1. Orthogonal protecting groups for N(alpha)-amino and C-terminal carboxyl functions in solid-phase peptide synthesis
F Albericio Biopolymers. 2000;55(2):123-39. doi: 10.1002/1097-0282(2000)55:23.0.CO;2-F.
For the controlled synthesis of even the simplest dipeptide, the N(alpha)-amino group of one of the amino acids and the C-terminal carboxyl group of the other should both be blocked with suitable protecting groups. Formation of the desired amide bond can now occur upon activation of the free carboxyl group. After coupling, peptide synthesis can be continued by removal of either of the two protecting groups and coupling with the free C-terminus or N(alpha)-amino group of another protected amino acid. When three functional amino acids are present in the sequence, the side chain of these residues also has to be protected. It is important that there is a high degree of compatibility between the different types of protecting groups such that one type may be removed selectively in the presence of the others. At the end of the synthesis, the protecting groups must be removed to give the desired peptide. Thus, it is clear that the protection scheme adopted is of the utmost importance and makes the difference between success and failure in a given synthesis. Since R. B. Merrifield introduced the solid-phase strategy for the synthesis of peptides, this prerequisite has been readily accepted. This strategy is usually carried out using two main protection schemes: the tert-butoxycarbonyl/benzyl and the 9-flourenylmethoxycarbonyl/tert-butyl methods. However, for the solid-phase preparation of complex or fragile peptides, as well as for the construction of libraries of peptides or small molecules using a combinatorial approach, a range of other protecting groups is also needed. This review summarizes other protecting groups for both the N(alpha)-amino and C-terminal carboxyl functions.
2. Preparation, isolation, and characterization of Nalpha-Fmoc-peptide isocyanates: solution synthesis of oligo-alpha-peptidyl ureas
Vommina V Sureshbabu, Basanagoud S Patil, Rao Venkataramanarao J Org Chem. 2006 Sep 29;71(20):7697-705. doi: 10.1021/jo0611723.
The N(alpha)-Fmoc-peptide isocyanates 3a-q, 4a-c, and 5a-c were prepared by the Curtius rearrangement of N(alpha)-Fmoc-peptide acid azides in toluene under thermal, microwave, and ultrasonic conditions. All the N(alpha)-Fmoc-oligo-peptide isocyanates made were isolated as stable crystalline solids with 71 to 94% yield and were fully characterized by 1H NMR, 13C NMR, and mass spectroscopy. Their utility for the synthesis of oligo-alpha-peptidyl ureas 7a-f and 8a-c by the divergent coupling approach was demonstrated. The coupling of N(alpha)-Fmoc-dipeptide isocyanates with amino acid ester or with N,O-bis(trimethylsilyl)amino acids resulted in N(alpha)-Fmoc-tripeptidyl urea ester and acids containing one each of peptide bond and urea bond. The divergent approach is extended to the synthesis of tetrapeptidyl ureas by the 2 + 2 strategy using bis-TMS-peptide acid as an amino component. To incorporate urea bonds in adjacent positions, N(alpha)-Fmoc-peptidyl urea isocyanates 9a-d were prepared and employed in the synthesis of three tetrapeptidyl ureas 10a-b and 11 containing one peptide bond and two urea bonds in series from the N-terminal end. The protocol was then employed for the synthesis of five urea analogues 13-15, 18, and 21 of [Leu5]enkephalin containing urea bonds at the 2, 3, 4 positions as well as at the 2, 4 and 2, 3, 4 positions. The analogue 2l was made by the convergent synthesis by the N --> C terminal chain extension. Finally, two urea analogues 22 and 23 of repeat units of bioelasto polymers, namely Val-Pro-Gly-Val-Gly-OH and Pro-Gly-Val-Gly-Val-OH, were synthesized incorporating the urea bond by the concomitant isocyanate generation and urea bond formation under thermal conditions.
3. Automated peptide-resin deprotection/cleavage by a robotic workstation
R N Zuckermann, S C Banville Pept Res. 1992 May-Jun;5(3):169-74.
A robotic workstation has been constructed that automates the deprotection of peptides with trifluoroacetic acid-labile side-chain protecting groups and cleavage from acid-labile resins. The workstation was constructed around a Zymark robot arm and is integrated with a peptide synthesis workstation. Peptide resin samples are deprotected and cleaved with a trifluoroacetic acid/scavenger cocktail that accommodates all common protecting groups used with Fmoc chemistry. Aqueous-ether continuous extraction is used to remove the scavengers and reaction by-products. The apparatus cleaves a 50-500-mg sample of peptide resin every 2 hours and provides the peptides as an aqueous solution in 10% HOAc. Crude peptides obtained with this apparatus are free from residual scavengers and range in yield from 50%-80%. This method is applicable to all but the most hydrophobic peptides and minimizes human contact with the noxious cleavage reagents.
