At CellMosaic, we offer custom synthesis of natural peptides, as well as modified peptides such as peptidomimetics, peptides with orthogonal functional groups, fully protected peptides, chimeric peptides, fluorescent or biotin-labeled peptides, peptide–oligo conjugates, and peptide-antibody/protein/enzyme conjugates. We use a PS3™ peptide synthesizer (Protein Technologies, Inc.) for milligram to gram scale synthesis. This machine is simple to use and flexible enough to allow manual intervention, which is critical for synthesizing high quality peptides with modified groups. In order to obtain the highest quality, all of our peptides are synthesized using the most efficient HATU or HCTU coupling reagent, in contrast to the less efficient HBTU generally used by other peptide houses to reduce costs. We use an Fmoc peptide protocol with modifications deemed necessary to make the highest quality racemization-free peptide.
Quote Request for Peptide Synthesis: please send us the peptide sequence and indicate where you would like to incorporate the functional group.
Cysteine has a free thiol group that can be specifically modified or conjugated with other molecules through well-established thiol chemistry; for example, by reacting with another thiol compound to form a cleavable disulfide bonded conjugate, or by reacting with a maleimide or an alkyl halide compound to form a stable thiol ether-bonded conjugate. N-terminal Cys-containing peptides have been used frequently for coupling with an immunogen carrier, such as KLH or BSA, for immunization purposes. Racemization of Cys residues is known to occur during standard HBTU and HATU coupling. This racemization can lead up to 40% byproduct and, in many cases, the racemized products are difficult to purify using standard reverse phase HPLC. To minimize the racemization of Cys residues (and histidine residues) during peptide synthesis, we intervene and perform manual couplings for all Cys and His residues. The outcome of such processes is high quality Cys or His-containing peptides. For peptides containing several Cys residues, we can selectively protect and deprotect Cys for labeling and conjugation. We can also selectively manage inter- or intramolecular disulfide bonds using orthogonal protecting groups for Cys.
Below is an example of a Cys-containing peptide synthesized using the standard automatic procedure (left) vs. manual intervention and special coupling (right)
Aminooxy (-ONH2) is known to react quantitatively with ketone or aldehyde-containing compounds under very mild near physiological conditions. The resulting oxime compound is stable for most biological applications. The oxime bond can be further reduced to a more stable imine bond. Despite their usefulness, aminooxy peptides are not easy to make. Commercially available Fmoc protected aminooxy amino acid analogs suffer from double acylation during peptide synthesis. However, using the di-Boc protected aminooxy amino acid results in very poor yield of peptide during TFA cleavage because of acidic decomposition. At CellMosaic, we address this issue by synthesizing a special aminooxy monomer for peptide synthesis and using a clean cleavage cocktail.
Keto peptides have frequently been synthesized at CellMosaic. Keto peptides can react with any aminooxy or hydrazine-containing compound.
Selective Modification of Peptides through a C-Terminal Acid
Peptides can be selectively labeled or conjugated through the C-terminal carboxylic acid. To perform such a conjugation for peptides containing few internal Asp and Glu residues and other functional groups, such as free amine and thiol, the peptide needs to be fully protected during modification and conjugation. Usually, trityl chloride or Rink acid resin can be used for synthesizing such fully protected peptide acids. The fully protected peptide acid can then react with any molecule of interest in solution.