title: Microwave-assisted solid-phase peptide synthesis at 60 degrees C: alternative conditions with low enantiomerization.
abstract: Several conditions have been used in the coupling reaction of stepwise SPPS at elevated temperature (SPPS-ET), but we have elected the following as our first choice: 2.5-fold molar excess of 0.04-0.08 M Boc or Fmoc-amino acid derivative, equimolar amount of DIC/HOBt (1:1) or TBTU/DIPEA (1:3), 25% DMSO/toluene, 60 degrees C, conventional heating. In this study, aimed to further examine enantiomerization under such condition and study the applicability of our protocols to microwave-SPPS, peptides containing L-Ser, L-His, L-Cys and/or L-Met were manually synthesized traditionally, at 60 degrees C using conventional heating and at 60 degrees C using microwave heating. Detailed assessment of all crude peptides (in their intact and/or fully hydrolyzed forms) revealed that, except for the microwave-assisted coupling of L-Cys, all other reactions occurred with low levels of amino acid enantiomerization (<2%). Therefore, herein we (i) provide new evidences that our protocols for SPPS at 60 degrees C using conventional heating are suitable for routine use, (ii) demonstrate their appropriateness for microwave-assisted SPPS by Boc and Fmoc chemistries, (iii) disclose advantages and limitations of the three synthetic approaches employed. Thus, this study complements our past research on SPPS-ET and suggests alternative conditions for microwave-assisted SPPS.
authors.affiliation: Department of Biochemistry, Institute of Chemistry, University of São Paulo, P.O. Box 26077, 05513-970, São Paulo, Brazil.
authors.forename: Carina
authors.initials: C
authors.lastname: Loffredo
authors.forename: Nilson A
authors.initials: NA
authors.lastname: Assunção
authors.forename: Juergen
authors.initials: J
authors.lastname: Gerhardt
authors.forename: M Terêsa Machini
authors.initials: MT
authors.lastname: Miranda
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry; D002766:Cholecystokinin / Q000737:chemistry / Q000235:genetics; D002853:Chromatography, Liquid; D019075:Electrophoresis, Capillary; D008401:Gas Chromatography-Mass Spectrometry; D006361:Heating; D008872:Microwaves; D008969:Molecular Sequence Data; D010455:Peptides / Q000138:chemical synthesis* / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification; D013237:Stereoisomerism; D053719:Tandem Mass Spectrometry; D013696:Temperature
chemical_list: D000596:Amino Acids; D010455:Peptides; D002766:Cholecystokinin