Stereospecific Pseudoproline Ligation of N-Terminal Serine, Threonine, or Cysteine-Containing Unprotected Peptides
摘要:
This paper describes an orthogonal and stereospecific method for ligating free peptide segments to form a monosubstituted pseudoproline bond with a hydroxymethyl moiety at the C2 carbon. The pseudoproline ligation, comprising both the oxaproline and thiaproline ligations, initially involves an imine capture of a peptidyl glycoaldehyde ester with an N-terminal cysteine, serine, or threonine peptide segment and then two spontaneous cyclization reactions. The thiazolidine or oxazolidine ester formed in the first cyclization undergoes an O,N-acyl transfer to form an pseudoproline bond. The thiaproline ligation can be carried out exclusively with unprotected peptide segments in both aqueous and nonaqueous pyridine-acetic acid conditions. However, the oxaproline ligation is best performed in a nonaqueous pyridine-acetic acid mixture with unprotected peptide segments except for those containing N-terminal nucleophilic amino acids such as Cys, His, and Trp. Pseudoproline ligation is not only regioselective but also stereospecific. 2D H-1 NMR studies of dipeptide models, Z-Xaa-psi Pro-OMe, indicate that the newly created C2 stereocenter of the pseudoproline ring affords only an li-epimer and the C2-hydroxymethyl-substituted pseudoproline exhibits high preference for cis conformation. Three of the model peptides have more than 50% cis isomers. Finally, this novel method has been used successfully in ligating two segments of 24 and 35 amino acids under mild conditions to synthesize three analogues of bactenecin 7, an antimicrobial peptide containing 59 amino acid residues.
Bidirectional Tandem Pseudoproline Ligations of Proline-Rich Helical Peptides
摘要:
We have developed a bidirectional ligation strategy for preparing proline-rich peptides that couples three unprotected segments in tandem to form two pseudoproline bonds (thia- or oxaproline) without the need for a protection scheme. Ligation in the C-->N direction exploits the regioselectivity of an amino terminal (NT)-Cys in forming a thiaproline bond over an NT-Ser or NT-Thr peptide in forming an oxaproline bond with a peptide that bears a carboxyl terminal (CT)-glycoaldehyde ester. Thus, successive ligations of three unprotected segments in a predetermined order formed a thiaproline and then an oxaproline bond. However, ligation through the N-->C direction is flexible. An NT-Cys, NT-Ser, or NT-Thr segment bearing a CT-glycerol ester as a masked CT-glycoaldehyde was used to form a pseudoproline bond with another CT-glycoaldehyde ester segment. Oxidative activation of the glycerol ester product to a CT-glycoaldehyde ester effected another round of pseudoproline ligation with an NT-Ser, NT-Thr, or NT-Cys segment. This sequential process could be extended for ligating three or more segments. Optimized conditions for this bidirectional strategy were applied successfully to syntheses of five analogues of a proline-rich helical antimicrobial peptide, the 59-residue bactenecin 7 (Bac 7), using three segments containing 24, 14, and 21 amino acids, respectively. CD spectra showed that Pac 7 and its analogues displayed typical polyproline II helical structures in phosphate buffers. Furthermore, the psi Pro-containing analogues exhibited antibacterial activity similar to Bac 7.
Stereospecific Pseudoproline Ligation of N-Terminal Serine, Threonine, or Cysteine-Containing Unprotected Peptides
作者:James P. Tam、Zhenwei Miao
DOI:10.1021/ja991153t
日期:1999.10.1
This paper describes an orthogonal and stereospecific method for ligating free peptide segments to form a monosubstituted pseudoproline bond with a hydroxymethyl moiety at the C2 carbon. The pseudoproline ligation, comprising both the oxaproline and thiaproline ligations, initially involves an imine capture of a peptidyl glycoaldehyde ester with an N-terminal cysteine, serine, or threonine peptide segment and then two spontaneous cyclization reactions. The thiazolidine or oxazolidine ester formed in the first cyclization undergoes an O,N-acyl transfer to form an pseudoproline bond. The thiaproline ligation can be carried out exclusively with unprotected peptide segments in both aqueous and nonaqueous pyridine-acetic acid conditions. However, the oxaproline ligation is best performed in a nonaqueous pyridine-acetic acid mixture with unprotected peptide segments except for those containing N-terminal nucleophilic amino acids such as Cys, His, and Trp. Pseudoproline ligation is not only regioselective but also stereospecific. 2D H-1 NMR studies of dipeptide models, Z-Xaa-psi Pro-OMe, indicate that the newly created C2 stereocenter of the pseudoproline ring affords only an li-epimer and the C2-hydroxymethyl-substituted pseudoproline exhibits high preference for cis conformation. Three of the model peptides have more than 50% cis isomers. Finally, this novel method has been used successfully in ligating two segments of 24 and 35 amino acids under mild conditions to synthesize three analogues of bactenecin 7, an antimicrobial peptide containing 59 amino acid residues.
Bidirectional Tandem Pseudoproline Ligations of Proline-Rich Helical Peptides
作者:Zhenwei Miao、James P. Tam
DOI:10.1021/ja000128g
日期:2000.5.1
We have developed a bidirectional ligation strategy for preparing proline-rich peptides that couples three unprotected segments in tandem to form two pseudoproline bonds (thia- or oxaproline) without the need for a protection scheme. Ligation in the C-->N direction exploits the regioselectivity of an amino terminal (NT)-Cys in forming a thiaproline bond over an NT-Ser or NT-Thr peptide in forming an oxaproline bond with a peptide that bears a carboxyl terminal (CT)-glycoaldehyde ester. Thus, successive ligations of three unprotected segments in a predetermined order formed a thiaproline and then an oxaproline bond. However, ligation through the N-->C direction is flexible. An NT-Cys, NT-Ser, or NT-Thr segment bearing a CT-glycerol ester as a masked CT-glycoaldehyde was used to form a pseudoproline bond with another CT-glycoaldehyde ester segment. Oxidative activation of the glycerol ester product to a CT-glycoaldehyde ester effected another round of pseudoproline ligation with an NT-Ser, NT-Thr, or NT-Cys segment. This sequential process could be extended for ligating three or more segments. Optimized conditions for this bidirectional strategy were applied successfully to syntheses of five analogues of a proline-rich helical antimicrobial peptide, the 59-residue bactenecin 7 (Bac 7), using three segments containing 24, 14, and 21 amino acids, respectively. CD spectra showed that Pac 7 and its analogues displayed typical polyproline II helical structures in phosphate buffers. Furthermore, the psi Pro-containing analogues exhibited antibacterial activity similar to Bac 7.