This article summarizes common technical questions related to peptide synthesis, purity, solubility, storage, purification, difficult sequences, resin loading, and Fmoc solid-phase peptide synthesis procedures.
In peptide raw material business and technical communication, customers often ask about synthesis principles, purity requirements, solubility, storage conditions, purification methods, and difficult sequence handling. The following Peptide Q&A is intended to provide a practical reference for product understanding and technical support discussions.
Solid-phase peptide synthesis is a major breakthrough in peptide chemistry. Its biggest advantage is that intermediate products do not need to be purified one by one, allowing the synthesis process to proceed continuously and supporting automation. Most automated peptide synthesis today is based on solid-phase synthesis.
Using Fmoc chemistry, the carboxyl group of the C-terminal amino acid of the target peptide is first covalently attached to an insoluble polymer resin. The amino group of this amino acid then serves as the starting point for peptide chain elongation. It reacts with activated carboxyl groups of other amino acids to form peptide bonds. By repeating this process, the peptide is assembled step by step. Depending on amino acid composition, post-treatment and purification methods may differ.
Generally, a length of about 10-15 amino acids is suitable. A longer peptide may provide a better immunization effect, but synthesis cost also increases. For MAP peptides, a length above 15 amino acids is usually preferred for better results. Peptides below 10 amino acids often show weaker immunization performance.
In general, for immunization purposes, a peptide purity of 70-85% is usually sufficient.
No. It is difficult to predict peptide solubility accurately or determine the best solvent in advance. Poor solubility does not necessarily mean there is a problem with peptide synthesis.
The peptides we provide are in powder form, usually off-white or grayish-white. Depending on composition, the color may vary. For long-term storage, peptides should be protected from light and stored at -20°C. For short-term storage, 4°C is acceptable. Short-term room-temperature shipment is also possible.
Peptide dissolution can be complex, and it is often difficult to identify the correct solvent immediately. It is strongly recommended to test a small amount first and never dissolve the entire sample before identifying a suitable solvent.
Crude and desalted peptides may contain both peptide and non-peptide impurities, such as truncated peptides and residual processing reagents like DTT or TFA.
Even after HPLC purification, some impurities may remain. These are mainly shorter peptides and trace amounts of TFA.
Peptide synthesis must consider length, charge, and hydrophobicity. As peptide length increases, crude purity and yield generally decrease, while purification becomes more difficult and synthesis failure risk rises.
For functional peptide regions, the sequence itself may not be changed. However, in some cases, auxiliary amino acids may need to be added upstream or downstream to improve solubility and balance hydrophobicity. If a peptide is too short, synthesis can also be difficult. For peptides below 5 amino acids, hydrophobic amino acids are often needed to make post-treatment easier. Peptides under 15 residues usually give satisfactory yield and purity.
Peptides containing Cys, Met, or Trp are often more difficult to synthesize and harder to obtain at high purity because these groups are relatively unstable and prone to oxidation. These peptides require particular care during use and storage, and repeated opening of containers should be avoided.
Peptide synthesis differs significantly from oligonucleotide synthesis. Some peptide sequences are genuinely difficult or impossible to synthesize efficiently. When amino acids such as Val, Ile, Tyr, Phe, Trp, Leu, Gln, or Thr are adjacent or repeated, the peptide chain may not fully expand on the resin, which lowers synthesis efficiency.
Examples of sequences that often reduce synthesis efficiency and product purity include repeated Pro, Ser-Ser, repeated Asp, and stretches of four consecutive Gly residues.
Peptides are generally purified using reverse-phase columns such as C8 or C18, with detection around 214 nm. The buffer system usually contains TFA at pH 2.0. Buffer A is commonly 0.1% TFA in water, and Buffer B is 1% TFA in acetonitrile at pH 2.0.
Before purification, Buffer A is used for dissolution. If solubility is poor, Buffer B may be used first, followed by dilution with Buffer A. For highly hydrophobic peptides, a small amount of formic acid or acetic acid may also be required. If the crude product is short (below 15 amino acids), a main HPLC peak is usually present and often corresponds to the full-length peptide. For longer peptides (above 20 amino acids), if no dominant peak appears, HPLC should be combined with mass spectrometry to identify the correct fraction.
In solid-phase peptide synthesis, free amino groups on the resin are detected to evaluate coupling efficiency. This is known as the Kaiser test. If free amino groups are present, the color turns blue, or red-brown in the case of Pro, Ser, or His residues.
The pyridine should be treated with ninhydrin and redistilled before use. To perform the test, take a small amount of resin, add 2-3 drops each of A, B, and C, and heat at 100°C for 1-2 minutes. If the solution or resin turns blue or red-brown, free amino groups are still present. If no color appears, coupling is complete.
Other methods for detecting free amino groups include TNBS, picric acid, and bromophenol blue tests.
Solid-phase peptide synthesis may fail or proceed inefficiently due to difficult sequences. One major reason is that certain sequences form beta-sheet structures on the resin, changing swelling behavior and burying reactive sites. Reported improvement methods include:
Wash 0.1 mmol resin with DMF. Dissolve 0.244 g (+)-biotin (1 mmol, MW 244.3) in 5 mL DMF-DMSO (1:1) solution. A little warming is necessary. Add 2.1 mL 0.45 M HBTU/HOBt solution and 0.3 mL DIEA to the solution prepared in step 2. Add the activated biotin solution to the resin and let stir overnight. Check resin to make sure coupling is complete as evidenced by negative ninhydrin test (colorless). Wash resin with DMF-DMSO (1:1) (2x) to remove excess (+)-biotin. Wash resin with DMF (2x) and DCM (2x). Let the resin dry before proceeding to cleavage.
Weigh 10 g 2-chlorotrityl chloride resin (15 mmol) in a reaction vessel, wash with DMF (2x), swell the resin in 50 mL DMF for 10 min, drain vessel. Weigh 10 mmol Fmoc-amino acid in a test tube, dissolve Fmoc-amino acid in 40 mL DMF, transfer the solution into the reaction vessel above, add 8.7 mL DIEA (50 mmol), swirl mixture for 30 min at room temperature. Add 5 mL methanol into the reaction vessel and swirl for 5 min. Drain and wash with DMF (5x). Check substitution. Add 50 mL 20% piperidine to remove the Fmoc group. Swirl mixture for 30 min. Wash with DMF (5x), DCM (2x), put resin on tissue paper over a foam pad and let dry at room temperature overnight under the hood. Cover the resin with another piece of tissue paper, press lightly to break aggregates. Weigh loaded resin. Pack in appropriate container.
Weigh duplicate samples of 5 to 10 mg loaded resin in an eppendorf tube, add 1.00 mL 20% piperidine/DMF, shake for 20 min, centrifuge down the resin. Transfer 100 µL of the above solution into a tube containing 10 mL DMF, mix well. Pipette 2 mL DMF into each of the two cells (reference cell and sample cell), set spectrophotometer to zero. Empty the sample cell, transfer 2 mL of the solution from step 2 into the sample cell, check absorbance. Subs = 101(A)/7.8(w) A = absorbance w = mg of resin Check absorbance three times at 301 nm, calculate average substitution.
Wash resin with DMF (4x) and then drain completely. Add approximately 10 mL 20% piperidine/DMF to resin. Shake for one min and drain. Add another 10 mL 20% piperidine/DMF. Shake for 30 min. Drain reaction vessel and wash resin with DMF (4x). Make sure there is no piperidine remaining. Check beads using ninhydrin test, beads should be blue. Coupling Step - Prepare the following solution: 1 mmol Fmoc-amino acid 2.1 mL 0.45 M HBTU/HOBT (1mmol) 348 µL DIEA (2 mmol) Add above solution to the resin and shake for a minimum of 30 min. This coupling step can be longer if desired. Drain reaction vessel and wash resin with DMF (4x). Perform Ninhydrin test: If negative (colorless), proceed to step 2 and continue synthesis. If positive (blue), return to step 5 and re-couple the same Fmoc-amino acid. Increase the coupling time if necessary.