Common causes of peptide analysis failure include: 1) oxidation; 2) poor peptide solubility; 3) inappropriate storage conditions; 4) TFA counterion contamination; and 5) biological contamination.
1) Oxidation
Symptoms of oxidation:
Over time, peptide activity decreases or disappears; the peptide color changes from white to yellow, tan, or brown.
Peptides susceptible to oxidation include sequences containing Cys, Trp, and Met amino acids.
Preventative Measures:
Design Optimization: Use Ser instead of Cys and Nle instead of Met. Storage and Protection of Peptides: Aliquot the peptide (to avoid repeated aliquoting); protect the peptide with argon and store it in a sealed container. Protection during Peptide Analysis: Add reducing agents (antioxidants): DTT, TCEP, β-mercaptoethanol; use argon to protect the buffer; replace the buffer and peptide with argon-filled water in an oxygen-free chamber; and perform analysis in an oxygen-free environment.
2) Poor Peptide Solubility
Symptoms of Poor Peptide Solubility:
Peptide is turbid or precipitates after dissolution.
Sequences with Poor Peptide Solubility: Peptide sequences >5 AA in length and containing Trp, Phe, Tyr, Met, Ile, Leu, or Val for more than 50% of the peptide sequence.
Solutions for Poor Peptide Solubility:
Follow solubility guidelines when dissolving; Perform solubility testing when synthesizing peptides; Replace non-essential hydrophobic amino acids with polar or charged amino acids; Introduce O-acyl groups (e.g., PEG); Introduce hydrophilic linkers (e.g., Lys-Lys-Lys-Lys); Frameshifting can be performed for peptide libraries.
3) Improper Storage Conditions
Symptoms of Improper Peptide Storage:
Peptide activity decreases or disappears over time; Lyophilized peptides transform from powder to crystals;
Susceptible Sequences: Peptide sequences containing Ser, Thr, Lys, Gly, or Arg amino acids.
Solution:
Aliquot the peptide (this will avoid repeated aliquoting); store the peptide at -20°C in a dark, dry place; avoid repeated freeze-thaw cycles;
4) TFA Counterion Contamination
Symptoms of TFA Counterion Contamination:
Unstable cell or tissue activity and enzyme assay results; IR spectra; Reduced mass spectrometry response; Peptide degradation;
Sequences susceptible to TFA counterion contamination: All sequences, especially those containing the positively charged amino acids Lys, His, and Arg.
Solution:
Select a TFA removal service or desalting treatment;
5) Biocontamination
Symptoms of Biocontamination:
Endotoxin contamination; Unwanted immune response;
Sequences susceptible to biocontamination: All sequences.
Solution:
Select an endotoxin removal service; Open the peptide container in a sterile hood;