The most common and commercially available BFC comprise esters (-nitrophenyl, pentafluorophenyl, -hydroxysuccinimide, sulfo--hydroxysuccinimide), isothiocyanates, maleimides, hydrazides, α-haloamides for the reaction with nucleophilic functional groups (-NH2, -SH, -OH) of vector molecules and formation of amide, urea, thiourea, Schiff-base, or thioester bond . Methods have been developed for the conjugation with peptides by solid-phase peptide synthesis (SPPS) resulting in defined position and number of chelate moieties. The outcome of the conjugation in solution wherein peptides and proteins comprise several reactive sites is very often a mixture of bioconjugate molecules with various content of the chelator. Such heterogeneity may cause the interpretation ambiguity of the performance of such imaging agents. Regioselective conjugation to antibodies was achieved by enzymatic reaction with lysine and glutamine residues using bacterial and human tissue transglutaminase as catalysts .
The chemical synthesis of proteins provides synthetic chemists with an interesting challenge and supports biological research through the generation of proteins that are not produced naturally. Although it offers advantages, studies of solid phase peptide synthesis have established limits for this technique: researchers can only prepare peptides up to 50 amino acids in length in sufficient yields and purity. Therefore, researchers have developed techniques to condense peptide segments to build longer polypeptide chains.
The progress of total chemical protein synthesis has been ..
Tyrocidine A is an antibiotic cyclic decapeptide produced by the bacteria Bacillus brevis. When imidazole aqueous organic solution was applied to the synthesis of tyrocidine A, the cyclization was complete after reacting at room temperature overnight with more than 90% of the desired product (). The reaction worked more efficiently than our randomly selected peptide thioester 4a–i. Tyrocidine A has a confirmed β-turn and β-pleated sheet conformation. We hypothesize that the β-turn structure facilitates the cyclization. To test the effect of the conformation of the linear peptide on cyclization, peptides having the same sequence but residue stereoisomers were synthesized. Model pentapeptide thioesters H-Asp-Leu-Thr-Phe-Gly-SBzl, H-Asp-Leu-Thr-D-Phe-Gly-SBzl, and H-Asp-D-Leu-Thr-D-Phe-Gly-SBzl were synthesized and then cyclized under the general cyclization condition. After 72 h reaction, no linear precursory were detected by LC-MS in all three cases. The results of cyclization of these three peptides were highly sequence dependent (). Cyclization of peptide thioester made up of all -amino acids gave 95% of the desired cyclic product and only 5% of the hydrolysis byproduct. Under the same conditions, cyclization of the peptide thioester with -amino acid yielded 74% of the hydrolysis product, 5% of the desired cyclic product and 21% of the cyclic dimer. The peptide thioester with -amino acid yielded 78% of the cyclic monomer, 2% of the cyclic dimer and 20% of the hydrolysis product. Circular dichroism (CD) study of these purified linear peptide thioesters in aqueous methanol showed that the conformations changed with -amino acid replacement of residues (). The peptide thioester having all--residues gave a positive peak at approximately 193 nm, and a negative peak at approximately 204 nm, suggesting the β-turn structure of the linear peptide thioester. The CD spectra of peptide thioester having -sequence gave a strong negative peak around 196 nm, suggesting a random coil structure. The CD spectra of -peptide thioester was more complicated and had the characteristics of both an α-helical and random coil conformation. Comparison of the CD spectra with the results of cyclization indicates that the cyclization efficiency is correlated to the conformation of linear peptide thioester. Molecular dynamics simulations and quantum mechanics calculations were also applied to these three peptide thioesters. The results are consistent with the cyclization tendencies.
a standard procedure for peptide thioester synthesis by ..
Cyclic peptides are important synthetic targets because of their potential usefulness as promising lead compounds in the drug discovery process. Their constrained conformation may result in increased resistance to proteolysis and higher receptor-binding affinities compared to their linear analogues. Typically, head-to-tail amide-cyclized peptides are synthesized by cyclization of a linear precursor either in solution or anchored to a resin support. Classical approaches for the synthesis of the cyclic peptides rely on the use of orthogonally protected linear precursors which are selectively activated and cyclized. The success rate of the macrocyclization step in these synthesis schemes is heavily dependent on the peptide coupling reagents., Cyclization via these conventional carboxylate activation chemistries is often inefficient, providing a complex mixture of linear and cyclic molecules, as well as oligomers.