Solid-Phase Synthesis of Difficult Peptide Sequences at Elevated Temperatures: A Critical Comparison of Microwave and Conventional Heating Technologies

Abstract

The Fmoc/t-Bu solid-phase synthesis of three difficult peptide sequences (a 9-mer, 15-mer, and 24-mer) was performed using N,N'-diisopropylcarbodiimide/1-hydroxybenzotriazole as coupling reagent on polystyrene, Tentagel, and ChemMatrix resins. In order to obtain an insight into the specific role of the elevated temperature and/or the electromagnetic field for peptide syntheses carried out using microwave irradiation, peptide couplings and Fmoc-deprotection steps were studied under microwave and conventionally heated conditions at the same temperature. While room temperature couplings/deprotections generally produced the difficult peptides in rather poor quality, excellent peptide purities were obtained using microwave heating at a temperature of 86 degrees C for both the coupling and deprotection steps in only 10 and 2.5 min reaction time, respectively. While for most amino acids no significant racemization was observed, the high coupling temperatures led to considerable levels of racemization for the sensitive amino acids His and Cys. It was demonstrated for all three peptide sequences that when performing the coupling/deprotection steps at the same reaction temperature using conventional heating, nearly identical results in terms of both peptide purity and racemization levels were obtained. It therefore appears that the main effect of microwave irradiation applied to solid-phase peptide synthesis is a purely thermal effect not related to the electromagnetic field.

Peptide synthesis, characterization and 68Ga-radiolabeling of NOTA-conjugated ubiquicidin fragments for prospective infection imaging with PET/CT

“…This is supported by the observation that no such difficulties were encountered in the synthesis of either UBI30-41 or its NOTA-conjugate, both lacking the threonine residue at the amino terminal. The most common reason for persistent difficulty in synthesizing a peptide using microwave-assisted solid phase synthesis are unfavorable intramolecular and/or intermolecular interactions of the nascent peptides, which may result in secondary structure conformations which inhibit coupling of additional amino acids [23][24][25].…”

Microwave Assisted Reactions in Organic Chemistry: A Review of Recent Advances

“…However, if there is no idea about exact internal reaction temperature, this type of reactions cannot be carried out or they may give incorrect outputs (Herrero, Kremsner, & Kappe, 2008;Bacsa et al, 2008). A single-mode high field-density microwave reactor (2.45 GHz, 850 W) along with internal fiber-optic temperature control had been used in the reactions to monitor the reaction temperature accurately in a small scale(0.5-2 mL) (Obermayer, Gutmann, & Kappe, 2009;Gutmann et al, 2010).…”

“…They have also used microwave instrumentation that has no internal probe to monitor the actual reaction temperature directly. Instead, they have used external IR sensors to monitor the surface temperature of the reaction vessel (Herrero, Kremsner, & Kappe, 2008;Bacsa et al, 2008) reported unreliability of these sensors with highly viscous reaction mixtures.…”

Aqueous Microwave-Assisted Solid-Phase Synthesis Using Boc-Amino Acid Nanoparticles

“…microwave (MW) irradiation has become a preferred choice for the organic chemist [1,[14][15][16]. There are many examples of the application of MW irradiation in peptide synthesis, including automated peptide synthesizers equipped with MW capability [17][18][19][20][21][22]. The main advantages of MW-assisted chemistry are shorter reaction times and higher yields.…”