Synthetic approaches to peptides containing the l-Gln-l-Val-D(S)-Dmt motif

The pseudoprolines S-Dmo (5,5-dimethyl-4-oxaproline) and R-Dmt (5,5-dimethyl-4-thiaproline) have been used to study the effects of forcing a fully cis conformation in peptides. Synthesis of peptides containing these (which have the same configuration as L-Pro) is straightforward. However, synthesis of peptides containing S-Dmt is difficult, owing to the rapid cyclisation of L-Aaa-S-Dmt amides and esters to form the corresponding diketopiperazines (DKP); thus the intermediacy of L-Aaa-S-Dmt amides and esters must be avoided in the synthetic sequence. Peptides containing the L-Gln-L-Val-D(S)-Dmt motif are particularly difficult, owing to the insolubility of coupling partners containing Gln. Introduction of Gln as N-Boc-pyroglutamate overcame the latter difficulty and the dipeptide active ester BocPygValOC(6)F(5) coupled in good yield with S-DmtOH. BocPygVal-S- DmtNH(CH(2))(2)C(6)H(4)NO(2) was converted quantitatively to BocGlnVal-S-DmtNH(CH(2))(2)C(6)H(4)NO(2) with ammonia, demonstrating the utility of this approach. Two peptide derivatives (CbzSerLysLeuGlnVal-S-DmtNH(CH(2))(2)C(6)H(4)NO(2) and CbzSerSerLysLeuGlnVal-S- DmtNH(CH(2))(2)C(6)H(4)NO(2)) were assembled, using these new methods of coupling a dipeptide acid active ester with S-DmtOH and introduction of Gln as Pyg, followed by conventional peptide couplings. The presence of the Val caused these peptides to be cleaved very slowly by prostate-specific antigen (PSA) at Leu Gln, rather than the expected Gln Val.

Synthesis and solubility properties of peptide fragments of human hemoglobin alpha-chain (123-136)

The solubility prediction method for protected peptides was successfully applied to relatively small peptide fragments of human hemoglobin alpha-chain (123-136) which contained various polar amino acid residues such as Asp(OBzl), Glu(OBzl), Lys(Z), Ser(Bzl), and Thr(Bzl). As reported previously for hydrophobic peptides and human proinsulin C-peptide fragments, solubility data indicated that the insolubility of protected peptides having a mean value of Pc value below 0.90 appeared to begin at the octa- or nonapeptide sequence level and that beta-sheet structure played an important role in the insolubility of peptides. When a peptide has a beta-sheet structure in the solid state, we can clearly determine the critical chain length for peptide insolubility, the solubility dependence on solvent properties, and the solubility independence of amino acid compositions of peptides.