1.Comparative study of methods to couple hindered peptides.
Spencer JR1, Antonenko VV, Delaet NG, Goodman M. Int J Pept Protein Res. 1992 Sep-Oct;40(3-4):282-93.
A comparative study of modern coupling reactions involving Boc-protected amino acid derivatives and dipeptides with N-terminal alpha,alpha-dialkylation and N-methylation was carried out. The coupling reactions were run using either equimolar amounts of the amino and activated carboxyl components or an excess of the activated carboxyl component. Yields of the target tripeptide Boc-Phe-Xaa-Phe-OBzl (Xaa = (NMe)Ala, (NMe)Aib, or (NMe) alpha Ac5c) were compared. Less than 10% of the product was obtained from methods utilizing pivaloyl mixed anhydride, pentafluorophenyl ester or acyl fluoride activation when Xaa = (NMe)Aib and (NMe) alpha Ac5c. At room temperature, significant yields of these two products were obtained from reactions which utilized an excess of the HBTU reagent (O-benzotriazol-1-yl-N,N,N',N'-tetramethyluronium hexafluorophosphate), the PyBroP reagent (bromo-tris-pyrrolidino-phosphonium hexafluorophosphate) or Boc-Phe-NCA (Boc-protected phenylalanine N-carboxyanhydride).
2.A convenient incorporation of conformationally constrained 5,5-dimethylproline into the ribonuclease A 89-124 sequence by condensation of synthetic peptide fragments.
Cerovský V1, Welker E, Scheraga HA. J Pept Res. 2003 Mar;61(3):140-51.
The presence of l-5,5-dimethylproline (dmP) within an amino acid sequence results in the formation of an X-dmP peptide bond predominantly locked in a cis conformation. However, the common use of this unnatural amino acid has been hampered by the difficulty of the economical incorporation of the dmP residue into longer peptide segments due to the steric hindrance imposed by the dimethyl moieties. Here, we describe synthesis of the C-terminal 36-residue peptide, corresponding to the 89-124 sequence of bovine pancreatic ribonuclease A (RNase A), in which dmP is incorporated as a substitute for Pro93. The peptide was assembled by condensation of protected 5- and 31-residue peptide fragments, which were synthesized by solid-phase peptide methodology using fluorenylmethyloxycarbonyl chemistry. We focused on optimizing the synthesis of the Fmoc-Ser(tBu)-Ser(tBu)-Lys(Boc)-Tyr(tBu)-dmP-OH pentapeptide (residues 89-93) with efficient acylation of the sterically hindered dmP residue.
