What is a counterion?

Most peptides bind counterions and are lyophilized in the form of the corresponding salts. Exceptions are those that do not have basic amino acids such as Arg, Lys, His, and those with a blocked N-terminus (such as by acetylation). Synthetic peptides that are purified by RP-HPLC are usually obtained as TFA (CF3COO or trifluoroacetate), AcO (acetate) or HCl (hydochloride) salts. Basic amino acid residues and the free N-terminal amine of the peptide are protonated and have TFA or AcO or HCl counterions depending upon the final lyophilization buffer. The vast majority of peptide APIs are produced as acetate salts. On rare occasions, they may be prepared as HCl or Na+ salts. APi can accommodate your salt form requirements.

Read also about net peptide content.

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