standard trifluoromethanesulphonic acid (TFMSA) cleavage using elemental analysis as well as chromatographic (HPLC) and spectroscopic (IR) methods. The deprotection of benzyloxycarbonyl and benzyl groups in synthetic bioactive peptides was analyzed using LC-MS and MS/MS experiments. In a 5 min microwave-assisted TFA reaction at low temperature, the majority of product is released from the resin, making the analytical scale MW-assisted procedure a method of choice in monitoring the reactions carried out on Merrifield resin due to the short reaction time and compatibility with HPLC and ESI-MS conditions.

Site-selective solid phase synthesis of carbonylated peptides

“…After extending the time of cleavage of the peptide, the deprotection was complete (see below) and the signal at m / z 564.267 was not observed. We found that incorporation of a carbonylated amino acid at the N-terminus does not allow removing the acetal group even after 24 h of incubation, in the mixture used for cleavage or microwave-assisted TFA (Kluczyk et al 2010 ) cleavage from the resin. The probable reason is the proximity of the amino group which easily gets protonated.…”

Microwave heating in solid-phase peptide synthesis

“…55,78,88 The cleavage time was decreased from 2-5 hours down to minutes. Moreover, two papers by Clearhout et al 50 and Kluczyk et al 252 describe how TFA under microwave irradiation can substitute the use for HF in the cleavage of peptides from the Merrifield resin and meta-dialkoxy-BAL PS, respectively, however, the methods suffer from lower yield. Recently, a fully side-chain protected linear version of a cyclotide was cleaved from a NovaSyn TGT resin (PEG-PS-copolymer functionalised with 4-carboxy-tritylchloride) using AcOH/TFE/DCM (1/1/8) for 45 min at B40 1C.…”

Conventional and microwave‐assisted SPPS approach: a comparative synthesis of PTHrP(1–34)NH2

“…Therefore, we decided to monitor the progress of PTHrP(1–34)NH 2 synthesis by UPLC‐ESI‐MS analyses of small aliquots of cleaved peptide fragments obtained by MW‐assisted mini‐cleavages. The application of MW‐assisted mini‐cleavages of resin‐bound peptides has been proposed as a fast, reliable method to monitor SPPS11. After specific coupling cycles, suspected to be difficult, we stopped the synthesizer and withdrew a small aliquot for analysis by UPLC‐ESI‐MS.…”